Galaxy Collisions: Forging New Worlds from Cosmic Crashes

Galaxy Collisions

Forging New Worlds from Cosmic Crashes

Curtis Struck Struck

Galaxy Collisions Forging New Worlds from Cosmic Crashes Crashes

«2 Springer

Published in association with

Praxis Publishing Chichester, UK

Professor Curtis Struck Iowa State University Ames Iowa USA

SPRINGER±PRAXIS BOOKS IN POPULAR ASTRONOMY SUBJECT ADVISORY EDITOR: John Mason, M.B.E., B.Sc., M.Sc., Ph.D. ISBN 978-0-387-85370-3 e-ISBN 978-0-387-85371-0 DOI 10.1007/978-0-387-85371-0 Springer New York Dordrecht Heidelberg London Library of Congress Control Number: 2011921115

# Springer Science+Business Media, LLC 2011 All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York, NY 10013, USA) except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. Cover design: Jim Wilkie Project copy editor: Dr John Mason Typesetting: BookEns, Royston, Herts., UK Printed on acid-free paper Springer is a part of Springer Science+Business Media (springer.com)

Contents

Preface Acknowledgments List of Illustrations

vii xi xiii

1

Exploring the w o r l d of galaxies 1.1 Finding the galaxies " 1.2 Galaxy properties: classification and evolving views 1.3 Modern views: universal classes and scalings 1.4 Discovering collisions and interactions 1.5 Collision basics

1 3 11 18 23 27

2

Head-on collisions and rings of fire 2.1 Early stage archetypes 2.2 Forging the rare rings 2.3 Why rings? 2.4 How precise a bullseye? Angles and offsets 2.5 Taffy galaxies and shovels: companions with gas disks 2.6 Twice-made rings 2.7 Bejeweled rings 2.8 The mysterious cartwheel: prototype or anomaly? 2.9 Other ways to make rings

31 31 32 40 41 44 47 49 51 53

3

Swinging on by: the tidal twists 3.1 Tides near and far 3.2 Discovering the whirlpool spirals 3.3 Into the tidal whirlpool: understanding the mechanisms 3.4 The foaming waves 3.5 The eye galaxies 3.6 Making bars 3.7 Probing the darkness with tails and dwarfs 3.8 Retrograde encounters: dancing backwards

57 57 62 64 68 71 72 75 86

4

Major mergers and their colossal consequences 4.1 Ellipticals from mergers? 4.2 A sticky aside on dynamical friction 4.3 Bringing a little data to the subject 4.4 Relaxing in the merger: quickly and violently!

93 93 96 98 100

vi

Galaxy Collisions 4.5 4.6 4.7 4.8 4.9 4.10

Starbursts, super-starbursts and mergers A new generation of merger studies Starbursts, winds and nuclear activity Growing quasars Merger models with feedback Ellipticals and mergers again

103 110 114 119 123 130

5

W h a t happens to t h e little guy: minor mergers 5.1 Classifications and destructive processes 5.2 Global dynamical friction and anti-friction 5.3 Cumulative effects

139 139 145 149

6

The 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8

secret of fire: triggered star f o r m a t i o n Counting young stars The spreading fire Attacking the flames with the latest tools Glowing embers: induced star cluster formation Fading embers: star cluster disruption How to start the fire Forecasting fires How did it burn?

155 155 162 167 170 178 182 187 195

7

Not 7.1 7.2 7.3 7.4

always in a galaxy far, far away Our Local Group of galaxies Collisions here and now: the Magellanic Clouds Collisions now and then: contrails in the halo . . . And those yet to come

201 201 203 215 226

8

A w i d e r view 8.1 In the beginning... 8.2 Many different evolutionary pathways in groups 8.3 The bright lights of the galaxy metropolis

233 233 239 251

9

Epilogue: the galacto biology of collisions

261

Glossary Resources Index

273 277 279

Preface

Sparks fly as an ancient blacksmith hammers a piece of glowing metal on his anvil. Similarly, interstellar gases are flung off the outer disk of a galaxy as a consequence of tidal forces following a close encounter with another. The gases gather in a vast plume, compressed in some regions, where gravitational forces squeeze out new star clusters, the galactic "sparks." The smith returns his metal to the fire to be reheated. As hundreds of millions of years pass, gas in the great tidal plume falls back onto the galaxy. Some of it finds its way into the core, and fuels the hot fires of a galactic starburst, whose "coals" are hundreds of millions of newly formed stars. The smith works his metal repeatedly, tempering and strengthening it. The collision induces compression waves, which propagate through the galaxy disk, inducing more star formation, and ultimately producing a mature disk, with relatively little leftover gas. For thousands of years the blacksmith was the master of a metal-based technology. He was as powerful and magical to his contemporaries as a modern day "rocket scientist." His power was in his knowledge not in his ability to muster political or military resources. He was an important part of the process that led to more complex civilizations possessing much more sophisticated technologies. Galaxy collisions also play a key role in the great story of the evolution of galaxies, and the development of structure in the universe. Both stories involve the ancient "element" of fire in some sense, and both involve the refinement of metals from seemingly unpromising raw materials. In metallurgy it is the chemical refinement of raw ores. In evolving galaxies, it is the nuclear production of the chemical elements beyond helium, which astronomers call "metals." Color images of disturbed galaxies with knots of bright young star clusters suggest an extension of the metaphor to fine jewelry. Nonetheless, despite this rather stretched metaphor, the fact remains that the world of galaxies is very different to ours. Galaxies appear ghostly, especially when seen with the naked eye through the lenses of a small telescope. However, this is far from the truth. Galaxies are the heftiest structures in the universe, except for clusters and superclusters of galaxies. The metals produced by the generations of stars that define their evolution allowed the formation of solar systems, planets, and life forms like us. Although they appear barely visible and ephemeral on the night sky, the story of their evolution underlies the story of ours. At first sight, galaxies appear to be very isolated entities. At least this is the way it looked to many of the astronomers who first cataloged them in the last century. It took some time to realize that galaxies can in fact fall together and

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Galaxy Collisions

collide. The buildup of galaxies through collisions is very vigorous, even unremittingly violent, in the early ages of the universe. Only recently have we discovered that a continuing rain of small galaxies onto larger ones like our own Milky Way continues to the present time. The story of single-celled life is the continual division and propagation of daughter cells. To a large degree, the life story of galaxies is the exact opposite; continual buildup, especially in the early days. The great story of the buildup of the largest structures in the universe is studied in two general ways. At the present time, the primary way is via largescale studies of the statistical averages (and characteristic deviations from these averages) of many individual cases, using both observations and sophisticated computer models. We can learn much about the properties and kinds of galaxies through the ages of the universe in this way. We can learn more about the details, such as the buildup of successive populations of stars (and perhaps planets), in the second way, which is the detailed study of how specific types of collisions and mergers change galaxies. The first way is the shortest route to understanding the big picture of galaxy evolution, though the statistical details can sometimes be rather dry. The second way carries the risk of what scientists sometimes call butterfly collecting or botany - collecting many pretty examples, but not seeing the whole picture. On the other hand, there are paths to big picture truths through the study of many interesting galaxy family sagas. We will try to explore both paths, without losing our way, in the following chapters. Galaxy collisions are a slippery topic on several levels. Firstly, it is rather hard to envision such a collision. Like continental drift, the objects and the process are just too big. Also like continental drift, the process takes a very long time. However, our minds can get around those difficulties in the same way that they get around many others, by ignoring them! In this book, I will talk about the vast galaxies with the same easy familiarity that I talk about my car, and I will describe their collision processes as though they occurred over a timescale similar to that of a typical television drama. This approach takes out a lot of the awe, but it is a practical necessity. The second reason that the subject of this book is a bit slippery is that it involves some complex dynamical processes. One my greatest challenges has been to explain these clearly, while ruthlessly striving not to get sucked into the whirlpool of complications. We have really learned an enormous amount about these distant phenomena, using a variety of powerful physical and mathematical tools. However, most readers of this book will be more interested in the big picture (appropriately for this topic!) than the fine print. I have worked hard on this, but no doubt I have fallen short in some places. A third slippery aspect of galaxy collisions is that it is not so much a selfcontained subject as a nexus or meeting place of many other topics in astronomy and astrophysics. A little knowledge of a lot of these topics is very helpful, and again I have wrestled with providing the minimum amount of background that is necessary, and to avoid sidetracks, even if many of them are actually beautiful byways. There is another side to this coin. Because of the nature of the field,

Preface

ix

many of the contributions have been made by researchers in related fields pursuing a sidetrack that leads in, and often relatively quickly out, of the field of galaxy collisions. There is a core of researchers who have spent most of their careers in this field (even if it does sometimes feel more like a nomadic camp rather than a settled community), in contrast to more self-contained areas of science. Altogether, thousands of both the nomads and settled farmers of research have contributed to this field. One of the unhappiest parts of writing a book like this is accepting that even major contributions will get only a brief mention, and many significant ones none at all. Not to mention the fact that what is major is still somewhat in the eyes of the beholder in this very active field. A consolation for these woes is the possibility of inciting an interest, or at least some curiosity, in readers new to the field. I hope this tourist brochure of the world of galaxies and their interactions will be useful as a starting point for deeper explorations. That is the primary goal of this book. As we will see, galaxy collisions occur throughout the world of galaxies, so the study of collisions is inevitably an exploration of the galaxy world. It is very natural to pursue that exploration on a path that is parallel to the evolution of a typical collision and merger. Before beginning on that path, Chapter 1 provides some general background on the history of the discovery of galaxies, some of their systematic properties, and how they have been discovered. In this chapter, I also define some basic terminology that is used throughout the book, including Hubble's galaxy types. In Chapters 2 and 3 we begin the journey in earnest, by exploring the early stages of galaxy collisions. Many of the most beautiful forms in the world of galaxies are found in this area, and we will study a number of these individual systems. Galaxy collisions build up galaxies, because most collisions end in the merger of the collision partners. We explore this process and its consequences in Chapters 4 and 5. The former chapter treats mergers between near equals; the latter treats the capture of smaller objects by bigger ones. One of the most spectacular consequences of galaxy collisions is the increase in the rate of star formation induced through the merger process. We consider the statistics and the physical processes behind this in Chapter 6. We will also consider the role of mergers and induced star formation in creating the phenomenon of active galactic nuclei in that chapter. Armed with a basic understanding of galaxies and their evolution in collisions, we return to our own Local Group of galaxies in Chapter 7 in search of evidence of collisions in its history. While we find that the Local Group has been a fairly quiet village in the world of galaxies, it appears that future development is inevitable. Finally, in Chapter 8 we take a broad look across many scales in the galaxy world. In so doing we get a better view of the environments of galaxy collisions, their cosmological context, and how the collision process is repeated on the larger scale of galaxy groups and clusters. I have attempted to avoid unnecessary jargon and abbreviations in this book

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where possible. When it is cumbersome not to, I have tried to confine the technical terms to the chapter or section where they are most relevant and used in context. However, a number of these technical terms are collected and defined in the Glossary for easy reference. There are relatively few general reading sources on galaxy collisions; this is part of the reason that this book was written, but a number of such sources have been listed in the Resources section at the end of the book. These are sources that are relevant to every chapter in the book, while some more specific references will be given at the end of each chapter. It is hoped that all of these resources will provide a starting point for readers interested in digging deeper.

Acknowledgments

This book is a small and humble distillate from a huge range of inputs and experiences of the author over several decades. I am afraid that these inputs are too many to remember, let alone properly acknowledge individually. I apologize to many for that, and also for any errors or inadequacies in my treatment of the rich materials that have been given to me. However, a few collective nods of appreciation are due. First, to the researchers, research organizations, and publishers who have given me permis­ sion to use images and other material. This field is graced by hauntingly beautiful images, which provide constant inspiration. Many of these resources would not be available were it not for the professional and general outreach work of the NASA Extragalactic Database, the arXiv preprint server, the Space Telescope Science Institute, the JPL Infrared Processing and Analysis Center, and others cited in the text and figures below. Next, I would like to give my warmest thanks to my research collaborators, Iowa State colleagues, and students, who are a huge source of continuing inspiration and education. How many of my blind spots have been illuminated in our conversations over the years! How much have you challenged me and kept me on track! Similarly, I am very grateful to Dr John Mason, Clive Horwood, and the editorial staff at Springer-Praxis for all their help. Though it is a cliche, I found this project much more difficult than I expected, and I found the professional and personal interruptions much more numerous than planned. John and Clive, thanks for your tolerance of my delays, mistakes of inexperience and gentle management overall. Finally, I am very mindful of the personal support provided by friends and family not only during this project, but also for my work that, over the years, provided a basis for it. Certainly, my immediate family endured neglect and occasional grumpiness as I struggled with the chapters below. But a much wider circle provided support, even with simple encouragement along the lines of, "I'd like to read it when you finish," which held out the possibility that I might actually finish. I hope you all find a little something of interest and inspiration in the world of colliding galaxies. Curtis Struck October 2010

Illustrations

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.1 2.2 2.3 2.4 2.5 2.6 2.8 2.9 2.10 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10

William Herschel's map of the Milky Way Lord Rosse's sketch of the Whirlpool galaxy The Hubble Diagram Hubble's sketch of his "tuning fork" scheme for classifying galaxies A montage of images of galaxies representing the Hubble classes A mid-infrared view of the Hubble classification scheme A prototypical rotation curve of a large disk galaxy Hubble Heritage image of the Whirlpool galaxy A collection of colliding galaxy forms HST image of the Cartwheel colliding ring galaxy A montage of ring galaxy images obtained from the Hubble Space Telescope Plot of how the orbital (epicyclic) radius changes with time The so-called "Sacred Mushroom" ring galaxy Early computer models of the effects of ring galaxy type collisions Contour map of hydrogen gas strewn between the two galaxies of the "Taffy" system Hoag's object, which has a broad, annular ring around a central bulge A beautiful false-color image of the Cartwheel ring galaxy NGC 4650a, a polar ring galaxy, whose ring is perpendicular to its inner disk The directions and relative magnitude of tidal forces acting on a spherical body The so-called "gravity well f u n n e l " The restricted 3-body effective potential diagram A very deep optical image of the Whirlpool galaxy, M51 An early computer model of the tidal production of the spiral arms in M51 Hubble Heritage image of the interacting system NGC 2207/IC 2163 Appearance and subsequent evolution of a bar component initiated by a galaxy collision A sampler of optical images of tidal tails from the Arp Atlas A computer model of the formation of the tail in IC 2163 The long tidal tails of the merger remnant NGC 7252

6 7 10 12 13 14 16 24 26 33 34 37 38 43 45 46 52 54 58 60 61 63 65 71 74 76 77 78

xiv 3.11 3.12 3.13 3.14 3.15 3.16 3.17 3.18 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 5.1 5.2 5.3 5.4 5.5 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8

Galaxy Collisions Star formation regions in the tidal dwarf galaxy at the end of the southern tail Antennae system False color optical image of Arp 105, the Guitar system The star-forming region called Ambartsumian's knot in Arp 105 Ultraviolet image of the Arp 305 system from the GALEX satellite SDSS image of the Arp 24 system SDSS image of M l 0 1 and its smaller companion galaxy NGC 5474 Near-infrared image of M l 01 Hubble Heritage image of the complex spiral arms of NGC 4622 The Toomre merger sequence Schematic illustrating Chandrasekhar's wake theory of dynamical friction An enhanced view of the merger remnant NGC 7252 Four famous merger remnants and ultraluminous infrared galaxies (ULIRGs) Hubble Heritage image of the Cigar galaxy, M82 A mid-infrared image of the starburst ring galaxy NGC 1097 Example of a multi-component merger model Multi-wavelength views of the relatively nearby merger remnant NGC 5128 HST images of quasar host galaxies Schematic merger-to-Active Galactic Nucleus evolutionary scenarios Sample star formation history from a 1996 merger model Images of the massive Millennium Simulation of large-scale structure formation Star-forming regions and young star clusters in the Antennae system Deep photographic image of the shell galaxy NGC 3923 Hubble Heritage image of the inner parts of the Black-Eye galaxy Two sample resonant orbits, of the type that counter dynamical friction Frictional orbital energy inputs to a star cluster orbiting in a galaxy halo Snapshots of a model of a near-prograde minor merger A red (continuum) image of the nearby spiral galaxy NGC 628 compared with a Ha emission line image Aerial view of the VLA radio telescopes Artist's impression of the ALMA telescopes SDSS images of a sample of tidal bridges and tails Hubble Heritage image of the Tarantula nebula and the R136a star cluster Images of the several clumpy tails and bridges HST image of the NGC 7714 in the Arp 284 system Optical image of the Eagle nebula star-forming region in the Milky Way

80 82 83 84 87 88 89 90 95 97 99 104 106 109 112 117 118 123 128 134 136 141 144 148 149 151 159 160 161 163 172 177 181 183

Illustrations 6.9 6.10 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11 7.12 7.13 8.1 8.2 8.3 8.4 8.5 8.6 8.7 9.1 9.2

Ultraviolet image of Arp 261 system from the GALEX satellite Results of various studies of the cosmic star formation rate versus redshift A three-dimensional map of the locations of Local Group galaxies Images of several Local Group galaxies to scale Composite image of the Milky Way, the Magellanic Clouds, and the Magellanic Stream 21 cm hydrogen gas emission from the Magellanic Clouds, Stream, and Leading Arm Schematic of a model of the Magellanic system Bound and unbound orbits of the Magellanic Clouds around the Milky Way The 2.5-meter Sloan Digital Sky Survey telescope at Apache Point Observatory SDSS image of the Draco dwarf spheroidal galaxy SDSS image of the so-called "Field of Streams" Map of red giant stars, derived from the 2MASS infrared sky survey Giant stellar loops around the edge-on disk galaxy NGC 5907 Dwarf satellites of the Andromeda galaxy, M31 The formation and development of the streams and shells around M31 Snapshots of a supercomputer model of elliptical galaxy formation HST images of the Coma cluster of galaxies The Hercules galaxy cluster Contours superimposed on an optical image of the M81 group HST image of Stephen's Quintet False color rendition of multi-waveband observations of Stephen's Quintet Optical image of the Bullet cluster of galaxies Eight high redshift galaxies derived from the Hubble Ultra Deep Field Portion of the famous Hubble Ultra Deep Field

xv 185 196 202 203 206 208 212 213 216 218 220 221 225 227 229 236 240 241 244 246 248 258 262 270

1

Exploring the World of Galaxies

In this book, we are going to explore the world of the galaxies and, in particular, study their interactions. For most people this is a very unfamiliar world. I mentioned in the Preface that galaxies have a ghostly or ephemeral appearance in small telescopes. Their appearance in our culture is also somewhat insubstantial. Most of us learned in school that the faint fuzzy band of light that can be seen stretching across the sky on summer evenings is a manifestation of the millions of distant stars contained within our own Milky Way galaxy. Some of us are old enough to remember cars or televisions with names like "Galaxie" or "Quasar." (As far as I know, no company has yet named their product "super-massive black hole," though I have known some that might be appropriately described that way.) A perusal of the astronomy coffee table books at the local bookstore will reveal many beautiful space telescope images of galaxies and galaxy collisions, including some that I have had the pleasure to work with. Similar images abound in science fiction movies and TV shows. These images can inspire, but no single image can convey the activity that spans huge ranges of spatial and temporal scales in galaxy collisions. Moreover, all of these scales are remote. In terms of size and mass and characteristic times, galaxies (at about 10 22 meters across with masses of about 10 42 kg) are even more remote from human scales than atoms (at about 10"10 m and 10"26 kg). However, great or tiny size is not necessarily a barrier to our understanding of the importance of a phenomenon in our universe. Submicroscopic germs and global climates are very much on our minds these days. Yet knowledge of the role of galaxies in our cosmic evolution, and the possibility of their continuing influence on our planet, has only been obtained within the past couple of decades, and is still somewhat tentative. Thus, it may be a bit early to expect to see teenagers sporting t-shirt slogans like "Love your galaxy" or "No fear of galaxy collisions." We can begin the process of familiarization with the galaxy world by using a number of metaphors to more familiar ideas and things in our world. For example, we can compare the scientific exploration of it in the past few centuries to other famous explorations. One aspect of this exploration is similar to the European discovery of the North American continent, and of the first travels of humans out of Africa. That is, an impression of vast emptiness. The average distance between galaxies is about ten times their physical extent, and a much larger multiple of the size of their visible stellar components. Another aspect of galaxy interactions is like Jane Goodall's famous explora­ tion of the social and technological world of chimpanzees. Most likely, humans C. Struck, Galaxy Collisions: Forging New Worlds from Cosmic Crashes, Springer Praxis Books, DOI 10.1007/978-0-387-85371-01, © Springer Science+Business Media, LLC 2011

1

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Galaxy Collisions

have known about chimpanzees since the time of their own origin, but who bothered to look at how they behaved in detail? Similarly, humans have always had the Milky Way to look at, but the scientific study of it was long neglected, though in fairness it took a long time to develop the tools which were necessary to pursue that study in any detail. As we will see below, even when galaxies began to be investigated in detail, the notion of collisions and interactions was initially dismissed. The study of the world of galaxies can sometimes seem like Alice's exploration of Wonderland - very confusing! For example, galaxies and components of galaxies move in a three-dimensional world without the constraints of ground or air resistance. These motions are governed by laws of gravity that are formally the same as on the surface of the Earth, but the practical consequences of these laws are very different. Still another aspect of the strangeness of the galaxy world comes from the fact that galaxies are composite objects with a great deal of nearly empty space between the stars, gas clouds and particles of dark matter that populate them. Despite their isolation, these objects are all bound together by the galaxy's gravity, and generally move on similar orbits. Comparisons with flying flocks of birds or traveling schools of fish come to mind. In many subfields of astronomy, the composite nature of galaxies is overlooked. For example, in studies of specific constituents, like stars or star clusters, the galaxy may be viewed as essentially a vessel holding the object of study. This is like studies of an individual fish caught from a school. For cosmological studies, the global properties of galaxies may be viewed only as signposts providing information on the overall structure and history of the universe. This is like the fisherman searching for schools of fish, interested only in their size, location and other bulk properties, not in the schooling behavior of the fish. We will learn a bit more about the constituents of galaxies on one hand, and their distribution across the space-time universe, on the other. However, most of our attention will be focused on the schools and their interactions. There is another point of comparison between the ocean world and the galaxy world. The great galaxy explorer Edwin Hubble put it as follows in his book The Realm of the Nebulae. "Toward the end of the nineteenth century, however, the accumulation of observational data brought into prominence the problem of the status of the nebulae and, with it, the theory of island universes as a possible solution." We will talk more about Hubble, nebulae and island universes shortly. I am afraid that a few parts of the exploration of galaxies and their evolution may seem as difficult as many early explorations on Earth. A full understanding of some aspects of galaxy interactions is technically difficult. Fortunately, most of the world of galaxies is truly a paradise to behold, with many exemplary individuals who have much to teach us. As a guide to this world, I will do my best to show you some of the great sights. Yet, a goal of this book is to get beyond the beautiful images to some of the new knowledge of what galaxies are, how they came to be, and how processes on the galactic scale can affect beings on small planets orbiting relatively ordinary

Exploring the W o r l d of Galaxies

3

stars within them. When we take stock of an individual human life, we mainly remember a handful of pivotal events or accomplishments, rather than the slowly changing day-to-day rituals of that life. It is not entirely obvious that the sum of the latter are much less important than any one of the former events, but it cannot be denied that the single pivotal events are important. The lives of galaxies have both kinds of events. Major collisions are certainly pivotal. Small micro-collisions are more routine events. As with humans, the cumulative effects of many small events, or what are called "secular" changes in dynamics, may be as important as the pivotal ones. However, that doesn't diminish the importance of the big changes. The focus of this book will be on explaining the big changes in the lives of galaxies induced by collisions.

1.1 Finding t h e galaxies

Our knowledge of galaxies has been hard won. Indeed, the history of the discovery of the nature of the Milky Way and other galaxies might more accurately be described as a comedy of errors rather than as the steady progress of science. It is still a great intellectual achievement, and we shouldn't expect the resolution of great mysteries of nature to be easy. For a long time the primary reason for this comedy was that astronomers did not have sufficient sensitivity and resolution in their instruments to see very many galaxies, or details within galaxies. More recently we have learned that the bulk of the mass in galaxies may be invisible dark matter. That is, it doesn't emit light or any other electromagnetic radiation, so that we can only detect its presence indirectly. Galaxies are still very hard to see! If galaxies are so big, and contain so much stuff, why is this so? To answer this question it is worth looking back briefly at the history of the discoverers, and the technologies they had to work with. From the beginning of humankind, four galaxies have been visible to naked eye observers. Two of them, the Magellanic Clouds are seen from the southern hemisphere, and so not easily observable to most of the early civilizations in the northern hemisphere. The Milky Way is visible from all parts of the Earth, but the true nature of this broad fuzzy band was very hard to discern, at least before the invention of the telescope. Recorded speculations on the nature of the Milky Way date back to classical Greek times. With even a small crude telescope, Galileo was able to resolve the milkiness into numerous stars about 400 years ago. Then the immediate question was - do we see this dense band of stars because it is a dense band of stars, or because it is the only region of the sky where an infinite distribution of stars is not obscured by some intervening material. Given our current knowledge of galaxies and galaxy disks, the latter idea seems a bit crazy. Without that knowledge, or even a basic understanding of Newton's universal law of gravity, it was very reasonable to begin with the idea that stars were distributed uniformly and that this distribution extended out to infinity, or at least to some great distance. In ancient scenarios the Earth was at the center of that distribution.

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Galaxy Collisions

When seventeenth century observations like Galileo's seemed to contradict that idea, the door was opened to attempts to patch the torn theory, or develop new ones. The most significant developments came in the mid-eighteenth century. Among the most well known are the speculative theories of Thomas Wright, an English teacher, astronomer, artist, architect and amateur theologian, who published his ideas in a book called An Original Theory or New Hypothesis of the Universe in 1750. His basic picture of cosmological structure was that the universe of stars was formed in a relatively thin spherical shell of enormous radius. Since our Solar System lies within the shell, when we look in a direction nearly tangential to the shell, and thus, through it, we see many more stars than when we look perpendicular to the shell along a radius. The former case corresponds to the Milky Way, while the latter case corresponds to what we would now call looking toward the galactic poles. Wright did also discuss the fact that a distribution of planar rings could account for the observed stellar distribution of stars on the night sky. While this conception brought him close to the modern view of the Milky Way, it was a secondary theory for Wright. Remarkably, Wright also expanded his notion of a universe as a shell to include the possibility of many other universes. These could exist on larger or smaller shells concentric to our universe, or as completely independent spheres. The modern idea of a multiverse comprising many universes has some common features, though based on completely different philosophical underpinnings. The great philosopher Immanuel Kant began his career with work in the physical sciences, including what we would now call cosmology, among others. He acquired a summary description of Wright's book, though without the artistic and descriptive illustrations. Perhaps because of this, he thought Wright's primary model for the universe (and the Milky Way) was disk-like. He adopted this model and sought to elaborate it. In particular, he argued for the idea that the so-called nebulous stars, which would soon be called nebulae, are other comparable star systems. Many of these had a roughly elliptical form, and Kant realized that this could be explained as the result of seeing disks of stars from many different points of view. In essence, Kant argued that the nebulous stars were what would much later be called island universes or galaxies. Scientists and historians of later times have dismissed Wright's and Kant's work as too philosophical or speculative. Like Kepler's, or even Newton's work before him, Wright's work did have a strong basis in theological considerations. It is certainly true that neither Wright nor Kant pursued a lifetime of astronomical observation in an attempt to test their theories. They were the general theorists. Anyone with even modest familiarity with the literature of modern theoretical physics knows the key role of symmetry principles there. Like the theological arguments of earlier centuries, symmetry arguments do not rest on much more than their intrinsic simplicity, beauty, and order. At least that is true initially; later their predictions can be tested against observation. Eventually, the questions of what is the Milky Way, and what are the nebulae or galaxies, also had to be answered by observation, and by people who would

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devote their lives to those observations. Such people included William and Caroline Herschel, who also began their work in the eighteenth century. The Herschels and others pursued two lines of study that were crucial to answering the questions above. The first direction has been called the natural history of galaxies (or nebulae). That is, the Herschels and Charles Messier in France undertook to describe and catalog all the nebulae visible through their telescopes (and the Herschels made some of the best telescopes of the time). Interestingly, their primary goals were not to do natural history or answer the questions above, but to identify fuzzy objects in the sky that could be easily mistaken for comets. Comet hunting and comet studies were then cutting edge science, and would continue to be so for some time. The second direction that was very important for William Herschel was estimating the distances of the stars, or at least their relative distances. Like the nature of the Milky Way this had been a long unsolved problem. It would be a great focus of nineteenth century astronomy, and ultimately the answer to the question of the nature of the nebulae would be answered by determinations of their distances in the twentieth century. The Herschels could only make a start on this long scientific crusade, but their methodology and determination made for a very good beginning. In the late eighteenth century, the Herschels had no model tools for determining stellar distances. It would be more than a half a century before stellar parallaxes were finally measured, allowing distances to a small number of nearby stars to be determined accurately. This method, which is essentially the same as trigonometric surveying, would provide the first step for calibrating other distance estimators, which could be extended to still greater distances. The Herschels had none of these techniques. All they had to work with were assumptions and star counts. Among the former they adopted the notions that the stellar system was finite, that they could see to the end of it, and that stars were relatively smoothly distributed within it. With these assumptions star counts were proportional to the distance to the edge of the stellar system in any given direction. Based on this methodology, they derived a model for the Milky Way shown in Figure 1.1. By modern standards it looks more like an amoeba than a galaxy. It is incorrect in several fundamental ways, including the fact that the Sun is near the center of the system, rather than toward the edge of the stellar disk, which we now know to be the case. The artistry of the figure is also much less than that of Thomas Wright. However, the diagram is based on data, and that fact alone makes it a great advance on the theological speculations of Wright. In this brief historical overview, I will not say much about the nineteenth century. It was a time for the beginnings of important technological develop­ ments, including larger telescopes, the use of photography to record observa­ tions and the use of spectroscopy to learn about what the stars were made of. The first two developments were very important for the study of the intrinsically faint galaxies; bigger telescopes collect more light, and photographic plates could collect it for much longer time spans than the eye. In this century, astronomy

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Galaxy Collisions

Figure 1.1 William Herschel's map of the Milky Way, based on star counts (from Wikipedia Commons, originally published in the Philosophical Transactions of the Royal Society, 1 785).

moved from the study of the Solar System to the stars. By the end of the nineteenth century, large catalogs of stars and their characteristics were being assembled. Studies of stellar parallaxes to determine distances and of the orbits of binary star systems were the norm. These investigations, especially the developing methodologies of distance determination, were prerequisites to gaining an understanding of the nature of the nebulae. Moreover, although they were improving, the observational tools were not yet up to the task of understanding galaxies. There were exceptions to this generalization. One of the most oft cited is the Third Earl of Rosse (William Parsons) with his huge 72-inch "Leviathan" telescope at Birr Castle in Ireland. He discovered the spiral arms of the object Messier 51, now known as the Whirlpool galaxy (Figure 1.2). Later he discovered spirals in other nebulae, so that the "spiral nebulae" became a class. Incidentally, when Rosse observed a small companion galaxy at the end of one of the spiral arms in M51 he made the first telescopic discovery of colliding galaxies. (Prehistoric naked eye observers of the Magellanic Clouds were the first to discover colliding galaxies, but they had much less understanding of what they were seeing than Rosse.) On the other hand, comparison of Figures 1.2 and 1.8 (below) show that his observations only gave an impression, rather than an accurate map of the interaction. The twentieth century dawned and astronomers like Jacobus Kapteyn were applying statistical tools to determine the structure of the Milky Way from extensive observations of stars. However, the nature of Rosse's spiral nebulae was still far from clear. The confusion of the time is evident in University of Chicago Professor F. R. Moulton's encyclopedic text An Introduction to Astronomy, published in 1916. Some quotes from the section on the spiral nebulae: . . . they seem to be vast swarms of incandescent solid or liquid particles, perhaps many with larger masses, surrounded by gaseous materials. There is difficulty in explaining their luminosity, though Lockyer attempted to account for the light of all nebulae by ascribing it to heat generated by the collisions of meteorites of which he supposed they are largely composed. . . . The suggestion has been

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Figure 1.2 Lord Rosse's sketch of the Whirlpool galaxy (M51), as seen with his 72-inch "Leviathan" telescope (from Wikipedia Commons, original from 1845).

made that a spiral nebula may develop when a star is visited closely by another star, or when a group of stars passes near another group of stars. In these quotes the good professor paints a picture of the spirals as a galactic phenomenon, existing on roughly a stellar scale. Indeed his description sounds rather close to a modern one describing a forming star or proto-star cluster. But he isn't quite sure in the end. He writes: There is one fact, which is opposed to the suggested explanation of spiral nebulae, and that is, as Slipher first found, their radial velocities average very great. For example, the Great Andromeda Nebula is approaching the Solar System at the rate of 200 miles per second... These quotes show a stunning ignorance of galaxies, less than a century ago, compared with our present knowledge. However, in the last quote one can smell the first smoke of the coming revolution. At about the time of Moulton's text, the major breakthrough for under-

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Galaxy Collisions

standing the nature of galaxies was coming from studies of a rare class of stars, the Cepheid variables. In the first decade of the twentieth century, Henrietta Leavitt, working at Harvard College Observatory, made her seminal discovery that the pulsation period of Cepheid variables correlated very well with their intrinsic luminosities. With this correlation, Cepheids became the most important of what astronomers call "standard candles," i.e., objects whose intrinsic total brightness or luminosity can be determined by some independent means. Thus, like candles all factory made to a set standard, we know their brightness, once it has been determined for some subset. Ejnar Hertzsprung, using stars in the Milky Way, whose distances could be determined by other means, calibrated Leavitt's correlation between Cepheid periods and luminosities. Hertzsprung then used the correlation to estimate the distance to the Magellanic Clouds. His estimate was poor by modern standards, but good enough to show that the Clouds were too far away to be considered star clusters within the Milky Way. New distance determinations showed that the globular star clusters were also much more distant than previously thought. Yet the globular cluster system seemed to be centered on the Milky Way. Were they part of the Milky Way system or separate? And thus, was the Milky Way much larger than formerly believed? If so, how could the spiral nebulae be like the Milky Way unless they were also much bigger, and as a result, much more distant than previously envisaged? This was a time of great confusion and controversy about these interwoven questions. The details make for a very interesting historical story. For our purposes, it suffices to know that Edwin Hubble broke the logjam in 1924. He did this by discovering Cepheids in the nearby spiral galaxies M31 (the Andromeda galaxy) and M33 (called the Pinwheel, in the constellation of Triangulum), and using the period-luminosity relation to estimate the distances. The distances he derived were large, and showed that these galaxies were indeed island universes comparable with the Milky Way. An interesting aside is that Hubble's distances to these and other galaxies were underestimated by about a factor of two. Walter Baade resolved this discrepancy in the 1940s when he discovered that there were two classes of Cepheid with slightly different periodluminosity relations. One class had been used to calibrate the relation, while those found in the external galaxies belonged to the other class. The details of Cepheid calibration continue to be worked on to this day, though that work is now about details and smaller corrections. In any case, although the debates continued for a few more years, with this discovery Hubble resolved the nature of the spiral nebulae and showed that the Milky Way was not alone, but rather a member of a huge population, spread across a vast volume of space. Almost any scientist would be happy with one such momentous discovery in their career, but Hubble's work had not reached its peak. He went on to consider not only the sizes and distances of galaxies, but how they moved relative to the Milky Way. Because of their great distance the motion of galaxies across the sky is nearly impossible to observe. Only recently have we been able to measure it for a few of the closest galaxies; such observations were far beyond the capability of

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Hubble's instruments. However, he (actually Vesto Slipher and others) could measure the motion of galaxies toward or away from us. This was achieved by use of the Doppler shift phenomenon. The Doppler shift is the slight change of the wavelength of a spectral line for objects moving toward or away from the observer. Ordinarily this wavelength shift is very small; only if the relative velocity between source and observer is close to the speed of light is it large. However, this relative velocity can be measured quite accurately in astronomical spectra. The relative velocity compared to the speed of light is proportional to the relative wavelength shift. Both the velocity and the wavelength shift are often called the redshift. It is actually only red if the source and observer are moving away from each other. If they are moving toward each other the spectral line is shifted to a shorter wavelength, giving a blue shift. A priori, Hubble seems to have expected that the galaxies would be moving randomly through the universe, and so he expected about equal numbers of red and blue shifts to be observed. Yet, with few exceptions, the measured shifts were red. That result was strange enough, but when Hubble plotted redshift against distance for a small sample of galaxies (first in 1929, then with more data in the 1930s), he discovered something even more significant - the redshifts (or recession velocities) increased linearly with distance. The relation can be written, v = Hd, now called the Hubble Law, where v is the recession velocity, d is the distance, and H is the so-called Hubble constant (with units of inverse time or frequency). The Hubble Law says that the farther away a galaxy is, the faster it is moving away from us (Figure 1.3). This was pretty mysterious... unless you were Einstein. Actually, it was not so much Einstein as other scientists applying his equations of General Relativity to cosmology, who realized that this is the signature of universal expansion. The other galaxies were not all traveling away from the Milky Way, they are just participating in the overall expansion, or space-time stretching of our universe. Hubble was rather lucky to make this discovery. If the Milky Way resided within a giant cluster of galaxies, like about half of all galaxies do, the observations might have realized the simple expectation of just as many galaxies coming toward us as moving away. This is because the enormous selfgravity of such a cluster overcomes the universal expansion, and most member galaxies are moving randomly (and at high speeds) relative to each other. With such a small sample of nearby galaxies, Hubble wasn't really sampling on a large enough scale to give a proper measure of the cosmological expansion, but he did get lucky (Figure 1.3). In the late twentieth century one of the primary scientific goals of the Hubble Space Telescope was to detect Cepheids and measure galaxy distances out as far as the Virgo cluster of galaxies (the nearest great cluster), in order to get a better measure of the rate of cosmological expansion. These studies are very much in the tradition of the master by his telescopic namesake. The distances to galaxies are great; millions of light years for some of the nearest, billions of light years for the most distant. We know that because in the time since Hubble's work a number of other standard candles have been found and calibrated. Some of these (especially supernova explosions) can be used out

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Galaxy Collisions

Figure 1.3 The Hubble Diagram, depicting the linear Hubble Law relation between galaxy distance and redshift. The diagram to the upper left is a version of Hubble's original. It fits within the range of the two points closest to the origin in the diagram on the lower right. The labels on the latter refer to different techniques used to estimate galaxy distances. This latter diagram is the result of extensive observations with the Hubble Space Telescope (from Wikipedia Commons/W.C. Keel (upper left) and W.L Freedman, et al., Astrophys. /., Vol. 553 (lower right)).

to far greater distances than the Cepheids. Once calibrated, Hubble's Law can itself be used for distance determination - measure the recession velocity of a distant galaxy, calibrate the Hubble constant from relatively nearby galaxies, and solve the equation for the distance to the distant galaxy. Cosmologists are not fond of this procedure because they would like to use independent distance estimates to look for deviations from the Hubble Law, which can tell us interesting things about the nature and evolution of the cosmic expansion. However, the procedure is a handy tool for quickly estimating galaxy distances. The travel time for light to reach us from distant galaxies depends directly on the distance between us, which depends on the recession velocity. Therefore, the redshift is also a measure of how long ago the light left a distant galaxy. Roughly speaking a redshift (e.g., relative wavelength shift) of 1 means the light left the distant galaxy when the universe was about half its present age of around 13.6

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Gyr.1 We observe a very few galaxies out to redshifts of about 6, whose light left those galaxies when the universe was less than a Gyr old. Since we can now observe galaxies with such long light travel times, we can do galaxy archaeology. That is, the typical structures of those distant galaxies can be viewed as artifacts from the history of galaxy evolution. That is not the topic of this book. However, the subject of galaxy collisions is intimately tied up with that of galaxy evolution, so the latter topic will reappear many times below. For now, let us return to the mid-twentieth century to note that Hubble spent much of his later career studying the structure of galaxies at very low redshifts by modern standards. He was looking for a rational classification scheme for galaxy structure. He did not live to complete and publish this work. His student, Alan Sandage, and many others continued it in the second half of the twentieth century.

1.2 Galaxies properties: classification and evolving views

The work of Baade, Hubble, Shapley, and soon many others in the mid-twentieth century, on galaxy structure, distances, groupings and the different populations of stars within them provided a strong foundation for the emerging field of extragalactic astronomy. As a result the developments from, say, the 1950s become more numerous and rapid. Many of those developments are not part of the main story of this short book, so we must limit ourselves to a brief consideration of the most relevant highlights of galaxy studies from that time to the present. A good place to start that discussion is with the classification scheme of galaxies that Hubble developed. This scheme is shown in Figure 1.4, which is taken from Hubble's book The Realm of the Nebulae. Hubble spent thousands of days (and nights) obtaining and studying photographic plate images of galaxies. Many of these images contain an enormous amount of detail. The intellectual discipline required to boil that vast amount of information down to such a simple tuning fork diagram is stunning. To a first approximation the diagram can be viewed as a one-dimensional sequence from quite round (or spherical in three-dimensions) galaxies on the far left to very flat galaxies on the right. The diagram doesn't look like that description, because the disk galaxies along the two tines of the fork are sketched in face-on views to show the nature of their spiral arms, rather than in thin edge-on views. However, flatness is not the most fundamental characteristic of galaxies, and the tuning fork contains much more information, some of it implicit. First, the galaxies in the handle of the fork are the so-called ellipticals. That is, elliptical in projection on the sky, but probably ellipsoidal in three dimensions.

Gyr, for gigayear, is equal to 1,000 million years, or 109 years.

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Galaxy Collisions

Figure 1.4 Hubble's sketch of his " t u n i n g fork" scheme for classifying galaxies, from his 1958 book The Realm of the Nebulae. The handle of the fork consists of elliptical galaxies with varying degrees of flatness, as projected on the sky. Each tine of the fork consists of spiral galaxies, with the openness of the spiral arms increasing to the right. The lower tine consists of spirals that also contain a bar component.

The visible parts of these galaxies consist mostly of very old stars, with few to no younger stars and interstellar gas clouds. For this reason the ellipticals are red in color. They are like extremely large star clusters, which vary in their degree of flattening. The (apparent) flattening of an elliptical can be characterized by the ratio of the longest to the shortest axis in the image, and that ratio ranges from 1 (the EO class) to about 0.3 (the E7 class). The latter value is somewhat arbitrary; it recognizes the practical fact that any flatter elliptical would probably be taken for an edge-on disk galaxy. It was originally thought that the flattest ellipticals must be rotating relatively rapidly like the disk galaxies. Spectral observations became sensitive enough in the 1970s to disprove this; almost all ellipticals rotate relatively slowly. That is, not rapidly enough for centrifugal forces to hold them up against their own self-gravity, again in contrast to the disk galaxies. In many ways, the ellipticals are the simplest galaxies. The disk galaxies are more complex in several ways. Firstly, they often have two strong spiral arms, as shown in Figures 1.4 and 1.5. Because of this they are called spiral galaxies as often as disk galaxies. In Hubble's scheme these galaxies are classified according to how tightly the spiral arms are wrapped. The most tightly wound are class Sa and the least tightly bound are class Sc. In addition to the disk component most spiral galaxies also have a central bulge component. For our purposes the bulge can be roughly viewed as a little elliptical galaxy in the center of the disk. This is not quite true; there are some moderate differences between ellipticals and bulges on average (and a subclass of bulges that are quite different). For example, bulges tend to be more round on average, and to rotate somewhat more rapidly than ellipticals. However, like ellipticals, bulges tend to be made up of old stars, with few young stars or gas clouds. On the other hand,

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Figure 1.5 A montage of images of galaxies representing the Hubble classes shown in the previous figure (montage from the website of William Keel, with images by Keel and his colleagues from the Digital Sky Survey. The DSS was produced by the Space Telescope Science Institute, under a grant from NASA).

spiral arms tend to host young stars or young star clusters, often making the disk galaxy blue, on average. The size of the bulge correlates with the spiral arm structure in the sense that tightly wound Sa types have large bulges, while loosely wound Sc types have small bulges. Hubble used both features as classification criteria, and if the two did not correlate so well the upper tine of the fork would not be a onedimensional line. Our modern understanding of the dependence of spiral wave structure on how the net gravitational attraction varies with radius in the disk, and the fact that the bulge contributes to this net gravity, helps us understand the correlation. The lower tine of the tuning fork represents a second class of disk galaxies, which are distinguished by having another major component besides disk and bulge, i.e., a bar. The bar component is a three-dimensional ellipsoid, which is much more elongated than a bulge, like an elliptical galaxy of type E3-E7. The lower tine parallels the upper with types SBa through SBc, where the capital B stands for barred. Bars are believed to consist mostly of stars on orbits that keep them within the bar most of the time. Observations of barred galaxies at different tilts or inclinations relative to the plane of the sky show that bars generally lie in the plane of the disk. In fact, their formation and evolution are thought to be intimately related to the disk, though that is not a story for this book. What distinguishes the bar from the disk is that the orbits of the stars are different in each, and like a great stirring bar, the bar moves through the disk. Actually, since the bar moves relatively slowly it is more accurate to say that disk stars orbit through the bar. As do gas clouds.

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Galaxy Collisions

Moving through the bar modifies the orbits of both disk stars and gas clouds, and in the latter case, this can lead to a compression of the gas clouds. By means that we will discuss later, star formation is enhanced in the compressed gas clouds, so bars often stimulate star formation. These statements are also true for spiral arms, which are also waves that move through the disk and compress the gas. We can see that the simple tuning fork does indeed encode a lot of information about the bulk properties of galaxies. Of course, there is a lot more information about galaxies that is not represented in the Hubble classification. I have already mentioned that there is much detail to be seen on good photographic images. This fact led to decades of refinement of Hubble's simple classification scheme. Elaborations included sub-classifications of spirals, recognition of ring structures on various scales, observations of multiple sets of spirals and rings, etc. Readers who are not fond of detailed classification systems

Figure 1.6 A mid-infrared view of the Hubble classification scheme as presented by the SINGS project collaboration using data from the Spitzer Space Telescope. Note the near reversal of blue and red colors relative to optical images (NASA/JPL-Caltech, Spitzer Science Center).

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will be relieved to know that the simple Hubble system will serve our needs throughout the remainder of this book. Another type of information that is not represented on the tuning fork is what galaxies look like in wavebands other than the optical. Hubble's scheme was, of course, based entirely on optical images. Galaxies can look very different in other wavebands. Figure 1.6, for example, shows the tuning fork in the mid-infrared. Specifically, it shows a color-coded combination of specific bands from the IRAC camera of the Spitzer Space Telescope. Analogous to the optical, the band covering the longest wavelengths is colored red, while that at the shortest wavelengths is colored blue. This seemingly logical choice results in blue elliptical galaxies and red spirals in the infrared. The reasons for this are that the red stars of ellipticals diminish in brightness at longer mid-infrared wavelengths, while hot interstellar dust around young star clusters increases in brightness. This is only one example of the peculiarities one must adjust to in non-optical galaxy images. We will see other examples of radio, infrared and X-ray images of colliding galaxies in later chapters. Classification details and non-optical views are not all that Hubble's scheme misses. In fact, all that we have been talking about so far concerns only about 10 percent of the matter in galaxies. Until the 1970s the most massive component of galaxies was unknown. I am referring of course to halos of dark matter that not only make up most of the mass of galaxies, but are also about 10 times larger than the visible structures of galaxies. Dark matter is indeed dark, mysterious and has never been directly observed, though physicists are searching for some possible forms of it, and rare interaction products, in deep underground laboratories. (They are underground to shield the detectors from cosmic ray particles, which penetrate the atmosphere, and could be a source of confusion.) So how do we know it exists if we have never seen it? The first clue came from spectrographic observations of galaxy disks, specifically, from the variation of Doppler shifts across the face of disk. These measurements must not be confused with the net Doppler shift of the entire galaxy due to the expansion of the universe, or other bulk motions. These spectral measurements were designed to look at motions within a galaxy disk. The primary motion is rotation around the center. If the disk is viewed partly tilted, not face-on, then because of the rotation we see the stars on one side of the disk moving away from us (red-shifted), but moving toward us (blue-shifted) on the other side relative to the center velocity. In the 1960s and 1970s, astronomers like Vera Rubin, Margaret and Geoffrey Burbidge, and others were looking for how the amount of red or blue shift varied with distance from the center. They expected it do decrease on both sides. That is, they expected the rotation velocities of stars to decrease with central distance. That's what happens with planets in the Solar System. As discovered by Kepler, the mean rotation velocity decreases as the square root of the distance from the Sun. As Newton explained, this is because almost all of the mass of the Solar System is contained in the Sun. If the mass of a celestial system is distributed, rather than concentrated in the center, then the rotation velocities will not decrease as rapidly with distance from the center. The galaxy spectroscopists

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Galaxy Collisions

Figure 1.7 A prototypical rotation curve of a large disk galaxy (NGC 3198). This particular rotation curve is notable because of its size of about 30 kpc. The data are marked as points with (small) error bars. The curve marked "Stars + Gas" shows the contribution of visible disk material, which becomes quite small at large radii, where dark matter begins to dominate. The curve labeled " G a s " is the contribution from interstellar gas clouds alone (from K.G. Begeman, Astron. & Astrophys., Vol. 223).

expected to see a little of this effect, because even though the stars in galaxies are distributed, they are still quite strongly concentrated toward the center. What they found is illustrated in Figure 1.7, which shows a graph of the rotation velocity of a rather large galaxy disk as a function of radius. At the very center, the rotation velocity is zero, and going outward it increases rapidly until we reach a radius containing a large fraction of the galaxy's stars (e.g., a characteristic or outer radius of the bulge if there is a large bulge). This is not a surprise, but beyond this peak the rotation velocity was expected to decrease. As Figure 1.7 shows, it did not. This result was found again and again, in galaxy after galaxy. The most natural explanation was that the gravitational matter was much more widely distributed than previously thought. However, since the stars and gas clouds are not that widely distributed, there must be an additional kind of matter, unseen, but holding all together. This last clause is important because the rotation velocities observed in the outer parts of disks were so high that the gravity of the visible stars would not be sufficient to hold onto those parts of the disk. Stars and gas clouds in the outer disk would fly free of the galaxy. After the landmark discovery that the spiral nebulae are galaxies like the Milky Way, the discovery of dark halos is probably the most important one in extragalactic astronomy, and it was quite unexpected. In galaxy disks, the gaseous component can extend out to distances several

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times greater than the stellar component. Evidently, the gas in the outer parts is not dense enough on average to form many stars. Thus, we speak of the gas disk versus the star disk. Using a spectral line from atomic hydrogen, radio astronomers observed the rotation characteristics of the extended gas disks, and found the same thing as the optical observers. Therefore, the dark matter halos extended beyond the star disk, and probably beyond the gas disk. Other types of observation have since confirmed that result, and the statement above about the size of dark halos. Altogether these observations suggest that the density of the dark matter does decrease with distance from the center, roughly as the inverse square of the distance (i.e., 1/r2), until it falls off more abruptly at its outer edge. We have been able to learn much about the distribution of dark matter in the last few decades, but have made little progress toward answering the key question - what is the dark matter? Describing the work that has been done on that question could easily take a chapter in this book. We'll limit ourselves to a few salient points. It probably does not consist of astronomical objects much bigger than large planets; they've been searched for. It could consist of smaller astronomical objects, though there are cosmological arguments against such objects made of ordinary atoms. It also cannot consist of microscopic black holes born out of the Big Bang. There are both astronomical observations and cosmological arguments arrayed against that possibility. Perhaps the most likely possibility is that it is an as yet unknown elementary particle. If so, it is both very common in the universe (adding up to all that mass), and very unforthcoming, avoiding interacting with known elementary particles that would make it detectable. This means that like the neutrino it must have no electrical charge, and no color or nuclear charge, since interactions via those charges are very strong. It can only interact via gravity and the weak nuclear force that underlies radioactivity. Neutrinos satisfy these properties, but experimental limits on their mass are too low for them to be a significant constituent of the dark matter. However, unified and supersymmetric theories of particle physics allow for the existence of many candidate particles. Another possibility is that dark matter is an illusion (an invisible illusion?) resulting from an incorrect theory of gravity. Maybe Newton and Einstein didn't have the last word on the nature of gravity, and contrary to their predictions gravity is just stronger in the outer parts of galaxies, and on other large scales. Actually, a good deal of study of alternate gravity theories has yielded several plausible ways to generate such effects that are consistent with many observational constraints. Work on such theories will surely continue at least until dark matter particles are detected; if they are not, such theories may yet carry the day. Most astronomers and physicists are skeptical about them, if only because they require modifications to Einstein's beautiful theory of General Relativity. I will say no more about them in this book, and assume the existence of dark matter, mostly to avoid discussing the issue repeatedly. I should note, however, that observations of colliding galaxies do play a role in constraining all of these theories.

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Galaxy Collisions

The most important point of connection is that large dark halos make galaxies much bigger and more massive than their luminous components. As we shall see, this makes it much easier for them to capture each other onto collision trajectories. It also makes it possible for galaxy groups and clusters to hold together in a universe that is not only expanding, but accelerating due to the effects of dark energy. Dark energy is another dark topic that we will not delve into here, since its effects are most important on larger scales than those of galaxy collisions.

1.3 M o d e r n views: universal classes and sea lings

In discussing the tuning fork above I noted that it is evidently a one-dimensional classification of galaxies, albeit with two branches and a lot of extra information implicitly encoded. This statement is rather vague, in an area where scientists would like to be much more precise. Just how many (observable) parameters does it take to describe the family of galaxies, and to what degree of accuracy? It is understandable that these issues were not very clear in the early days of the field, and even later while the field remained biased by "seeing" primarily with optical images. However, the state-of-the-art has progressed greatly in the last few decades. Many of the same issues confronted stellar astronomers in the early decades of the twentieth century, when galaxies astronomers were still wrestling with the nature of the spiral nebulae. Their story provided a guide to the kind of answers that might be found to the questions above, and so, is worth briefly recalling. Progress came very quickly following Ejnar Hertzsprung and Henry Norris Russell's discovery of the stellar main sequence, and the utility of what came to be called the HR diagram. The latter is a graph of stellar luminosity versus temperature, or in observational terms stellar color versus magnitude. In this graph most of the stars fall on or near a single curve, the main sequence. This is remarkable because it means that almost all of the global properties of these stars, e.g., mass, radius, luminosity, color in various bands, are determined by one such property, e.g., surface temperature. The reason for this is that stars are selfgravitating gas spheres in near equilibrium, emitting an almost thermal spectrum of radiation. In equilibrium, and with much symmetry, the number of independent parameters is small, i.e., one. Actually, not quite one, there are other factors that can move stars off the main sequence, including the amount of rotation and the abundance of heavy elements. However, these variables generally have a modest, secondary effect. About 10 percent of all stars are not found on the main sequence in the HR diagram, and in fact, are much farther from it than can be plausibly accounted for by one of the secondary factors. A great triumph of twentieth century astrophysics was to explain the properties of these stars as the result of different internal structures realized in different evolutionary phases, either before or after they reside on the main sequence. In sum, most stars can be approximately

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described as a one-parameter family, the main sequence, but almost all stars can be described as a two-parameter family with evolutionary state, or relative age, as the second parameter. So what about galaxies; are their bulk parameters described by one, two, three, or many parameters? Answers to that question seem to be solidifying now, but they didn't come as rapidly as in the case of stars. I don't know exactly why, but I sometimes suspect that part of the reason is because galaxies all look so different. Perhaps, at least subconsciously, we find it hard to swallow that their basic structure could be explained by changes in just a few variables. It is certainly true that regularities among galaxy properties were first found among restricted Hubble types, where the galaxies look more alike. However, I think the real reason that galaxy relationships weren't discovered earlier was that they couldn't be, because the telescopes and instruments were not up to the task. The 1970s saw very significant improvements in optical spectrographs and galaxy spectroscopy. We have already discussed one important consequence optical rotation curves and the discovery of dark matter. Let us consider a couple of others. In the case of rotation curves, the relevant quantity was the Doppler shift of spectral lines. Another property of spectral lines is their width in wavelength or frequency units. There are a number of physical causes for spectral line width. In the case of an emission region in galaxies the primary cause is the net relative motion of individual sources; e.g., the random motions of stars in the nucleus of a galaxy, with many stellar redshifts and blueshifts superposed to make a wide line, rather than a shifted line, as when all the stars share a common redshift. In 1976, Sandra M. Faber and Robert E. Jackson published observations of line widths in the cores of elliptical galaxies, and discovered the tendency of those widths to increase with the luminosity of the galaxy. What came to be called the Faber-Jackson relation was one of the first quantitative regularities to be discovered in a class of galaxies. Physically, it means that the motions of the stars in elliptical galaxy cores, which are primarily random rather than circular as in disks, are more vigorous in big, bright ellipticals. In 1981, Faber, along with David Burstein, and Cambridge University astronomers Roberto Terlevich and Roger L. Davies, published a paper, which in the current age, we might call an upgrade. It again provided data on the relation between luminosity and velocity dispersion in ellipticals, but added another relationship. This was between luminosity and the average abundance of heavy elements in the ellipticals. The latter quantity is related to the maturity of a galaxy. This is because a greater heavy element abundance is thought to be the result of more previous generations of stars, which produce the heavy elements, and return some of them to the interstellar gas to be incorporated in new generations of stars. The correlation suggests that bigger, brighter ellipticals experienced more such stellar processing. These correlations and some others they found were very interesting, but the statistical analysis of the data showed something even more exciting. The correlations apparently could be boiled down to only two independent variables

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Galaxy Collisions

or degrees of freedom. The family of ellipticals was not quite as simple as the oneparameter stellar main sequence, but almost; it was revealed to be primarily a two-parameter family. In addition to these papers, a great deal of other work was done at that time and since. Astronomers now call these relationships between elliptical galaxies the Fundamental Plane of ellipticals. This is an abstraction. It envisions a space of many different dimensions, each corresponding to a bulk property of ellipticals (luminosity, central velocity dispersion, mean size, etc.). Since in ellipticals these properties are mostly determined by just two degrees of freedom, they lie on a two-dimensional Fundamental Plane in this space. (More generally, it is a two dimensional curved surface, but the distinction is not crucial here.) The spiral galaxies have not been neglected in these kinds of studies. Another very important paper of the time was R. Brent Tully and J. Richard Fisher's 1977 study of the correlation between the widths of radio hydrogen emission lines in galaxies and their total luminosities. The hydrogen line (21 centimeter wavelength) widths are analogous to the velocity dispersions we have been discussing. However, in this case the line width is due to the combination of the redshifts from gas rotating away on one side of a galaxy disk, and the blue shifts from the other side. The radio telescopes of the time had spatial resolutions that were not much better than the size of the galaxy disk, so they could not track the Doppler shifts at various locations across the disk, but only get a sum of the emission from the whole disk. The width of that emission was determined by the highest and lowest red and blue shifts, and the corresponding highest values of velocities of approach and recession. Given the constancy of rotation curves discovered in the optical spectra, the correlation meant that the higher the luminosity of a disk galaxy, the faster the rotation speed in its disk. Later work showed that there is a slightly different "Tully-Fisher" relation for each of the different Hubble spiral types. Tully and Fisher's paper was a work of synthesis, of assembling the necessary data from a variety of published sources. This is a method of research that is very common at the present, but was less so in those relatively data-starved times. In fact, others had anticipated the correlation we have been discussing, but Tully and Fisher's paper became justifiably famous for another reason. They were able to show that the correlation was good enough to be used for galaxy distance determinations. One could measure the hydrogen velocity width of a distant galaxy, assume it obeyed the calibrated correlation, derive its luminosity from the correlation, and use that to estimate the distance. In other words, the TullyFisher relation allows one to use the total galaxy brightness as a standard candle. In subsequent years, this distance determination technique has become very important. One feature of all of the papers by Faber and Jackson, Tully and Fisher, and Terlevich etal. impresses me greatly. That is, by modern standards, the data they had available to them were very poor, usually just a couple of dozen data points. This was almost as bad as the data Hubble had available to formulate his famous law. In each case, the astronomers made an intellectual leap, took the dare

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against their career prospects, and published. Many other scientists have done this too, but we will not be talking about them, because their bets turned out to be wrong. Like the Faber-Jackson work, the Tully-Fisher paper presaged later develop­ ments in the scaling of galaxies of a given class. Subsequent work has shown that the global properties of disk galaxies are, like ellipticals, approximately a twoparameter family. According to some recent work this is not a coincidence. In 2008, Dennis Zaritsky, Ann I. Zabludoff, and Anthony H. Gonzalez published a paper boldly titled "Toward Equations of Galactic Structure," meaning equations relating the properties of all Hubble types of galaxies. In essence, they present evidence for a universal Fundamental Plane for galaxies. This is not a complete surprise since Faber-Jackson and Tully-Fisher relations are very similar, they just involve different types of velocity. But one can ask, are those velocities really that different? The literal answer to this leading question is that, yes, in the type of motion they represent (random versus rotational) they are different. However, in both cases the motion represents a kind of force supporting the stellar component against the gravity that, on its own, would happily pull all the stars together into a great black hole. In the cores of ellipticals the random motion represents an effective pressure; in the disks centrifugal force resists gravity. One of the tricks the authors above used to affect their synthesis was to use a net velocity consisting of a weighted sum of both of the velocities used previously. In essence they are saying that it doesn't matter whether it is pressure or centrifugal force that balances gravity, or a combination, we will still get a Fundamental Plane. They provide evidence that their Fundamental Plane applies to galaxies ranging from the very smallest dwarf galaxies to the very largest galaxies found in galaxy clusters. This evidence is new and it will take time for astronomers to evaluate and digest it, but that process promises to teach us much about the order and unity of galaxies. So far, in this section we have been talking about the regularities in, and the order and unity of, the bulk properties of galaxies. However, those regularities are intimately connected to, and the result of structural regularities within, the components of galaxies. To complete our overview of galaxy structure we need to review very briefly these internal structures. On this topic, I would like to order the discussion contrary to its historical development, and begin with some of the most recent results. These concern the structure of dark halos, which were not even known when work began on the visible components of galaxies. The most important aspect of halo structure is the distribution of dark matter within them. We have already discussed the fact that it is widely distributed, and not concentrated in the centers of galaxies. In fact, its distribution, on average, can be described much more precisely. In many galaxies, the density, e.g., the mass of dark matter per cubic meter, decreases roughly as the inverse of the distance from the halo center squared (1/d 2 ), at least out to some outer radius, where it falls off very rapidly. This rate of density decrease is just that required to provide the extra gravity needed to maintain the

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Galaxy Collisions

flat rotation curves, which is one of the reasons we know that the density falls off this way. The reasoning of the previous sentence is circular (its purpose is only to point out consistency), so it is good that there are other kinds of evidence for the form of the halo density profile, as it is called. Halo structure can, and has, been studied in great detail on the computer in large-scale structure formation simulations, which will be talked about later in this book. These confirm that the density profiles are universal, meaning basically the same in all types of galaxy in essentially all environments. They also allow the construction of functions to fit the density profiles that are much more accurate than the simple inverse radius squared formula of the previous paragraph. This formula and even some of the more sophisticated ones predict an infinite density rise at the halo center. This is not possible, but the questions of whether the halo density rises very rapidly in galaxy cores or levels off, and exactly what processes determine this, do not yet have definite answers. Other questions can also be investigated with these models, such as what is the shape of dark halos? A very short answer is that their shape is generally, but not always, rather spherical. Yet another very important question is what is the nature of the orbits of halo particles? Another rather approximate answer is that the very large numbers of halo particles are distributed more or less randomly over all orbits consistent with constraints, like that on their energy. All of these questions involve complexities well beyond the scope of this book and are subjects of very active research. The matter distribution, shape and orbit structure of the visible components of galaxies are much more accessible to direct observation, though there are often limitations of instrument sensitivity and resolution to consider. In brief, the density of stars in disks and bulges fall off nearly exponentially, while the gas density in disks generally falls off more slowly with distance from the center. As already noted, orbits of both stars and gas clouds are nearly circular in most disks. Stellar orbits in bulges are a mix of overall rotation around a single axis (the disk axis) and a random part, like halo particles. The presence of a strong bar can modify motions and density profiles in disks. The overarching conclusion here is that the structures of components within galaxies, and the form of regularities among them are universal. We will not discuss the causes of this uniformity in galaxy components, though some relevant points will be discussed in later chapters. I cannot resist pointing out that with high-redshift observations, some evolutionary processes can be studied directly over time, and their causes investigated in increasing detail. However, the main point for us is that isolated individual galaxies when viewed as physical systems do not seem to be as complicated as they could be. Evidently they are not that much more complicated than the main sequence stars, despite their apparent diversity. The situation is reminiscent of humans, who have a wide range of superficial appearances, but the bulk of whose genetic material is identical. Of course, even individual stars would not all seem so very similar if we could resolve their surfaces, as we can resolve nearby galaxies. This "universal similarity" and simplicity of galaxies is initially surprising, but

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is of fundamental importance for studying galaxy evolution in general, and the effects of galaxy collisions in particular. If the initial states of galaxies involved in collisions were random, they would be unknowable, and it would be extremely difficult to figure out the effects of the collision. Nature has been much kinder to us.

1.4 Discovering collisions and interactions

The history of the study of galaxy collisions is much shorter than that of galaxies themselves. When nobody knew what exactly a galaxy was, and whether there were more of them than just the Milky Way, the question of whether they collided was not an obvious one! Moreover, most of the nearest, brightest galaxies were not obviously colliding. The Whirlpool galaxy (also called M51, see Figures 1.2 and 1.8), which will be discussed in later chapters, is a prominent exception, and M51 puzzled Charles Messier (its discoverer) and William Herschel. However, that puzzlement was as much about the nature of the spiral arms in the Whirlpool, rather than its connection to a nearby companion galaxy. (We now know that both phenomena are connected in that system.) Many other Messier (and Herschel) objects are now known to be interacting, based in part on conclusive evidence from radio astronomy, but the signs were not obvious before recent decades. The point is that, before collisions could be studied and a judgment made on their importance, examples had to be identified, but the nearest and first studied galaxies provided no reason for thinking about collisions. That motivation eventually came from a couple of different directions. First there was Hubble's galaxy classification program. While most galaxies could fit on the tuning fork, a few odd ones definitely could not, and had to be tossed in a separate bin labeled "peculiars." Harlow Shapley expressed the thinking of the time very well in his 1943 book Galaxies: There are also plate spirals... and frankly "pathological" types, (as Baade calls such freaks) like NGC 5128... and the ring-tail system, NGC 4038... The theories that sufficiently explain the relatively simple looking Sc spiral Messier 33, and the most common galaxies in Virgo, must have sufficient flexibility to take care of these aberrant types. The interpreter may need to resort to the assuming of collisions to find satisfactory causes. He will find some justification, because the individual galaxies are not so far separated but that encounters may have been fairly numerous, if the time scale has long enough... We are only at the threshold to the house of galactic knowledge, and within there are doubtless many dark and difficult rooms to explore and set in order.

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Galaxy Collisions

Figure 1.8 High resolution Hubble Heritage image of the Whirlpool galaxy M51 (NGC 5194) and its companion to the north (NGC 5195). Note the pink star-forming knots strung along the spiral arms, and the dark dust lanes connecting to the bridge and strewn across the face of the companion (NASA, ESA, and the Hubble Heritage Team (STScl/AURA), with processing by Robert Gendler).

This very prescient statement is surely one of the "rosy fingers" of light announcing the coming dawn of this field. For the interested reader, I note that NGC 5128 is the 5128th object in the New General Catalog (NGC) of nebulae and clusters of stars, which was based on earlier lists of objects made by the Herschels, and contains many nearby galaxies. Not all of Hubble's peculiars were colliding or interacting galaxies, but a large fraction of them were. As we will discuss in detail in the following chapters, many of the morphological peculiarities are the result of tidal forces acting between galaxies in close encounters. Collisions often result in mutated shapes, so looking for such shapes is often a good way to find colliding galaxies. In the quote above, Shapley alluded to a good way to find colliders - look in groups. Another prescient pioneer in this field, who was probably on Shapley's mind, was the Swedish astronomer, Bertil Lindblad. Lindblad studied galaxy groups and was discussing the possibility of collisions within them as early as the 1920s. Lindblad's work evidently inspired his colleague E. Holmberg, who in 1941 carried out the first simulation of a flyby encounter between a disk galaxy and a companion. This was an analog computation using light bulbs and light detecting photocells attached to the bulbs. Thus, each bulb could detect the

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intensity of light from other bulbs. The falloff of light received by a detector with increasing distance from a bulb mimicked the falloff of gravity with distance between two masses (Newton used this analogy too). Bulbs and detectors distributed on a disk represented stars in a galaxy and another bulb represented the companion galaxy. The latter was moved along a trajectory past the former, the tidal force at each step was estimated from the amount of light hitting the detectors, and the positions and velocities of each analogue disk star was adjusted accordingly. Holmberg found that as a result of the flyby, spirals like those of the Whirlpool were produced, so a collision could have produced that form at least. Lindblad's work was taken up in earnest by Fritz Zwicky in the 1940s and 1950s. Zwicky was a Swiss immigrant to Caltech. He undertook a great catalog of galaxy groups and associations, and was especially interested in "interconnected galaxies." He found many of these in his cataloging work. In 1959, he published an influential review article on "Multiple Galaxies," in which he presented many examples, and a kind of morphological classification for interacting galaxies (Figure 1.9). Zwicky was a great believer in many kinds of morphological analyses, but he was also interested in the underlying physics, and speculated on the role of tidal forces in producing these morphologies. Zwicky's classification scheme did not last, but his emphasis on the importance of "bridges and filaments" for the interpretation of colliding galaxies did. Zwicky's paper was a major step forward for the study of colliding galaxies, not so much for what he said, but because he showed that there were, in fact, a significant number of these objects (though they are rare), and that rather than being random freaks, one could fit them into a classification scheme. Zwicky was an extremely innovative thinker. His background in hydrodynamics helped him become an important figure in the early development of jet engines, and his imaginative thinking allowed him to propose the existence of neutron stars and dark matter in galaxy clusters long before other astronomers were ready to contemplate such notions. However, the field was very young in his time. One can only wonder what he might have come up with if he had been working a decade or two later. His work was an inspiration to Halton Arp and Boris A. Vorontsov-Velyaminov who assembled catalogs of peculiar, but mostly colliding, galaxies in the 1960s and early 1970s. They had an especially powerful tool at their disposal, the National Geographic Society Palomar Sky Survey. This survey produced photographic plates in blue and red color bands of all of the northern hemisphere sky, and extending some way into the southern hemisphere. The plates were taken using the 48-inch Schmidt telescope (a type of telescope with a wide field of view) at Mount Palomar Observatory over a period of 10 years, and completed in 1958, with some later additions. Interestingly, monetary contributions from Zwicky and his heiress wife had aided in the construction of Palomar Observatory, and Zwicky did much of his own work at the smaller telescopes there. Prints were made from the Palomar plates, distributed to many observatories and libraries, and ultimately digitized and made available on the internet.

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Galaxy Collisions

Figure 1.9 A collection of colliding galaxy forms, and especially "bridges and filaments" from Zwicky's (1959) review article on "Multiple Galaxies" (from Hundbuch der Physik, Vol. 53).

The photographic atlases of peculiar galaxies assembled by Arp and VV (as Vorontsov-Velyaminov was commonly referred to in the west) were made by scanning the Palomar prints and identifying relevant objects. For most of Arp's objects, better images were obtained at the 200-inch Mount Palomar telescope. These atlases became the basic tools of research in this field and the starting point of thousands of research projects in the decades since. The Arp Atlas is often described as though it was a book, but actually it was not. (Recently the atlas has been reproduced in a book about it, with historical commentary.) It was

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published in two ways. One was as an issue of the Astrophysical Journal Supplement. The format of that journal was fairly small at that time, and at about half a dozen to the page Arp's images are smaller still (see Figure 3.8 below for a photo montage of Arp Atlas galaxies). Later, Caltech published the atlas as a large format notebook with photographic prints of better quality. The atlas began with an extended introduction, which also served as the journal article. This text described in detail the procedures Arp used to select the 338 systems in the atlas, which were generally basic morphological features, with intentionally very little theoretical interpretation. He states, "Because so many of the physical processes pictured are not understood, no rigorous attempt at classification has been made." He also described the observing parameters of the follow-up imaging, and gave tables of these parameters, object positions, and recession velocities where available. For the curious, every page contained mysteries and questions about the origins and evolution of these objects. The story of the VV atlas is similar; it too was not published as a book, but as a series of journal articles over a number of years. The sky survey was extended to the southern hemisphere in the 1970s and 1980s with a Schmidt telescope designed to be comparable to that on Mount Palomar. In the 1980s Arp and Barry Madore assembled a southern atlas of peculiar galaxies. It contains many more objects, fainter and more distant thanks to improved technology, and also more categories. It was published as a book, actually as two books. The first contained the complete atlas in tabular form; the second contained images of a selection of atlas galaxies. Because there were so many objects in the atlas, the image atlas contains only a fraction of the total. However, now that both northern and southern surveys have been digitized, it is possible to obtain at least a survey image of any object in the catalog. With these tools and the interest they incited, the door was open to broad studies of colliding galaxies. Computer models of galaxy collisions also began to be produced in the 1960s and early 1970s, illustrating the evolution of certain types of collision. Those of the brothers Alar and Juri Toomre were especially influential, in part because they were able to explain many of the peculiar forms of the Arp and VV atlases. Alar Toomre's models in the late 1970s led him to the conclusion that collisions would usually result in mergers between the two galaxies, and that such mergers could have a significant effect on the story of galaxy evolution. This work convinced many astronomers that galaxy collisions were not just freaks of nature, but an essential aspect of the development of galaxies. The field was launched, and branched out in the many different directions we will consider in the following chapters. We will also review Toomre's pioneering works in more detail.

1.5 Collision basics

Before plunging into the deep waters of colliding galaxies studies, it is worth taking a few more minutes to clarify some basic, but fundamental questions. The

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Galaxy Collisions

first of these is, given that Lindblad saw evidence of collisions in galaxy groups so long ago, why did most astronomers persist in thinking that collisions were rare, unusual, and so, not worth much attention. The first and simple answer to this question is that on optical images of galaxies, collisions are rare. Only a few percent of galaxies show fairly prominent signs of interaction with another. In the early days it meant that only a small number were discovered. However, as ever more galaxies were observed, many more interacting systems were found. As Arp, and later Arp and Madore found, a few percent of a survey that includes many thousands of objects is still a substantial number. Beyond the psychological impact of small number statistics, there is another factor that almost certainly had a strong effect on the thinking of astronomers the island universe metaphor. The images of the Hubble Atlas were printed so that galaxies filled almost the whole field, but on survey plates showing a large field of view, galaxies look very small compared with the distances between them. Collisions between these far-flung islands seem very unlikely. In part this impression is a cosmic accident. Roughly half of all galaxies are located in clusters of galaxies where the distance between galaxies is much less than outside of such clusters. The Milky Way is located some distance from the nearest large cluster, located in the constellation Virgo. It is interesting to speculate that if the Milky Way was located within the Virgo cluster, then the island universe view of galaxies might not have developed. The more salient point, however, is that even though the average distance between galaxies appears very large, and random collisions between them seem unlikely, the collisions are not random. As we will discuss in later chapters, collisions are much more likely in clusters and groups, and both collisions and the group structure of galaxies were orchestrated by the structure of the early universe. Moreover, galaxies are not as far apart as they look. Dark halos are about ten times bigger than the visible galaxies, so the galaxy separation-to-size ratio is much smaller than it appeared in the early days, and even random collisions are not as unlikely as previously thought. Taken together, the group and halo effects make galaxy collisions almost inevitable, rather than unlikely. So again, we are lead to ask, why don't we see more of them? There is also a timing consideration. By normal standards galaxy collisions take a very long time to play out, from a few hundred million to a billion years (Gyr). However, compared with the total age of the universe, about 13.6 Gyr, this is a relatively short time. During the collision, the time when the two galaxies are very close, and exerting a strong influence on each other, is even shorter. Thus, even if every galaxy experiences a strong collision once in its lifetime, the effects may only be easily visible for a couple of hundred million years. This is only about one percent of its lifetime, which largely explains why when we look at an arbitrary galaxy we don't usually find it in collision. This despite the fact that even one collision, like a lost love, may have played a large role in its past. There are many further complexities to this argument that we will explore in later chapters. Before we can proceed to those chapters, we need to consider a couple of more fundamental questions. First, given the fact that galaxy collisions are too slow for

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us to actually see them happening, how do we know that they really do? The answer is that, lacking the ability to observe the collision dynamics, we must use inference and indirect information. We also must rely on Occam's razor, the scientific principle that instructs us to prefer the simplest explanations for natural phenomena. In the 1960s alternate explanations for some peculiar galaxies included the possibility that some were exploding galaxies, or that in some cases we were watching the birth of new galaxies, made from new matter coming into the universe from, for example, white holes. White holes are the speculative opposite of black holes. They can bring matter into the universe, rather than sucking it out. In the end, these theories seemed rather specialized, and not able to account for a wide range of observations in a wide variety of peculiar galaxies. As we will see in the rest of this book, collision theory has been able to do exactly that. It has also been able to identify different systems as being the result of the same kind of collision, but at different stages. This has brought physical principles to bear in creating organized classification schemes that make sense of the many "peculiarities." In this Zwicky's goal was realized and extended. All of this has been very much aided by the computer models. The computer models have also provided a number of predictions that can be compared with observation, as well as "retrodictions" that have explained new observations. Thus, the answer to the question is not easy; it involves an interwoven web of observation, modeling and theory. Exploring that web and describing many of its features is one of the main goals of this book. So finally, let us go and explore some colliding galaxies!

Further Reading

Kanipe, J., and Webb, D., The Arp Atlas of Peculiar Galaxies A Chronicle and Observers Guide, (Richmond, Willmann-Bell Inc.), 2006 Arp, H. C. and Madore, B. F. A Catalogue of Southern Peculiar Galaxies and Associations, Vols. I and II, (Cambridge, Cambridge University Press), 1987 Berendzen, R., Hart, R., and Seeley, D, Man Discovers the Galaxies, (New York, Science History Publications), 1976 Sandage, A., The Hubble Atlas of Galaxies (Washington, Carnegie Institution of Washington), 1961

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Head-on collisions and rings of fire

2.1 Early stage archetypes

In the next few chapters, we will begin to look in some detail at what happens in specific kinds of galaxy collisions. Galaxy collisions begin as elegant waltzes on the vast space and time scales of the galaxy world. They end more like mantis mating or corporate competitions, with the greater consuming or merging with the lesser. The literature in this field has always been somewhat colorful. In the 1970s, the process was commonly referred to as galactic cannibalism! We will delay consideration of the end games until Chapter 5. Color images of disturbed galaxies with knots of bright young star clusters suggest a calmer comparison with fine jewelry. This chapter is about how that beautiful jewelry is made relatively quickly on the galaxy collision forge. In this and the next chapter, we explore specific examples of early stage collisions in some detail. The examples have not been chosen at random; each defines an important, even archetypal, class of collision. They derive from several perpendicular axes in the space of galaxy collision orbits. Specifically, the angle between the orbital axis of the companion and the rotation axis of the primary disk changes by 90° or 180° in different cases. Since they are the result of somewhat special collisions, e.g., narrow ranges of orbital parameters, they are not entirely representative. However, many other classes of collision can be understood as combinations of these prototypes, so they help us make sense of more typical collisions. This last point also helps justify spending some time learning the intricate details of these collisions. The careful study of these rather special collisions provides the basis for a very broad understanding of collisions in general. This study is aided by the fact that the forms of these colliding galaxies usually provide very beautiful images, especially when derived from Hubble or Spitzer Space Telescope data. In these chapters and, in fact, up to the end of the book, we will focus on collisions involving two galaxies. It would be nice to be able to say that the reason for this focus is that collisions between three or more galaxies almost never happen. This was certainly the belief in the early days of the subject, when astronomers were amazed that the isolated island universes could find each other at all in the vast emptiness of intergalactic space. Now we know of several circumstances where such multiple collisions are not rare. A look at our own Local Group of galaxies shows that collisions with small dwarf companions may be almost constant in such a typical environment (see Chapter 7), but their

C. Struck, Galaxy Collisions: Forging New Worlds from Cosmic Crashes, Springer Praxis Books, DOI 10.1007/978-0-387-85371-02, © Springer Science+Business Media, LLC 2011

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effects on the parent galaxy are likely to be quite minor. On the other hand, collisions between comparably sized galaxies are common in so-called compact groups of galaxies. These groups are a somewhat unusual environment in our time, but they may have been much more typical in the early days of galaxy formation and evolution (see Chapter 8). Yet most of these facts have only been discovered within the last decade or two. The study of multiple collisions is not nearly as well developed as that of collisions between two comparable galaxies. However, it is not simply a case of having to stick to what we know. Since even in multiple collisions, most of the action happens two at a time, we can hope that what we learn about the prototypes described below can help us in the cases of multiple encounters. Most of the action is also stimulated by close passages of the two galaxies. For a wide range of values of the relevant collision parameters, the two galaxies move well apart after the first close pass, either never to return, or to return only after a significant respite. In the latter case, the second close passage is similar to the first, though because of the operation of dynamical friction (discussed later), they do not move as far apart, or stay apart as long. Subsequent close encounters usually result in a fairly prompt merger of the two. The relatively long time between the first and second close encounter, which occurs in many cases, allows the galaxies to respond to the abrupt disturbance in isolation. This allows them time to develop waveforms characteristic of the different types of collision that we will consider in this and the following chapter. In briefer encounters and in the merging process this is not the case; the two galaxies perturb each other continuously. In such cases, it is much harder to derive information about the collision from the visible waveforms. Thus, not only are the early and late stages separated in time, they have very different characteristics. In some ways, the early stages are more distinctive, simpler, and more informative, though the ramifications of the late stages for galaxy evolution are greater. Because of their distinctiveness, and what they tell us about the collision process, we will consider these "archetypal" collisions in some detail.

2.2 Forging t h e rare rings

Even Fritz Zwicky must have been surprised on the night in 1940 when he discovered what came to be known as the Cartwheel galaxy. Zwicky was known as an iconoclastic, anti-establishment astronomer. Nonetheless, we have already seen above that he contributed greatly to the study of colliding galaxies. In contrast, Edwin Hubble was every bit the establishment astronomer. Hubble used the great 100-inch Mount Wilson and 200-inch Palomar telescopes to acquire superb images of the prototype galaxies for the galaxy classification scheme he was developing. Hubble's tuning fork classification (described in Chapter 1) remains the foundation for galaxy classification today. Meanwhile, Zwicky surveyed groups and clusters of galaxies with images obtained on the 18-inch

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Figure 2.1 A Hubble Space Telescope image of the Cartwheel colliding ring galaxy. (NASA/STScl/AURA, K. Borne).

Schmidt telescope, the smallest telescope on Palomar mountain. (In more recent decades this historical telescope was used by Eugene Shoemaker and Eleanor Helin for the purpose of finding near-Earth orbiting asteroids.) Hubble used the great telescopes of the day for finding prototypes in the

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galaxy world, while Zwicky used a small telescope, not only to catalog clusters, but also to collect oddments. He presented images of a few of his collectables in his article in the commemorative volume for his old mentor, Theodore von Karmen, and contrasted them with some of Hubble's favorites. Despite its general symmetry, the Cartwheel was among the oddest of the odd (Figure 2.1). Its appearance is dominated by the presence of a large star-forming, blue-white ring. Its beauty is enhanced by red, inner ring structures and faint spiral arms connecting inner and outer rings. At first glance, the spirals are the only normal parts of the galaxy; the outer ring was an unprecedented structure in galaxies at the time. Zwicky speculated that it was some sort of unusual centrifugal effect, which is true as far as it goes. He ended the 1941 article with comments about how there was a lot left to learn about galaxy structure, which is still very true, albeit on a different level of understanding. In the following few decades, dozens more ring galaxies were discovered, and their images made available in peculiar galaxy catalogs, but their origin remained mysterious (Figure 2.2). In 1976, Roger Lynds and Alar Toomre used simple

Figure 2.2 A montage of ring galaxy images obtained from the Hubble Space Telescope. The images in the top row (Arp 148 and Arp 147), and the lower left (the Lindsey-Shapley ring) were produced by the Hubble Heritage project (NASA, ESA, and the Hubble Heritage Team (STScl/AURA, A. Evans,). Higdon, M. Livio. The image in the lower right panel (NGC 922) is from the study of A. Pellerin, et al., Astron. /., Vol. 1 39).

Head-on collisions and rings of fire

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computer models to demonstrate the basic physical process for forming ring galaxies. Their picture has been well confirmed by observational studies in the last few decades, and is now arguably one of the most well understood processes in galaxy dynamics. Lynds and Toomre's idea was that ring galaxies were formed in one of the simplest kinds of galaxy collision. In their model, the target galaxy has a large disk containing stars and gas clouds orbiting on circular or quasi-elliptical orbits, with outward centrifugal forces balancing the gravitational pull of material interior to the orbit. In their model, Lynds and Toomre proceed by letting an intruder galaxy fall right down the rotation axis of the target disk, through the disk and out the other side. During the time that the intruder is passing through the disk, its matter is superimposed on that of the target galaxy. The gravity of the latter is holding the stars and clouds in orbit. The extra matter means extra gravity, so the stars and clouds are pulled inward. In fact, once the inward motion of a star commences, it continues for some time after the intruder has passed through. However, with the extra gravitational pull removed, unbalanced centrifugal effects eventually slow the inward motion and reverse it. The star then accelerates outward, overshooting its initial orbital radius. Then the centrifugal forces are reduced, excess gravity pulls the star back for another cycle of in-and-out oscillation. All this despite the fact that the intruder is now long gone. This description of forces alternately pulling our representative star one way or another is one way to look at the orbital consequences of the encounter. Modern physics looks a bit askance at such traditional descriptions involving pseudoforces like centrifugal force. A somewhat different way of looking at it is to say that during the overlap period, the star experiences a brief, but strong, acceleration from the gravity of the intruder. This changes its energy and angular momentum, putting it on the inward phase of a precessing elliptical orbit. This new orbit includes inward and outward oscillations as well as angular motion around the disk center forever after. There is another word that I have used in a rather fast and loose way above, the preposition "through," as in "the intruder went through the target disk." What does it mean for one galaxy to go through another? Less than you might think! This is because most of the matter contained in galaxies is concentrated into a very, very small fraction of the volume of a galaxy. Suppose we could shrink the stars in a galaxy down to the size of grains of sand. Then the average distance between stars in a galaxy like ours would be about 140 km on this scale. Sand grains spaced 140 km apart do not make for a very good beach, but that's the way it is for stars in a galaxy. The size of the galaxy disk itself would be about a couple of million kilometers on this scale, or of the same order as the diameter of the Moon's orbit. We can try to envision the collision of two clouds of sand each the size of the Earth-Moon system, with sand grains separated by at least a 100 km. The result is that the two clouds pass through each other with almost no grains from one hitting any from the other. Thus, even in direct collisions between galaxies there

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are few or no star-star collisions. This is likely also true for dark matter particles, though we cannot be completely certain until we know more about them. This is not so true for the gas clouds in galaxy disks. Unfortunately, even the term "gas cloud" suggests a picture of the interstellar gas (like clouds in the sky) that is grossly oversimplified. This is not the place to pursue any very complete description of the interstellar gas in galaxy disks. It is sufficient to note that this gas exists in a wide range of phases, with characteristic temperatures ranging from a few Kelvin to millions of Kelvin, and with a similarly large range of densities. The neutral atomic hydrogen phase has temperatures and densities in about the middle of these large ranges, and contains a large fraction of all the interstellar gas. Looking face-on at a galaxy disk, radio telescope observations show that this component covers most of the surface. Therefore, if two comparably sized galaxy disks collided face-on and center-on-center, almost all the hydrogen clouds would be involved in a collision. Cold, dense molecular clouds are more compact, and less likely to collide. In summary, the simple rule of thumb is that in galaxy collisions the stars and dark matter pass by each other, but a large fraction of the gas elements collide in the overlapping parts of the gas disks. Note that in the simplest version of the Lynds and Toomre model the intruder is assumed to have no gas disk, e.g., like an elliptical galaxy, and the gas dynamic complications are avoided. Now we know something about what happens in a direct collision between galaxies, and how disk orbits are affected. This still does not explain where the ring comes from. For example, if all the stars in the disk orbited in and out together, then the disk would alternately expand and contract as a whole - but they do not move in sync. Like the Solar System, objects closer to the center have much shorter orbital times than objects farther out. (However, because of the distribution of matter in galaxies, the analogy is not exact.) This means that when the stars in the inner disk of a ring galaxy have completed their inward motion and rebounded outward, stars farther out in the disk are still moving inward, and stars in the outermost parts have just begun their inward motion. With stars moving out at one radius, but moving inward at a slightly larger radius, we get a region of compression. Moreover, once the outward moving stars stop and fall back, they move away from the compressed region, and into a low-density rarefied region. The compression zone, which is in fact a wave, moves outward (Figure 2.3). In fact, a series of waves are produced by this mechanism. Figure 2.3 shows another feature of the compression wave; it can be a zone of orbit crossing, where inward moving stars pass by outward moving stars. This is not true in the rarefied regions. Moreover, the star wave has relatively sharp edges. At those edges, stars are slowing down and reversing their direction of motion at the innermost or outermost excursions of their orbits, making so-called caustic edges. Figure 2.4 shows an example of a galaxy with a rather wide ring wave, with fairly sharp edges (the prototype so-called Sacred Mushroom galaxy). This system has not been studied in sufficient detail that we can be completely sure that it truly illustrates the predicted caustic effect, but it seems very likely.

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Figure 2.3 Plot of how the orbital (epicyclic) radius changes with time for sample stars at different initial radii. The units of both quantities are scaled to a specific (core) value. Note how initially all the stars move in together as a result of the extra gravitational pull of the companion. Later they 'rebound' outwards, losing some of their synchronous motion, and so squeezing together at small radii to form the first ring. As time passes the orbits get more out of phase forming orbit-crossing zone in later rings. Ultimately, these orbit-crossing zones overlap themselves, eventually smearing out the rings (figure by the author).

What about the gas clouds? Just as they do not fly past each other in collisions between two galaxy disks, their orbits cannot cross each other in the same way the stars do. Generally, they will collide with similar clouds in the ring compression zone. If not, they will still have to plow their way through a denser than average surrounding medium. This will result in strong cloud compressions, and probably a great deal of turbulence. Clouds may also be gathered into strong local centers of gravity, that is, dense, massive clouds. A great deal of star formation will be triggered in these massive clouds, but more on that later. Let us summarize what we have learned about ring galaxies. In essence, we have seen that a galaxy disk can be rung like a bell, following a sharp blow from

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Figure 2 . 4 The so-called Sacred Mushroom ring galaxy (Digital Sky Survey, produced by the Space Telescope Science Institute, under a grant from NASA).

an outside object. True, the details are very different. In the case of a bell, oscillations in the solid metal are translated into sound waves in the surrounding air. In the galactic case, hundreds of millions of stars and thousands of gas clouds have their orbits modified in such a way as to create an outward moving compression wave stretching tens of thousands of light years around the galactic center. The wave from a bell makes air molecules oscillate at a rate of hundreds of cycles per second. A ring wave moves through a galactic disk on a timescale of hundreds of millions of years. If a ring wave had been started in our galaxy at the beginning of the age of the dinosaurs, it would still be moving through the outer disk. However, in both cases natural oscillations are excited in the systems by an abrupt disturbance, despite the vast difference in scales. Another common analogy that astronomers use is that the ring waves are like the waves excited by a stone dropped in a pond. Again, the physical details are very different. In the water, molecules oscillate up and down in the wave, perpendicular to the surface of the pond, while in the ring wave stars oscillate in and out within the disk. However, the analogy is a good one since in both cases

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the particle motions lead to the repeated waves, though either dissipative or orbital mixing effects damp the waves after a few oscillations. Actually, in the case of galaxies, this is an extrapolation based on computer modeling, since we do not actually see more than one ring in more than a handful of galaxies. From the figure above, it appears that the Cartwheel is one of that handful. Nonetheless, even in this most famous example, the outward velocity of the inner ring has not been accurately measured and found to be in accord with the theory. If it is found to have no outward velocity, then it would likely not be an example of a second wave exited by the collision, but to have a different cause. I make this somewhat narrow point to highlight how hard it is to confirm any prediction of theory in astronomy. Frequently, it takes many laborious observations with different instruments, in many wavebands, simply to constrain the theoretical possibilities. We will see in the following sections that we do have confirmation of many aspects of the basic theory of ring galaxies. That is one of the reasons why these very rare objects are so valuable to the study of colliding galaxies. Before leaving this discussion of the basic theory of the rings, there are a couple more questions worthy of mention. The first is why we only have rings in disks, not the bulge or halo components of the galaxies? The answer highlights the different natures of these structures. Disk stars are supported against gravity by their rotational motion. Aside from their rotation, disk stars have only small components of random motion; this is why they do not rise very far out of the relatively thin disk. Bulge stars and halo particles, on the other hand, have largely random motions and much smaller overall rotational motions around the galactic center. This means that their orbits range from nearly circular to largely radial (in and out), they lie in many different planes, not a single one, and that within any plane they circulate in both directions. Thus, the radial disturbance of the intruder does not change the overall structure of orbits in the bulge and halo enough to form a discernable ring. In the specialist jargon, the bulge and halo particles are said to be dynamically hot. They move rapidly on random trajectories like the molecules in a hot gas. Disk stars are dynamically cold; they pursue largely ordered trajectories like the motion of a cold liquid in a pipe. We will see later that galaxy collisions tend to heat up galaxies, dynamically speaking. That is the basic idea of why there are rings in disks, and no rings in bulges, but we need to refine the idea a little to answer a related question - how big an intruder does it take to make a ring in a disk? If very small intruders could make bright rings we would not expect them to be as rare as they are. Only about a few percent of nearby galaxies are strongly interacting (though traces of weak or past interactions are more common), and ring galaxies make up a comparably small percentage of the strongly interacting galaxies. Moreover, it seems intuitively right that strong effects should have strong causes. So what prevents a very small intruder from making a wave in a cold disk? The answer is that the disks are not so very cold. The random velocities of disk stars in a galaxy like the Milky Way are roughly 10 percent of their orbital

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velocities, but not zero. Therefore, disturbances of much less than that amplitude, or much smaller intruder-target galaxy mass ratios, will yield no detectable wave.

2.3 Why rings? In the previous section we delved more deeply into the kind of "centrifugal effects," to use Zwicky's term, that are needed to create a colliding ring galaxy. A large part of the answer to the question, "how are ring galaxies made?" was given there. Granted that, you might still wonder whether it is worth paying so much attention to objects that are so rare in the universe of galaxies, hence "why rings?" I first asked myself that question in the early 1980s when I started working with my long-time collaborator Phillip Appleton, on models and observations of ring galaxies. Actually, at the time we had a pretty straightforward answer - they would be relatively easy to model. At the start of this work, Phil was working at the University of Manchester, which had very good computer resources for the time, and he was working with other experts in the kind of simulation codes required, so we had hopes of making very good models. At the same time, we had very high ambitions to study many aspects of the physics of colliding galaxies, but it seemed wise to start with relatively simple cases. In retrospect, our research was a stimulating mix of some conservative good sense, and some unrealistically great expectations. In the intervening years, many of our expectations have been met in studies of many kinds of collision, as a result of the work of many modelers. Nonetheless, our basic point was that, given the relative simplicity of the problem, not to mention the work of Lynds and Toomre, this was one case where we knew in advance what observed structure came out of a specific kind of collision. With such a promising starting point, we hoped that detailed computer modeling could be compared with high quality observations, and provide an unprecedented level of detail about the processes involved in galaxy collisions. We were (and still are!) particularly interested in learning about two areas. The first of these was how density waves produced in galaxy collisions triggered star formation. This one will be a recurring theme in this book. The second was how the multi-temperature, multi-phase interstellar gas responded to such waves. I think it is fair to say that there is a great deal yet to be learned about general spiral density waves in galaxies, and this ignorance stands in the way of studies of star formation in such waves. In the case of ring waves, they are simple (approximately circular and usually propagating outward), and we know about when, where and how they formed. We know the basic motion of disk stars in the wave, and can test ideas about the behavior of the gas via the comparison of computer models to observations. Despite these advantages, nature has still stacked the deck against us. First, the observations are very limited in several ways. Ring galaxies are rare, so only a

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limited number have ever been cataloged (albeit with many more discovered in the last decade), and very few are close enough to study in any detail. There are other ways besides collisions to make rings in galaxy disks, and it takes some detailed study to confirm that that is the case in any particular system. Specifically, rings produced by other means generally do not expand outward. Thus, a pattern of spectral line shifts in the stars or gas indicative of outward expansion is good evidence. On the other hand, the absence of a collision partner is not. The partner could have been disrupted, could have merged promptly into the primary bulge, or could simply lie behind a luminous component of the primary. In all, less than about a few dozen colliding ring galaxies have been studied well enough to confirm their nature, while millions of galaxies have now been catalogued. We have only a modest number of known cases, and we only get one snapshot of a long evolution in each case. This is like trying to do the several dozen steps in the assembly of a piece of furniture or a complicated child's toy with directions for only two or three of the (randomly chosen) steps. A parent's holiday nightmare! However, when it comes to deciphering ring galaxies, we have no young child demanding that we get it done now. Even with limited examples, we have time to work it out. Until recently, the real barrier was lack of detailed information on those systems. In the first case, there was a lack of spatial resolution in the observations. We would like to be able to resolve structure at least down to the scale of the most massive gas clouds, about a few hundred light years across. In the optical waveband the launch (and repair and several refurbishments) of the Hubble Space Telescope made this possible (see Figure 2.2). In the last few years a new generation of large ground-based telescopes have been constructed which can also do very well. A second obstacle was that until recently our view was generally limited to the optical waveband. In the last couple of decades, detailed maps of radio emission have revealed the distribution of hot, ionized gas and warm atomic hydrogen in many of the closest colliding galaxies of all types. As will be described below, satellite observatories now provide a wealth of information in the infrared, ultraviolet, and X-ray bands, as well. Thirdly, the resolution of our computer models was also limited. The computational speed and memory capacity of computers has increased enormously in recent decades, to the point where there are no longer major limitations in computing models of colliding galaxies. These considerations apply to the study of colliding galaxies in general.

2.4 How precise a bullseye? Angles and offsets

The idealized Lynds and Toomre scenario for the formation of a ring galaxy was described above. It is a simple and elegant picture that clearly shows the basic physics of how rings are made in symmetric galaxy collisions. In the next few

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subsections, I would like to emphasize that rings can be produced in a wider range of collisions, when the idealizations of Lynds and Toomre have been relaxed. As a first example, consider the case when the companion galaxy does not move right along the symmetry axis of the primary disk, but impacts the center of that disk at an angle. In reality, this angle is a little bit difficult to define, because the companion orbit is generally curved like a hyperbola or an ellipse, and so, the companion's precise direction of motion relative to the primary disk axis is continuously changing. However, since near the time of impact, the relative speed of the two galaxies is high, and the encounter is brief, details of the direction changes along the curved orbit are not too important. Generally, the companion orbit is confined to a plane. The angle between the primary disk plane and the companion orbit plane is well defined, and is called the orbital inclination. It is an important parameter. We will see in later sections that when the companion orbit is close to the plane of the primary disk, spiral or more complex waveforms are produced in the primary disk. So the question is what is the largest value of the inclination angle for which the collision still produces a recognizable ring? This question is complicated by the fact that the critical conditions for ring making depend on other properties of the galaxies besides the orbital inclination. These subtleties aside, the rough answer to the question above, as derived from computer models, is that the angle of attack can be quite large, of order 45° or so, and a ring can still be formed. Thus, impacts at a wide range of angles can still generate a ring, albeit a distorted one at the extremes. This considerably relaxes the symmetry of the Lynds and Toomre model. The reason for this is intuitively obvious, at least in hindsight. Even with a considerable inclination angle, the primary pull of the companion on disk stars or clouds is inward. It is not up or downward, because the pre-collision pull upward (for example), is roughly countered by the post-collision pull downward (though this also depends on details of the orbit). Regardless of the exact angle of attack, if the companion orbital plane is nearly perpendicular to the primary disk, then stars will not be pulled forward or backward in their orbits by any significant amount. The primary pull is inward in all cases. The next question to consider is how close to the center of the primary disk must the impact point be in order to generate a recognizable ring? Here again, simple intuition is in accord with the results of computer modeling. For example, if the impact point is halfway out in the primary disk, then all of the stars at larger radii will still feel an inward pull, and so a ring will form in the outer disk. It will be an asymmetric ring, stronger on the impact side, but a ring nonetheless. For stars orbiting at radii smaller than that of the impact point, the pull of the companion will be a tidal force, which we will discuss in detail in the next chapter. For the present, we simply note that this can generate spiral, rather than ring waves. The situation is very nicely illustrated by a series of computer models that Alar Toomre produced in the 1970s (Figure 2.5). Our discussion of this point has implicitly assumed that the companion is very compact, with all its mass concentrated near the center. This is generally

H e a d - o n collisions a n d rings o f f i r e

t=6

12

°

18

O

24

Figure 2.5 A collection of early computer models of the effects of ring galaxy type collisions, but with progressively larger offsets of the impact point from the disk center. Each row shows five snapshots of the wave development in a single model. The bottom row shows a symmetric ring-making collision, and the offset, and the spiral wave character, are greater in each successive row toward the top. The collision partner is assumed to be a featureless source (large circle) of gravitation. The disk stars are initially on circular orbits (AlarToomre, 1978, from Proceedings of I.A.U. Symposium No. 79).

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not true of the companion's dark halo matter. Because of the extended halo, even stars orbiting at radii smaller than the impact radius will feel excess mass and gravity near the primary center. Thus, even tidal spirals in the inner primary disk will have a circular ring-like component in their structure.

2.5 Taffy galaxies and shovels: companions w i t h gas disks

In the simple theory, we have also not considered the possibility that the companion has a gas and star disk of its own, though we did note the likelihood of gas cloud collisions in such cases. The existence of a companion disk introduces another parameter, the tilt angle of the companion disk relative to the primary disk. There are a number of interesting possibilities that I described with computer models in a journal paper in 1997. The most obvious is that the tilt angle is zero, and both disks have the same orientation. This means that at impact they smack straight on, with clouds in the two disks hitting at the full relative velocity of the two galaxies. Viewed from the primary, the impacting clouds of the companion try to lift up the primary clouds to follow the trajectory of the companion halo. With extra weight (in the primary's gravity field), they cannot keep up, and fall behind the companion halo. Some companion clouds pass through holes in the primary gas disk, lift no weight, and do not fall behind. Some companion clouds hit gas concentrations in the primary disk and are promptly captured. The net result is that gas is strewn between the two galaxies as they move apart, like the debris between two fully loaded trucks after a collision. This process is not purely theoretical. It is realized in a system aptly nicknamed the "Taffy" galaxies, where the hydrogen gas has been observed stretched out like the sticky candy between the two galaxies. Moreover, the primary galaxy appears to be a ring galaxy (Figure 2.6). (Note however, that the intergalactic taffy is best seen when the two galaxy disks are seen edge-on, making it hard to see the rings.) In fact, other examples are now known, but this is a rare class of galaxies. At the opposite extreme, the companion disk can be tilted perpendicular to the primary disk, minimizing the number of intergalactic cloud collisions. In fact, there are two subcases. The first is when the plane of the companion disk passes through the primary center near the time of impact. That is, a line connecting the two disk centers goes through the companion disk. When two galaxies experience a strong collision in a small galaxy group, or in isolation, they are generally weakly gravitationally bound to each other or weakly unbound, but both are bound to the group as a whole. This point will come up repeatedly in later chapters. In this circumstance, their relative orbital speeds near the time of impact are not very different from the orbital speeds of stars and gas clouds in the disk of the primary. This means that in the time it takes the companion disk to pass through the primary disk, there will also be time for clouds in the primary disk to orbit into the overlap region and hit the

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Figure 2.6 A contour map of the hydrogen gas strewn between the two galaxies of the "Taffy" system. Black and white are reversed in this image, so the sky is white, and the two galaxies are the dark blobs at the left and right sides of the white contours (from Y. Gao, et o/., Astron. /., Vol. 1 26, HI data originally obtained by ).). Condon, et al.).

companion. In essence the companion gas disk acts like a shovel, briefly (on galaxy timescales), scooping up primary clouds. The value of the mass scooped up depends on the relative velocity component in the plane of the primary disk. It will generally be more than the mass contained in the direct impact area of the primary disk. In all but the most favorable cases, it will be less than the total gas mass of the companion. If it is relatively little, then the gas will be captured by the companion. If it is a large mass, then we will have taffy between the galaxies for a time, but most of it will fall back to the primary.

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The last case to consider is when, at impact, the companion disk is both perpendicular to the primary disk and perpendicular to the line of centers. In this case, the shovel is turned sideways relative to the direction of motion, so it scoops up very little. In all of these cases, assuming that the companion disk is smaller than the primary, a part of the primary gas disk is relatively undisturbed, at least until material starts falling back out of the taffy bridge. Therefore, as long as the orbital inclination and the impact offset are not too great, as per the previous subsection, we expect a ring wave in the disk stars, and in the unperturbed part of the primary gas disk. The system Arp 284 provides an example of some of these ideas. My research collaborator Beverly Smith, of East Tennessee State, has studied this system for many years. The primary is a lop-sided ring galaxy, which also has spiral arms. We see the companion edge-on (Figure 2.7). We will discuss shortly how star formation is concentrated in the rings of many ring galaxies. However, in this case, there is little or no on-going star formation in the ring. This result of optical observations is confirmed by recent Spitzer Space Telescope observations, which show a low level of hot dust in the ring. Heated dust inevitably accompanies star formation. Beverly's radio telescope observations show that there is plenty of interstellar hydrogen gas in the system, but it is not concentrated in the ring where it

Figure 2.7 As in the previous figure, contours of hydrogen gas are shown on the black/white reversed image of the galaxies, here the Arp 284 system. In this system, tidal torque or swing has moved a huge amount of gas out of the disk boundaries of the galaxies, in contrast to the splash of directly colliding gas clouds in taffy systems (Beverly ). Smith).

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could fuel star formation. In fact, the majority of it is not even contained within the galaxy disks, but has been thrown out into a bridge between the galaxies and into great loops or tails. Some of the gas splashed out of the primary may have come from the region where the remaining disk stars are concentrated in the ring wave. Thus, the fuel for star formation has been removed. What goes up, must come down, at least where gravity rules. The collisions that make ring galaxies involve deep penetrations, and as we will see in later chapters, orbital energy is dissipated in such collisions. As a result, after the encounter the two galaxies do not move very far apart, unless their initial relative velocity was very high. Thus, they are usually doomed to fall back together and merge. The gas splashed into the space between them will generally fall back into one or the other even before they merge. This infalling gas is generally some mixture of material originating from both galaxies. Consequently, it spins around an axis that is different from that of either of the original disks. In physical terms, it has a different angular momentum vector. This means that when it is accreted onto a galaxy it forms a disk that is tilted relative to what remains of the original. It has also generally lost a good deal of its spin (angular momentum), and is concentrated in the center. This means that the primary galaxy, for example, may have both an inner and an outer gas disk with different tilts. If the companion lost most of its gas in the collision, it will have a reformed gas disk, tilted relative to any remnant stellar disk. Then the galaxies collide again and merge, and things get really messy! There is a class of beautiful galaxies with large annular disks and central bulges, and often with waves within the annular disks. These are called Hoag's objects, after Art Hoag the discoverer of the first (Figure 2.8). They may be the result of mergers between unequal progenitors. If the smaller companion orbited close to the symmetry axis of the primary (i.e., a ring making orbit), then the otherwise messy merger may have preserved the larger outer disk. Hoag's objects have not been studied much, so this explanation is only one conjecture among several in the literature. Much more observational and modeling work will be required to explain them fully.

2.6 Twice-made rings

I would like to take the discussion back a bit, to the case of simpler companions without gas disks, on simple orbits close to the rotation axis of the primary disk. The reason for backtracking is to consider yet one more class of rings. It is like cases that we have already covered, but now we will follow it a little farther ahead in time. We discussed above how after the collisions the two galaxies do not generally move too far apart, and fall back for a second collision. Still more orbital energy is dissipated in this second collision, and the two galaxies usually merge. However, there are situations, e.g., where the companion is relatively small, when the merger does not occur until the third time around.

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Figure 2.8 Hoag's object, which has a broad, annular ring around a central bulge. The nature of this ring is probably not the same as that in colliding ring galaxies, though its origin may be related. (NASA, ESA, and the Hubble Heritage Team (STScl/AURA), R. Lucas).

In this case, and with the simple companion on a nearly symmetric orbit, the second collision would not be so different from the first, and we would generate a new set of ring waves in the primary disk. However, this set of rings will not be exactly like the first set. In the discussion above, we assumed that the radial motions of stars and gas clouds in the disk plane were very small before the first collision. Rings were formed because the collision induced coherent radial motions in the disk. The assumption of no initial radial motions is generally not valid when the second collision occurs. It may remain a good approximation for the gas clouds, because shock waves and wave dissipation in general may damp their radial motions before the

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second collision. However, as shown by Figure 2.3 above, this is not the case for the stars. In the inner part of the disk, some stars are going in while others are going out, and we say that the phases of the radial motion are mixed. This is not the case in the outer disk, where rings are still visible. Out there radial phase still varies with distance from the disk center. In either location, when the companion passes through and delivers its extra gravitational pull the second time, the effect of that pull will depend on the radial phase of the star feeling it. For stars that happen to be in phase with the unbalanced outward centrifugal acceleration, the two forces may approximately cancel, ending the radial excursions of those stars. For stars that happen to be accelerating inward, the two forces will add, yielding much bigger radial excursions. Intermediate effects will result in stars at intermediate phases. As a result, we get very broad and very weak rings in the inner disk. They may not even be visible against the background. As ring waves propagate through the outer disk they will get broad and strong, only to disappear at radii where the net radial motions of most stars is small, and then reappear at larger radii. The result is analogous to moving along the interference pattern produced by the classic physics experiment in which light waves pass through two slits before hitting a screen. Here we have two sets of interfering ring waves. It must be acknowledged that this phenomenon has not been observed, or at least not recognized. We have made a lot of assumptions, so such systems must be rare even among the rare ring galaxies. Finding and interpreting them is difficult for other reasons as well. They are worth considering because some of the same processes will be involved in the much more common cases where the companion merges in the second collision, or very soon afterwards. This helps us understand objects that look like colliding ring galaxies, but without any visible companion. On the negative side, the complexity of second sets of rings, and in particular their dependence on the whole interaction history, means that we can derive little information from their observed strength and spacing. Hopefully, by this point I have succeeded in convincing you that although colliding ring galaxies may be one of the simplest kinds of colliding galaxy, there are many, many variations on the theme, and some of those variations are rather complex. Like shoppers in a very large jewelry store, we have looked at many rings in a variety of cases. We are startled then when the clerk walks up to us and asks if we would like to look at some with jewels mounted in them, because so far we haven't.

2.7 Bejeweled rings

The blue jewels of the ring galaxies are massive, young star clusters that form within the rings. Of course, we have already seen them in a number of the illustrations above. In many systems, they dominate the appearance of the ring, more like pearls on a necklace than a few jewels on a ring. They are also absent in some rings. However, they are present in essentially any ring that contains

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interstellar gas as well as old stars. In this section we will begin to try to understand the how and why of this phenomenon, which we will explore more deeply in a later chapter. As with most aspects of the rings this will be a preview to other kinds of colliding galaxy, where the same kind of thing occurs. Indeed, the "beads on a string", or "pearls on a necklace" metaphors are old cliches for clumps of young, blue stars in spiral waves in galaxy disks, but as usual the rings provide relatively simple examples. When we discussed the gravitational/centrifugal nature of ring waves, we noted that ring waves can be orbit-crossing zones for stars, but not for clouds; they will collide with each other. From the point of view of the interstellar clouds, ring waves look like multiple car crash pileups on a busy thoroughfare, and they never hear it coming. I say, "hear it coming" instead of "see it coming" because this interstellar pileup is an extreme version of the same kind of gas compressions we experience as sound waves in our atmosphere. Actually, they are more like the sonic booms produced by supersonic aircraft. Most of the mass of the interstellar gas has temperatures less than 10,000 K, and sound speeds of less than 3 km/s. The cool molecular clouds have temperatures and sound speeds much lower than this. Ring waves propagate out through galaxy disks at speeds of about 50-100 km/s, that is, at highly supersonic speeds. Thus, no sound wave could be sent from the wave front to warn outer disk clouds, and they do not hear it coming. (A light signal would work of course, so they could see it coming, but we need not pursue this explanation of a weak joke any further!) Highly supersonic collisions dissipate a lot of energy. One does not even want to think about a head-on collision between two supersonic aircraft, but in terms of relative speed compared to the local sound speed (the so-called Mach number), our colliding ring wave clouds are going more than ten times faster than such aircraft. When two material objects hit at high speeds the outcomes range between two extremes: the splat or the explosion. In such high-speed collisions, solids and liquids would be vaporized, and gases heated to very high temperatures (e.g., millions of degrees K). Unless these gases can cool faster than they can move away from the crash sites, they will explode outward. This would be the case for such a crash on Earth. In the case of waves in galaxies, the scale of the crash site is so huge, of order thousands of light years, that it takes a considerable time for the gases to expand away from the region. In fact, in many cases there is time for the gases to cool before they blow up, and out of the vicinity. The caveat is that if there is no explosion, then the high pressure produced behind the impact shock is not reduced in the way it would in an explosive outflow. According to the ideal gas law, high pressure requires either a hot gas at low density or a cooler gas with a correspondingly higher density. As the ring wave's gaseous shock runs through the cloudy, multiphase interstellar gas, it generates the whole continuum of possible high-pressure outcomes. Warm-tohot, low-density gas will permeate the whole region, and its high pressure will compress the cooler, denser clouds. As a result of this pressure, and the

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gravitational collapse that it may trigger, these clouds will compress the densest sub-clumps within them. These sub-clumps will become gravitationally unstable, and their collapse will generate many new proto-stars on a scale millions of times smaller than the width of the ring wave. One is reminded of how carbon deposits are squeezed into diamonds by the enormous pressures in the interior of the Earth. However, the analogy is not really very good. In the ring wave, we believe stars form within sub-clumps, contained in clumps, contained in clouds, contained in groups of clouds in the dense post-shock gas. Thus, we expect the stars to form in small associations or larger clusters, with these contained in levels of still larger clusters. We should note that the higher levels of structure are probably not tightly bound by gravity, and so will fall apart after the ring wave passes and they find themselves in a less dense environment, and heated by the intense radiation of massive young stars. I should point out that this structure has not really been observed and analyzed in detail in any colliding ring galaxy. A partial analysis has been carried out, using Hubble Space Telescope observations of the Cartwheel ring galaxy by Phil Appleton and his students. They found hundreds of barely, or nearly, resolved star clusters in the Cartwheel. Many of them are obviously contained in still larger groupings, though this has not yet been quantified in any rigorous way. Such studies have been carried out on other types of colliding galaxy, as we will discuss below.

2.8 The mysterious Cartwheel: p r o t o t y p e or anomaly?

The Cartwheel galaxy itself, often called the prototype of the colliding ring galaxies (Figures 2.1 and 2.9), provides a good example of how even relatively well studied colliding galaxies can retain a great deal of mystery about their nature and origins. The Cartwheel mystery is a classic whodunit. There are a number of ring galaxies that appear to be products of a collision, but with no apparent companion. In the case of the Cartwheel there are three possible perpetrators. It appears, however, that the closest two are interacting with each other. This means that they both have similar orbits relative to the Cartwheel. In terms of explaining the Cartwheel, if one of them hit, then the other passed closely by. In that sense, these two count as one. However, in the early 1990s, James Higdon discovered an important piece of evidence suggesting that the hit-and-run was committed by the third, and apparently, considerably more distant companion. This was a faint plume of atomic hydrogen gas stretching from the Cartwheel to that third companion. It appears to be a long, stretched bridge between the two galaxies. Among the relatively nearby ring galaxies, it is uniquely long and thin, though its faintness means there is considerable uncertainty in such details. Moreover, we seem to be viewing this bridge from the side, so we cannot measure telltale velocity changes

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Figure 2.9 A beautiful false-color image of the Cartwheel ring galaxy assembled by Phil Appleton using data from several different wavebands, obtained from four NASA orbiting telescopes. Purple denotes X-ray emission from the Chandra X-ray Observatory, blue is ultraviolet emission from the GALEX (Galaxy Evolution Explorer) telescope, green is visible light from the Hubble Space Telescope, and red is infrared emission measured by the Spitzer Space Telescope (NASA, JPL-Caltech, Spitzer Science Center and P.N. Appleton).

along it. As a result, we are not entirely sure that it is a bridge, rather than tidal debris positioned by chance to look like a bridge. Even worse, the companion appears almost edge-on, so we do not any clues about the nature of the collision from its tidal morphologies. Ordinarily we might not be so cautious about the interpretation of these radio observations. However, new evidence implicates the closer companions. Observations made with the Chandra X-ray satellite have discovered a bridge of hot gas stretching from the closer companions to the opposite side of the Cartwheel ring. In the absence of on-going heating, and no heating source is obvious, this gas will cool quite quickly. For this reason, X-ray bridges between galaxies are rare, and are generally very young bridges if the heating comes from a collision. In the case of the Cartwheel it is hard to imagine any other heat source.

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Thus, the X-ray observations of hot gas suggest a recent collision with the nearby companions, but the radio observations of cool gas suggest a collision with the farther companion a longer time in the past. It seems that we need more clues. Yet other constraints are hard to come by, even when we have some good observational data. For example, the expansion speed and size of the ring have been estimated from the observations. Dividing the second by the first gives an expansion age. However, we only know the ring size projected onto the sky and one component of the expansion velocity. Since we do not know the projection angle, our estimate for the expansion age could be inaccurate. Similarly, we only know the projected separations and relative velocities between the Cartwheel and its companions, so an analogous estimate of the time since impact is also likely to be inaccurate. If we knew the several projection angles, we could compare the ring expansion time to the two separation times, and whichever companion has the separation time closest to the expansion time would be the perpetrator. Lacking these angles, it appears that we must file the Cartwheel case in the unsolved galactic mysteries file - at least until we find more evidence. This is unfortunate because the Cartwheel, despite its work-a-day name, is an unusually beautiful colliding galaxy, with its blue outer ring, red inner ring, and connecting spokes. Double rings and spokes are rather rare among the ring galaxies, and it would be very nice to know more about their origins. There are more clues, for example, the precise shape of the outer ring, which is not elliptical, i.e., a circle in projection. That tells us the collision was not perfectly symmetric. It appears from HST observations that the age of star clusters inside the outer ring increases with distance behind the ring. Accurate determinations of those ages (from future observations) would allow one to get a good estimate of the ring projection angle, and the true expansion age. (In fact, estimates have been made already in one study.) The pattern of the stellar motions within the Cartwheel disk has been measured and provides more clues. I have argued in a recent journal paper that taken together all of these clues suggest that the far companion was the ringmaking partner, but that these extra clues indicate a close fly-by collision with the other two. However, I have to admit the evidence is not conclusive. For the present, we must leave the Cartwheel story on the edge of view, a little fuzzy, and not completely resolved. We have come a long way since Zwicky and other pioneers, but in every case study we consider, we will have to leave questions to be answered in the future. Each case will teach us some general lessons about galaxy collisions and galaxy evolution, so we should not be too upset about our inability to "sweat (all) the details."

2.9 Other ways t o make rings

Since we have investigated collisional rings quite thoroughly in this chapter, it is worth ending with a mention of some other kinds.

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Figure 2 . 1 0 NGC 4650a is a polar ring galaxy, whose ring is perpendicular to its inner disk, and is likely the result of accretion, perhaps following the disruption of a smaller companion in a collision (NASA, ESA, and the Hubble Heritage Team (STScl/AURA), ). Gallagher).

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The first "other kind" of ring also originates in galaxy collisions, but collisions of a very different kind. These are the "Saturn's rings" of the galaxy world (Figure 2.10). They are formed when a small companion is disrupted, or partially disrupted, on the way to merger, leaving material behind in stable orbits. Of course, in such a violent dynamical process, gas and stars are not left behind on nice, circular orbits. Because they are collisionless, the residual stars will follow a wide range of orbits depending on the precise circumstances (e.g., position and momentum) of their liberation, and the details of the competing forces acting on them at that moment. Many of them may pursue orbits similar to that of the small companion galaxy itself at the time. If a large amount of gas is pulled out of the companion, the situation is rather different. Intersecting orbits will lead to dissipative cloud collisions, which will change the orbits of the gas clouds. Eventually, this will spread the gas out into a broad ring, and the cloud orbits will be more circular. The bumping and nudging between clouds will be minimized. At this point, we have made a ring of gas, which may be capable of forming new stars, and thus become a ring of stars and gas. This is not quite the end of the story, however. If the primary galaxy has a disk, or other non-spherical component, this component will exert torques on the ring material. The result will be precessional motions out of the orbital plane of the ring. The ring will "flap" with different periods and phases in different places. The result will be more cloud collisions and orbit evolution, until the torques are minimized. The torques are reduced to zero if the orbital plane of the ring is the same as the disk, or perpendicular to the disk plane. In the latter case, the ring intersects the poles of the disk, and galaxies with such rings are called "polar ring" galaxies. Polar rings are thought to be about as common as collisional rings. They are not as proficient at triggering star formation as collisional rings, so they are not as bright relative to the parent galaxy. This makes them somewhat harder to find. A couple of different kinds of rings are often associated with barred galaxies. These are the resonance rings, which come in two types, inner and outer. The origin of these rings is a rather technical topic, which we need not explore here. I will have more to say about them later, and for the present a couple of brief comments will suffice. Firstly, these rings are quite common, especially the inner resonance rings. Part of the reason for this is that they can be found in isolated as well as colliding galaxies. Secondly, at least in principle, they can be straightforwardly distinguished from the rings produced in the direct, head-on collisions discussed above. Resonance rings can be produced at essentially any radius in the disk of a galaxy, though they are typically found at small radii. They do not expand outward through the disk like those produced as waves in collisions. Because of the mechanisms that produce them, resonance rings remain near their radius of origin, or at least, expand or contract on a longer timescale than collision produced ring waves. Thus, spectroscopic measurements should generally reveal little or no expansion. This is also true of polar rings, but they are not generally confined to the disk like resonance rings. With these other cases, our survey of the simplest form of ornamentation in

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the world of galaxies, the rings, is complete. Even on this huge scale, nature makes much more elaborate jewelry, which we will examine next. Further Reading

Appleton, P.N., and Struck-Marcell, C , "Collisional Ring Galaxies," Fundamentals of Cosmic Physics, Vol. 16, p. I l l , 1996 Lynds, R., and Toomre, A. "On the Interpretation of Ring Galaxies: the Binary Ring System II Hz 4," Astrophysical Journal, Vol. 209, p. 382, 1976 Madore, B.F., Nelson, E., and Petrillo, K. "Atlas and Catalog of Collisional Ring Galaxies," Astrophysical Journal Supplement, Vol. 181, p. 572, 2009

3

Swinging on by: the tidal twists

We have seen in the previous chapter that colliding ring galaxies are the result of the direct pull of the companion galaxy's gravity as it passes through. Most galaxy "collisions" are not literally so. That is, the galaxy disks pass closely by each other, but they do not actually collide. They can still profoundly affect each other, primarily by means of forces that are analogous to those that cause ocean tides on Earth.

3.1 Tides near and far

I live, and was born and raised, in the central parts of a huge continent, and so I have no direct, daily experience of the tides as do those who live near the oceans. For those who live near a few special places on this planet, like the Bay of Fundy or the western side of Hudson Bay, where the sea tides have especially large effects, keeping track of them can be a survival issue. The Earth's atmosphere and crust also experience tides with effects we probably still have much to learn about. As I will explain more carefully in a moment, these tides result from the fact that the Moon's gravitational pull is stronger on the side of the Earth facing it than on the opposite side. The Earth also feels tidal forces exerted by the Sun that are about half the strength of those of the Moon. This fact is just the first of many complications that determine the size of ocean tides at any particular coastal location. The so-called astronomical tides acting on a planet as a whole are generally simpler. Looking around our Solar System, we can find examples of much stronger or much weaker tides than those felt by the Earth. The tides exerted by Mars' small asteroidal moons, Phobos and Deimos, are so small that they are hardly worth mentioning. Mercury orbits at about one-third the distance of the Earth from the Sun, so it experiences much stronger solar tides. As in the case of the Moon, the accumulated effect of these tides has been strong enough to modify Mercury's rotation. Jupiter's inner moon Io experiences tidal forces that are strong enough to heat and churn the silicon and sulfur magmas in its interior, and make it the most volcanically active body in the Solar System. How is one force able to generate all these different effects? I will try to answer that question in this section, in preparation for considering what tidal forces can do to galaxies. Physicists understand tidal forces as "differential gravity." In part, this is a reference to the calculus technique used to calculate them, but the root word "difference" is also key. The Moon's gravity pulls more strongly on the side of C. Struck, Galaxy Collisions: Forging New Worlds from Cosmic Crashes, Springer Praxis Books, DOI 10.1007/978-0-387-85371-03, © Springer Science+Business Media, LLC 2011

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Figure 3.1 Schematic of the directions and relative magnitude of tidal forces acting on a spherical astronomical body (seen in cross section) (derived from the Wikipedia article on Tidal Forces, Wikipedia Commons, in accord with the GNU Free Documentation License).

the Earth that faces it than it does on Earth's center, which is slightly farther away. The Moon pulls more strongly on the center than it does on the side of the Earth facing away from it, so in effect, the force difference is equivalent to the Moon pushing the far side away (relative to the center). When you combine the two effects the net result is that the Moon stretches the Earth along the line connecting their centers (Figure 3.1). At the points on the Earth's surface farthest from that center-line (at the top and bottom of Figure 3.1), the strength of the Moon's pull is about the same as that on the Earth's center. However, that force points in a slightly different direction from the one at the center. That force can be broken in two parts, one parallel to the force on the center, and a smaller part toward the center. With a slight extension of this reasoning we see that the tidal force compresses or squeezes the Earth in all directions perpendicular to the line connecting the Earth to the Moon. Thus, we have the two key facts about tidal forces applicable in all situations - they stretch the bodies along the line connecting the centers, and compress them in the other two directions. The Earth is not absolutely rigid, so tidal forces pull out a very slight bulge in the direction of the Moon, but on both the sides facing toward and away from the Moon. Actually, because the Earth rotates on its axis more rapidly than the Moon orbits the Earth, the tidal bulge is moved off the line of centers. When this happens a small component of the lunar gravitational force works to pull the bulge back in line, which has the effect of very gradually slowing down the Earth's rotation. This process has already gone to completion for the Moon, with the result that it turns once on its axis for each revolution around the Earth, and

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we only see one face of the Moon. Sometime in the far distant future the Moon will only be visible from one hemisphere of the Earth. This so-called synchronous rotation results from the action of tidal forces over millions of orbits. Over the age of the universe, two galaxies do not orbit each other more than a few tens of times, and generally much less, so this phenomenon is not relevant to the world of galaxies. The world of close binary stars lies in between that of planets and galaxies in spatial and temporal scales. Synchronous rotation is often achieved in such systems. So too is another effect not seen in the Earth-Moon system because those bodies are relatively compact and dense, and with a large separation compared to their sizes. This is not the case for the stars in many close binary systems. Stars are generally larger, more diffuse bodies than planets to begin with, but when they evolve to the giant stage their outer layers expand to hundreds of times their initial (main sequence) size. The differential tidal force across such a giant star can be comparable to the gravitational force of the star holding on to its outermost layers, leading to a phenomenon called Roche lobe overflow from the giant to its companion. We can understand the consequences of the situation better if we try to understand the motion of a small satellite orbiting around the two stars in a close binary system. Three forces drive the satellite's motion: the gravitational forces of the two stars, and the centrifugal force associated with its orbit around them. In the case of a satellite orbiting a single star, we can visualize the gravitational force field around the star as being like a funnel with the star stuck at the bottom. The orbiting satellite is like a coin or ball rolling around the top part of the funnel, except such a coin would experience friction and fall down the funnel. An orbiting satellite experiences no such friction. This funnel model is also similar to the large plastic yellow funnel often found in museums, which has a track that sets a coin rolling into the funnel in such a way that it orbits, and slowly spirals down into a collection bin (Figure 3.2). Notice that in this funnel model we have made a simplification; we have assumed that the funnel (or the single star) was not moving. The only motion was that of the orbiting satellite. We could set the museum funnel device moving across a relatively frictionless surface of ice, and arrange for the coin to drop after the motion was started, and while moving with the funnel. The motion of the coin should not be any different. Similarly we don't expect that the joint motion of the satellite and the star affects the orbit of the former. In physics, this simplification is codified as Newton's first law. We have also made another important simplification. A plane containing the circle at the top of the funnel is like the orbital plane of the satellite. The funnel slopes downward, but the steepness of the slope at any distance from the center represents the strength of the gravitational acceleration at a corresponding distance from the star. Satellites may orbit the star in other planes, tilted in different directions. Any one funnel model in our three dimensional world can only represent the two dimensions of one orbital plane plus the strength of the gravity, but not the extra dimensions of all possible satellite orbits.

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Figure 3.2 The so-called "gravity well f u n n e l , " found in many museums illustrates well the effect of a gravitational potential field on an orbiting body (Jimmy Dorff, Duke Univ.).

In the case of the satellite in a binary star system, a similar model can be constructed, though it has to account for the effects of two sources of gravity. We again have to start with a couple of simplifications. The first is the same as before, and since the overall motion of the system does not matter, we will adopt a frame of reference in which its center of mass is at rest. The second simplification is a little more subtle. In addition to a potentially very complex satellite orbit, the two stars are also orbiting each other. Imagine a double funnel device spinning on ice with coins dropping into it. The simplification is going into a reference frame that rotates with the two stars, since in that frame their motion is frozen. The coordinate axes of this frame are constantly turning, and thus changing direction, so it is an accelerated reference frame. This is true in the formal physics definition of acceleration, even though the reference frame is not gaining speed, as suggested in the common usage of this term. According to Newton's first law, accelerated reference frames are not as natural as frames at rest or in uniform motion. This unnaturalness can be seen in the equations of motion for the satellite in the rotating reference frame. They contain extra pseudo-force terms, which are not found in the corresponding equations in non-rotating frames. One of these terms is the so-called Coriolis force.

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Figure 3.3 The restricted 3-body effective potential diagram, in three-dimensional or contour diagram form, is analogous to the model of the previous figure, except for a relatively low-mass object orbiting the force field of two high-mass objects. Another difference, explained in the text is that this is the "effective potential" in a reference frame rotating with the line connecting the two massive bodies (derived from the Wikipedia article on Roche lobes, in accord with the GNU Free Documentation License, original source: http://hemel.waarneman.com/lnformatie/Sterren/ hoofstuk6.html#h6.2, author Marc van der Sluys).

Nonetheless, in this rotating frame we can define what is called an effective potential. This is a mathematical function that describes a surface, which in this case is the equivalent of the funnel in the case of a single star. (As in that case, we will confine our attention to a single orbital plane, here the orbital plane of the two stars, and use the third dimension to represent the strength of the effective force.) An example for the case of stars of equal mass is shown in Figure 3.3. Many intricate and beautiful orbits are possible on this type of surface. Their description fills many pages of books on celestial mechanics. We will limit ourselves to a few basic points. First of all, we do have individual funnels around each of the two stars. This means that satellites can still orbit either star, and be bound within that star's "sphere of influence." In that case the primary influence of the other star is to limit the range of distances over which satellite can orbit the first alone. Secondly, the funnels are warped into a kind of connection between the two stars. If a model of our effective potential was constructed of plastic, and we put it under the kitchen faucet, the funnels would fill with ever higher levels of water, until the water got high enough to fill the canal between the funnels.

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Thirdly, if we continued to run water into our plastic funnel, we would eventually get a nearly circular model lake with a couple of deep fishing holes. The circular shape of the outer boundary of this lake, reminds us that a satellite orbiting at a distance much greater than the separation of the two stars, effectively does not see that separation. Its orbit is like that of a satellite orbiting a single star with a mass equal to the two stars. The funnel model helps us to see and understand a great deal about the motions of satellites in the gravity field of two orbiting stars, which is a more complicated situation with stronger (and variable) gravitational forces. This model, and what we learn from it, takes us a good step towards understanding the gravitational dynamics of stars orbiting galaxies in collision. However, there are still more complications in galaxy collisions. The first is that, as we will see in later chapters, the centers of the two galaxies spiral into each other; they do not follow simple circular or elliptical orbits. This is one indication that the effective potential is more complicated, and in fact, changes with time. Moreover, the mass in galaxies is distributed, not concentrated at two points. The potential changes are effectively the result of the motion of many, many point masses, that is, the stars and dark matter particles in the galaxies. A two-funnel model in this case should be made of fabric rather than plastic to model complex stretches and folds. However, let us leave this model before we contort it too much, and get back to observations of galaxies, and later on to more realistic computer simulations.

3.2 Discovering t h e w h i r l p o o l spirals

Charles Messier was the first to record an observation of the great Whirlpool galaxy (Figure 3.4) on October 13, 1773. (I don't know if it was a Friday.) Messier was well-known as a comet hunter, and history records that he was observing one that evening when he apparently happened on a new nebula. As noted in Chapter 1, Messier is most famous for his catalog of just over 100 nebulae visible with small telescopes. He listed them as objects for comet hunters like himself to avoid, but many modern amateur astronomers in the northern hemisphere test their metal by trying to observe them all. The Whirlpool galaxy is object 51 on the list, or just M51 for short. Although he discovered the object, Messier never saw the whirlpool structure himself. He did note that it had two centers, each surrounded by an "atmosphere," and that the two atmospheres touched. The greatest discoverer of nebulae at that time was also the greatest astronomer of the day, William Herschel, mentioned in Chapter 1. A good part of Herschel's astronomical success was due to his great skill as a telescope maker. He had the largest and finest instruments, and knew what to do with them. Among his many accomplishments, Herschel mapped the structure of the Milky Way in far greater detail than anybody before him. He adopted a very uncommon view for the time, that many of the nebulae were greater collections of stars just like the Milky Way. He also had a theory for the evolution of these

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Figure 3.4 A very deep optical image of the Whirlpool galaxy, M 5 1 , which highlights the material flung out and scattered around the two galaxies by their extended interaction (Jon Christensen).

nebulae, involving birth, death, and rebirth. This theoretical outlook made him particularly eager to resolve the nebulae into stars, which he was able to do with many of Messier's nebulae, which we now recognize as star clusters in the Milky Way. He also thought he saw stars in M51 (in the 1780s) in addition to the nebulosity, but modern images suggest that his stars were unresolved clusters of young stars. The first to see the whirlpool structure, and coin the name, was William Parsons, the Third Earl of Rosse, in Ireland. Like Herschel before him, part of his astronomical success was the result of his telescope making. His "Leviathan" telescope had a 6-foot diameter mirror of speculum metal, truly remarkable for the time. With this instrument he could observe, sketch and later paint the beautiful spirals of the Whirlpool. From that time on it would be the prototype of spiral galaxies. New images of the great Whirlpool still have the power to capture our imagination. Recent image montages produced with the Hubble Space Telescope

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and the Spitzer Space Telescope rank among the most spectacular produced by those "factories" of iconic images. Like Lord Rosse's sketches they still beg many questions, especially, "what is the nature of the spirals and how are they produced?" In the hundred years or so from Lord Rosse in the mid-nineteenth century through James Jeans in the first half of the twentieth century there were many diverse theories regarding these questions. No theory stood on very firm ground until the generation of Shapley, Hubble and others found the distances to nearby galaxies, and confirmed that they were great stellar systems comparable to our own Milky Way. The history of M51 is wrapped up with that of the general question of the origin of spiral structures in galaxies, but the fact that one of its spirals leads up to a companion galaxy raises more questions. Could the companion be related to the spirals, or even the cause of them? If so, were there multiple causes or drivers of the spirals, some applicable to paired galaxies, and some to isolated galaxies? (We currently believe that is the case, as will become clear in later sections.) If not, were chance encounters able to generate spirals that persisted long after the encounter? These questions could not begin to be addressed until the work of Holmberg, and did not get very good answers until the early computer models, like those of Toomre and Toomre, as discussed in Chapter 1. Even these models did not answer all the questions, and modelers and other astronomers have worked to fill in the blanks in the time since then.

3.3 Into t h e t i d a l w h i r l p o o l : u n d e r s t a n d i n g the mechanisms

Holmberg, the Toomres, and other early modelers taught us that prograde collisions, with orbital planes nearly coincident with the disk plane of the primary galaxy, can turn that galaxy into an M51-type "whirlpool." Toomre's model images show this dynamic process in action (Figure 3.5), but it would also be worth taking a little time to get a better conceptual understanding of how it works. We can use the simple physical pictures that we started to develop in Section 3.1 to achieve this. Once again, we need a double funnel to represent the gravitational potentials of the two galaxies. However, in this case we cannot imagine the two funnels model as realized in a simple, rigid plastic setup. As mentioned above, the reason for this is that unlike the case of the double stars, two galaxies do not maintain the shape of their relative orbit. As we will see in later chapters, the galaxies are able to channel some of their orbital energy into internal energy, that is, the motions of stars or dark matter particles with the galaxies. As a result, their orbits decay, and they fall in towards the center of mass. Children bouncing in a circle on a mattress pretty quickly end up in a heap in the center, and the same is true for interacting galaxies. The bouncing children are a little too wild to allow for a discussion of some of

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-0.5

0

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Figure 3.5 An early computer model of the tidal production of the spiral arms in the M51 system by the Toomre brothers. The large, shaded dots represent the centers of the gravitational potentials of the two galaxies. Representative disk stars, initially on circular orbits the center of the primary galaxy, are show by open circles. Time is given in units of the orbital period of an outer disk star (from Alar and Juri Toomre, Astrophys. /., Vol. 178).

the details of this process, so let us switch to a more conventional metaphor heavy metal balls moving across a flexible rubber sheet. This metaphor is often used in discussions of the general relativistic warping of space-time by massive bodies. However, it is even more appropriate to the present discussion, because making such a model out of real materials would inevitably involve similar amounts of orbital dissipation as in the cases of galaxies or bouncing children. Moreover, this metaphor allows us to stretch our imaginations further. Imagine in the two funnels produced by the heavy balls that we have many very small 'BBs' circling the centers. These take the place of the circling coins in the plastic funnel, but we need to imagine that they do not spiral inward, at least on the timescales we are considering. These BBs represent the disk stars in the two galaxies. As our two metal balls reach the point of closest approach their funnelshaped depressions overlap, and many interesting things can happen. One of the first is that a BB orbiting one funnel can suddenly find itself deeper in the well of the second. Generally, it will not have sufficient angular momentum to maintain a circular orbit around the second at this distance, so

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it will start to fall toward it. In most cases, the BB will fall around the center, and to the opposite side of the second funnel, and thereafter continue on this in-andout orbit. The same thing happens when a companion galaxy flies past the primary disk galaxy in an orbital plane that is nearly the same as the plane of that disk. Note, for simplicity we can adopt the approximation we used with the ring galaxies, that is, assume that the companion galaxy consists of little more than the gravitational potential of its dark halo. Specifically, we neglect its disk components, for the present. What we have described with the BB is the gravitational capture of a star from the disk of the primary by the companion as it flies past the primary disk. To accomplish this theft, the companion must be nearly as massive as the primary, and the point of closest approach must be quite near the outer edge of the primary disk. A very rough estimate is that the ratio of the distance to the companion relative to the star's orbital radius around the primary must be comparable to the mass ratio of the companion to the primary. We have also not yet considered another important factor, that the encounter must be prograde. Even if our BB wants to fall into the second funnel, if that funnel moves past faster than the BB can begin its acceleration to it, the BB will remain in orbit around the first funnel. That orbit may be somewhat disturbed from its original shape, but it will be preserved nonetheless. Thus, to capture the BB the second funnel must not only provide a steeper slope, it must maintain that slope long enough for the BB to fall into it. This requires that the second funnel have an orbital velocity relative to the primary center that is about the same as that of the BB. More simply, the BB and the second funnel have to orbit together long enough for the BB to fall in. The same is true for the disk star attracted by the gravity of the companion galaxy. Generally, they have to orbit together in the same direction around the primary (prograde) to accomplish the capture; retrograde encounters occur too quickly. What else can happen on the rubber sheet with our moving funnels and BBs? When the second funnel moves through its point of closest approach to the first, one of the BBs might be located exactly at the balance point between the two funnels. In this case, the downwards slope towards either funnel center is the same, and the curvature goes to zero (flat!) at the location of this particular BB. Other BBs on one side of the special one will be pulled toward the second funnel, and the ones closest to it may be captured as we have seen. The others will have their orbits altered such that they start to fall towards the second funnel while briefly finding themselves on a steeper slope towards it. However, as the second funnel moves away, the slope toward the first will reassert its dominance and these BBs will fall back towards it. Overall, this is a good qualitative model for the tidal bridge that forms between two galaxies involved in the kind of prograde collision we are considering here. There is indeed a balance point such that disk stars closer to the primary remain securely bound to the primary. Stars well outside the balance point may be lost to the primary. Stars slightly outside the balance point temporarily feel a stronger attraction to the companion, but like most romantic

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affairs, it doesn't last. Some of these stars may be pulled a substantial distance from the primary, but they will fall back, and thereafter remain on very noncircular orbits. Stars interior to the balance point experience more of a perturbation of their orbits than the BBs in our rubber sheet model, because the gravitational pull of the companion is a long range force that cannot be fully represented by that model. We will have to consider their fate a little more carefully. An even more substantial failing of our rubber sheet model is the fact that the two heavy metal balls that form the two funnels are not significantly attracted to each other unless they get very close, in which case they promptly fall together. This is not the case with two interacting galaxies, which certainly feel each other's longrange gravitational attractions, and have their orbits continuously modified in response. These effects are severe enough to explain the induced spiral waves of M51 and the tidal tails opposite the companion bridge more evident in other systems. So, at this point we must abandon the convenient visualizations provided by the model, and stick to the galaxies. When we considered double stars and planets, we could sum up the immediate effect of tides with the statement that they stretched along the line connecting the bodies and compressed in the two perpendicular directions. Galaxy disks are different. There are virtually no pressure forces as in stars, or solid body forces as in planets, both of which constrain the ability of material elements to move very far from their neighbors, and mix with more distant elements. This is not the case for stars orbiting independently in galaxy disks. As a result, they respond very differently to the forces of the collision partner. When disk stars near the line connecting the centers of the two galaxies are pulled radially outward by the tidal forces, a temporary stretching of the disk in that direction may result. However, from the point of view of a single star, the disturbance is much like that in the case of ring galaxies. A radial disturbance (outward, unlike the inward pull in ring making collisions) leads to an epicyclic motion around the initially nearly circular orbit. Yet, only a narrow sector of the disk is affected in this way. With the passage of time, stars at different radii in this sector find themselves in very different parts (azimuths) of the disk as a result of the shearing; that is, the very different angular velocities at different radii. As a result, these stars generate no long-term coherent response. Roughly speaking, they mix back into the disk. Disk stars located well off the line of centers will be pulled inward to some degree (like the compression in the case of planetary tides). This will generate an epicyclic oscillation in stellar orbits just like that of ring galaxies, but as in the case of the stretching along the line-of-centers, this will lead to only transient effects. Far more important is the fact that a component of the companion's gravity will either pull stars located away from the line of centers ahead in their orbits, or drag them back against their orbital motion. In the first case, the orbital angular momentum of the stars is increased, in the second it is decreased. As a result, the size or mean radius of the orbits is increased in the first case and decreased in the second.

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If we assume that the gravitational pull of the companion on the disk stars operates for a short time (near the point of closest approach), then the velocity changes we have been discussing will be small impulses. Figure 3.1 in Section 3.1, though derived in a different context, also provides a schematic view of the direction and relative magnitude of the radial and azimuthal components of these impulses. On the other hand, I have already mentioned that one of the important facets of this type of collision is that near close approach the angular motion of the companion approximates that of a significant fraction of the disk stars. Therefore, the companion's pull on these stars is prolonged, not brief, at least over some range of radii in the disk. The result is generally an enhancement of the effects just described, not a significant change in their character. This means, for example, that the stars that are pulled ahead in their orbits can be effectively flung out to much larger radii than their initial orbit. When they reach their outermost radial distance (apocenter), they slow down. Neighboring stars pile up, forming a sharp density caustic, like the ring waves. Then they fall back, crossing the orbits of other stars still moving out to apocenter, again like the orbit crossing zone of ring waves. The result is a tidal tail, a roughly spiral structure, which often extends far beyond the original disk radius. To a large degree, this tidal tail is a material structure. That is, it largely consists of an ensemble of stars moving together on similar orbits, or orbits whose characteristics vary slowly with separation between the stars. Deeper within the original disk the tidal impulses are not so great and the effects are more modest. The primary result, a two-armed, spiral wave structure, is the same. However, these waves are probably not material waves, but dominantly phase waves that move through the gas and stars like the ring waves. Also, like the ring waves, they are probably transient, rather than steady waves. They are the combined result of the relatively coherent initial tidal disturbance, and self-gravitational effects within the inner disk. This discussion would seem to provide a way to distinguish between tidal tails and spirals, even though the spirals often extend continuously into bridges and tails. The distinction would be whether a given wave segment is a dominantly material or phase wave. This could be determined in numerical models, but is hard to decipher in the observation. A rough estimate is given by whether the segment seems to be outside or inside a still recognizable disk. Stars outside the nominal disk must have been materially thrown out there. However, given the vagueness of the term "recognizable" in a galaxy after a significant collision, this is a very rough guide!

3.4 The f o a m i n g waves

As we begin to understand the basic mechanisms of tidal bridge, spiral and tail formation, we learn a great deal about the morphology of colliding galaxies, at least in the early stages. One of the most obvious morphological characteristics

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of these structures is that they stand out because of the bright knots of star formation that often illuminate them. The ring galaxies have taught us a fundamental principle that we can hope to apply and test in these cases - waves compress the gas, building larger clouds and enhancing the gravitational forces that collapse them. To reverse the argument, the prominent star formation in these other tidal features suggests that they are also capable of gathering and compressing quantities of gas. The discussion above of the effects of tidal forces supports this notion, in a general way. That discussion was able to provide a plausible account of the mechanisms for the formation of the various tidal structures, but it cannot provide much information about the later evolution of those structures. A more complete understanding would include answers to questions such as: how and how much is the gas compressed in these waves; when is star formation enhanced within them; and how do the answers to these questions depend on the pre-collision properties of the galaxies and on the collision parameters? In some cases we are just beginning to understand some of the details of this later evolution, while in others we still know relatively little. The nature of spiral waves in galaxies has been studied intensely for at least 40 years, so we might expect that it would be easiest to answer these questions for the case of tidal spirals, rather than the bridges and tails. Even though that expectation has not been entirely realized, in this section we will focus attention on the spirals. Early studies of waves in galaxy disks emphasized that spiral forms were natural because of the strong shear in these disks. Recall that in talking about ring waves I used the old analogy of dropping a pebble in a pond and watching the ring waves propagate outwards. In the case of a shearing disk, it is more like dropping a stone into a whirlpool. If we overlook the fact that most whirlpools are turbulent, so we probably would not see what happened, we can imagine the propagation of the resulting wave. The part that moves inward encounters more rapidly moving water, and so is pushed ahead of the rest. The opposite is true of the part of the wave that moves outward. The result of this shearing is that the wave is stretched out into a spiral form. This same wave shearing process is also an important part of the explanation for the narrow ringlets in Saturn's rings. In that case, they are wrapped very tightly. In galaxies, the shear is not so strong, and the waves are rather lazily wrapped, which makes for a very beautiful appearance. While the appearance of many spiral waves in galaxies is quite open, the mathematical theory tells us that this is not permanent. These waves too will wind up, and they will do so around special resonant radii. These are the radii of so-called Lindblad resonances, where the period of the nearly circular orbits divided by the period of epicyclic motion is a whole number or simple fraction. You may remember we encountered epicycles as a representation of the radial motion of stars in ring galaxies. The orbits of stars in spiral density wave galaxies can also be approximated with an epicyclic component, because the waves perturb original circular orbits. In the case of a perfect ring galaxy, all the epicycles at a given radius are identical, regardless of their azimuthal position. In

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spiral disks, the epicycles at any radius have different "phases" at different azimuths. That is, they are out of sync, with stars at one point along a circle in the disk at their outermost radial excursion, while stars at a different point on the circle are at their innermost. To summarize, the orbits of stars in spiral wave galaxies have an epicyclic component, and at special radii this component is in resonance with the basic circular motion around the galaxy. The spiral waves have enhanced interactions with these resonant radii. In particular, the waves can wrap around them, like a fishing line around a reel, on a timescale comparable to a few orbital times or about one Gyr. The upshot of this discussion is that we do not expect tidally induced waves to last for more than a few times around the galaxy disk. Somewhat coincidentally, this is similar to the lifetime for ring waves before they disappear as a result of spreading and overlap. The discussion would also lead us to believe that since the waves are shearing and stretching, they should be getting weaker over this period, at least in the absence of some source of reinvigoration. However, this is not entirely true. There is a source of partial regeneration and prolonged strength for spiral waves. In the right circumstances, the shape and the propagation speed of the spiral wave may be such that the orbital circulation and epicyclic motion of a star at the wave crest match those of the wave such that the star remains on the wave crest. The star essentially surfs the wave. When this happens, there is an important consequence. The persistence of the star and its neighbors in the wave for a significant fraction of an orbital period implies that at any time a significant fraction of all the disk stars are near the peak of the wave. This in itself tells us that the wave is strong, but more than that, it suggests that the wave contains enough material to have a strong self-gravity to hold it together. Such gravitational cohesiveness further increases its strength. This positive feedback effect was discovered by the ubiquitous Alar Toomre and his student Thomas Zang. They named it "swing amplification." This effect may seem to be based on a coincidence, the "right circumstances" mentioned at the beginning of the last paragraph. That impression is not correct. The characteristics of the spiral wave are determined largely by the gravitational potential, and so is the epicyclic period of our surfing star. I will not go into the details, but merely use this fact to argue that swing amplification is not the result of an unlikely coincidence. Rather it is a reasonably likely outcome, which strengthens the spiral tidal wave. Our surfing star was set up for a very good run. A swing amplified wave will be able to compress the interstellar gas more strongly, helping us understand the vigorous formation of young star clusters like those in the spirals of M51. That point completes our discussion of the basic theory of prograde collisions and the waves they induce - the tidal twists of the section title. It does not even come close to covering the full story of Nature's dynamical creativity in these collisions. It is worth briefly considering a few of her elaborations before we go on to other topics.

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Figure 3.6 The Hubble Heritage Project image of the interacting system NGC 2207/IC 21 63 ("Owl Eyes"). This image is printed has south at the top in contrast to most astronomical images, which have north at the top. Computer models suggest that the galaxy on the has swung about 1 80° under and around the spiral. (NASA, ESA, and the Hubble Heritage Team (STScl/AURA), D. M. Elmegreen).

3.5 The eye galaxies

In Tolkien's Lord of the Rings fantasy, the dark lord Sauron is described as having the appearance of a great, disembodied, all-searching eye. There is also a class of galaxies, the oculars, with this appearance, though as far as astronomers know, they are not the source of any evil emanations. In fact, the ones that have been observed at high resolution are very beautiful; an example is shown in Figure 3.6. The ocular is the galaxy on the right hand side. Note the pinched oval, eyelid shape that distinguishes the class. Short tidal tails, like the one on the right in the figure, are also characteristic. This galaxy, IC 2163, is the most famous of the oculars, though part of what makes this image so striking is its equally beautiful spiral companion, NGC 2207. Because of the combination of a bright yellow central bulge and the extended, but lower surface brightness, spiral disk, NGC 2207 also has an eyelike appearance. It is not an ocular, however, since it lacks the oval ring, with arm segments coming together at nearly right angles at the ends of the oval. Most oculars have a close companion. Unlike Sauron they are not generally cyclopean. Unlike ring galaxies and M51 whirlpools, they were discovered on the computer at about the same time that they were recognized on the sky, and much more recently than the other classes. The paper that first described them as a distinct class and explained their origin was published in 1991 by the American husband and wife team Debra and Bruce Elmegreen and Swedish astronomers Maria Sundin and Bjorn Sundelius. This team spotted the characteristic shape in

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previously published papers of interacting galaxies, and studied them with more computer models. They also examined many survey images of nearby galaxies, and identified several dozen, candidate examples of oculars. In subsequent years, the NGC 2207/IC 2163 pairing has been studied in detail in all major wavebands, largely by a team including the Elmegreens, myself, and several other astronomers. Figure 3.6 is derived from Hubble Space Telescope observations obtained by this team and enhanced by the Hubble Heritage team of the Space Telescope Science Institute. In the original paper, the authors made a compelling case that oculars result from a prograde encounter, similar to the kind that produced M51. Without going very far into the dynamical details, I will just note a couple of general points about how the "eye" is made. At the beginning of this chapter I described how tidal forces acting on a body like a planet or a star, stretch that body along the line connecting to the body responsible for the forces, and squeeze or compress in the perpendicular directions. Later, in discussing the formation of tidal tails, we saw how this story is modified somewhat when tidal forces act on a disk of orbiting stars. Some stars are pulled ahead in their orbits, and slung outward in the tail. While some gain, others lose, and this is also true for the exchanges of angular momentum in flyby galaxy collisions. The stars that gain angular momentum fly outward, but those that are pulled back in their orbits, lose angular momentum and sink inwards. From here on the story for these stars is much the same as for ring galaxies. The inward motion of these stars overshoots their (new) equilibrium orbital radius, so they pile up in a caustic edge at an innermost radius. However, only a minority of the disk stars (and gas clouds) is involved in this "ringing," so a symmetric ring is not the result. In some cases, the caustics are only limited segments. In others, in combination with the complex motions of the rest of the disk stars, we get the ocular wave. Computer models show that oculars are phase waves; they propagate through the disk like the waves of ring galaxies. Also like symmetric ring waves, a second wave can form and propagate through the disk. Thus, they are a kind of ring wave produced in strong fly-by collisions, which are more famous for generating strong tidal tails and spiral waves like M51. Like many details of the early stages of galaxy collisions it is not yet entirely understood what determines when one waveform is dominant. It certainly depends on the mass distribution in the galaxy before the collision (and its gravitational potential), as well as the orbital trajectory through the collision. However, the oculars provide a nice example of how we can apply some of the simple dynamical ideas developed above to more complex structures produced in galaxy collisions.

3.6 Making bars

The oval form of the oculars is a step towards a more drastic and important transformation, which can be induced by galaxy collisions - the development of

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a bar component in a galaxy disk. In several papers in the late 1980s, Masafumi Noguchi was the first to investigate with computer models how the tidal distortions of prograde collisions can induce the formation of a bar component in a galaxy. I discussed barred galaxies briefly in Chapter 1, as a component of galaxy classification schemes. Before we can discuss the induced formation of a bar we should first consider in more detail just what a bar component of a galaxy is. For comparison, we remind ourselves that a galaxy disk is shaped like a thin pancake, made up of stars and gas clouds following approximately circular orbits. More technically, our pancake is an example of an oblate spheroid. This is a three dimensional figure which has a shortest axis along which the body is thinnest, and it extends out to the same distance along any two axes perpendicular to the shortest. The bulge component of galaxies is simpler; it is often nearly spherical (equal extent along all axes), and made up primarily of stars following random orbits. A bar is different from both these components. In shape it may be prolate, or triaxial. A prolate body is cigar-shaped, with a long extent along one axis, and two equal, short extents along perpendicular axes. Triaxial means that the two short extents are not exactly equal. A cigar-shaped component could conceivably point in any direction relative to the disk axis, but real bars are virtually always found in the plane of the disk. That is, their longest axis is embedded in the disk, and so, perpendicular to the disk axis. However, the evidence suggests that the shortest extent of most bars is greater than the disk thickness. In this way, bars are like bulges, though they stick to the disk plane. The stars in bars have a larger random velocity component than disk stars. Nonetheless, like disk stars, the primary part of their motion is circulation around the galaxy center. To remain in the bar, this rotation must occur on average in flattened elliptical orbits, not the circular orbits of the disk. Like the disk and the bulge, the self-gravity of the bar must be great enough to hold it together as a distinct component. Noguchi's early work indicated that the bar-like, tidal stretches and squeezes a disk experiences when a companion flies by in the prograde sense can lead to the formation of a bar with enough self-gravity to hold itself together for some time (Figure 3.7). I use the rather weak verb "indicated" here because there were a number of limitations in these early models. Firstly, they were not run for an especially long time, so although the bars appeared to hold together; it was not clear for how long. Secondly, the number of particles representing stars in the model was modest, which always makes one worry about the accuracy of the models. However, later, better-resolved models did find long-lived bars were produced in such collisions. Yet Noguchi's results raised a great many questions, a number of which were addressed in the following decade. In the 1990s, Noguchi undertook the study of differences between bars formed intrinsically, not as a result of a collision, and bars formed extrinsically as a result of a collision. By that time, it had long been known that disks with high surface densities, implying relatively high self-gravity, were subject to a strong barmaking instability. That is, models of high surface density disks, with matter

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Figure 3.7 Numerical model of the appearance and subsequent evolution of a bar component initiated by a galaxy collision (from E. Athanassoula, Lecture Notes in Physics, Vol. 474).

initially in circular orbits would spontaneously undergo a rapid, dynamic change to a state dominated by a large bar. If a galaxy just happens to have a highdensity disk, it can form an "intrinsic" bar. One way to acquire such a disk is through gradual external infall, e.g., of gas clouds from the halo, or from connected cosmological structures. Noguchi found that there were differences between intrinsic bars and the extrinsic bars formed in strong interactions. In his models, intrinsic bars had declining density profiles like the disks they were embedded in, while extrinsic bars had more nearly constant densities along their length. Intrinsic bars tended to rotate faster around the galaxy center than extrinsic bars. However, subsequent work has shown that intrinsic bars (which do not continue to be rapidly fed by infall) tend to slow down. Interestingly, he also found that the characteristics of bars formed in weak interactions were more

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like those of intrinsic bars, than bars formed in strong interactions. He concluded that in those cases, rather than driving bar formation, the collision forces merely pushed the disk over the edge of internal instability. In work done at about the same time, J. Chris Mihos and Lars Hernquist emphasized another important aspect of tidal bar formation - that it was much harder to induce bar formation in a galaxy with a substantial, compact bulge than in disks with weak bulges. This appears to contradict another of Noguchi's conclusions - that the bars in early-type galaxies are primarily produced in collisions. However, the structure of these bars is extrinsic, so they presumably were formed in disturbances strong enough to overcome the inhibitory effects of bulges. Unfortunately, since these works there has been little additional discussion of these related issues. For example, I am not aware of any results from large-scale models of galaxy formation about how bars form in different galaxy types. In fact, recent observational evidence suggests that bars are relatively insensitive to their environment, so their formation is generally intrinsic. Extrinsic bars may be rare. There is also recent observational evidence that the fraction of barred galaxies was less in the past (e.g., at redshifts of about 0.5), while collisions and mergers were probably more common in the past. This again suggests that most bars are intrinsic. Let me conclude this subsection by noting a couple more very cool effects of induced bars. The first is from a modeling study of about six years by Ingo Berentzen and a team of three other European and American astronomers. They did a series of numerical experiments in which a companion galaxy flies through the disk plane of a larger barred galaxy. In the various experiments, the companion impacted the disk at a variety of different points relative to the bar. They found strong interactions between the waves generated by the collision and the pre-existing bar, which is also a wave. One such interaction is the antithesis of the induced bar - pre-existing bars could be destroyed as a consequence of the interaction. A variety of other features could also be produced including rings, offset bars and asymmetric spirals. Bars can also be regenerated if there is not too much gaseous dissipation. To my mind, this is a fascinating set of dynamical phenomena, which has been little studied. A second important effect of induced bars is the fact that they can funnel a great deal of interstellar gas into the central regions of galaxies. All bars perturb the orbits of gas clouds and stars - in some cases generating converging orbits, which results in cloud collisions and energy losses in the gas. Angular momentum is also transferred outward, allowing the gas inflow. We will consider these processes further in a later chapter, and especially focus on the possibility that nuclear activity and starbursts can be generated by such inflows.

3.7 Probing t h e darkness w i t h tails and dwarfs

In considering the spirals, oculars, and bars, we have spent some time now with the interior effects of prograde collisions, though we certainly have not

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Figure 3.8 A sampler of optical images of tidal tails from the Arp Atlas (from Level 5 on the NASA Extragalactic Database).

exhausted the subject. Still, with many other aspects of the world of colliding galaxies to explore, we cannot linger. It is time to move from the torso to an examination of the limbs of these galaxies. The arms and legs of these galaxies are the tidal bridges and tails. These two structures are closely related, so for brevity, I will focus on tails. The first point about tails is that they come in a wide variety of shapes and sizes. Figure 3.8 shows a sampler of objects from The Arp Atlas of Peculiar Galaxies, chosen to illustrate this point. A good part of this variety is due to the effects of age. Since tails are made of material flung (or more correctly torqued) out of galaxy disks from a range of positions with a range of velocities, they tend to start out short and stubby. Given enough torque, they tend to end up long and thin. As in the disk they left behind, caustic waves or shocks can propagate through the tails. Although a lot of tails have been modeled on the computer, the regularities of wave compressions within them are not yet well understood definitely not as well understood as the waves in ring galaxies discussed above. Our understanding of the star formation induced within tails is also very limited,

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Figure 3.9 A computer model of the formation of the tail in IC 21 63 (see Figure 3.6). The gas particles in the initial disk are color-coded according to their radii (a rainbow disk), as can be seen at the moment of companion passage shown on the left. After the tail has developed, as shown at the right, the color-coding shows that particles at the outer edge of the tail originated in the outer disk, while those in the core of the tail came from deeper in the disk (figure by the author).

and I will have more to say about that in a later chapter. For now suffice it say that young star clusters can be found in tails of a wide range of ages, and the variety of those clusters is also interesting. Optical images of tails can be quite spectacular, but in some ways the best parts of tails are optically invisible. Because gas disks extend beyond the stellar disks in galaxies, and because models show that tails are, in essence, pinched out of disks (Figure 3.9), a large part of any tail is drawn from the outer gas disk. Thus, the outer parts of tails often consist almost entirely of gas (Figure 3.10). In many cases, you have not seen half of a tail until you have seen 21 cm radio observations of the gas. Tails can carry off a significant part of the spin or angular momentum of their parent disks, with consequences that we will discuss more in later chapters. Generally, it is very unlikely that the material in tails will escape its parent galaxy, or at least the merger remnant containing its parent galaxy. So what goes up comes back down, eventually, and this dog really does eat its tail. The timescale for material at the end of long tails to go out and return is very long, of order one to several Gyr. Therefore, one effect of tail formation is to steal gas, the fuel for star formation, out of a disk, and store it for a while before returning it. This galaxy scale hoarding seems at first to be a way to keep a galaxy disk from using its fuel relatively quickly, and to prolong star formation. However, it is a little more complicated than that. Most of the gas in the tail has come from the outer disk, where there was little star formation before the collision. It was already in storage. When it falls back onto the merger remnant, it will be falling into a stronger gravitational field than that of its parent galaxy, so we would

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Figure 3 . 1 0 The long tidal tails of the merger remnant NGC 7252 are detected via the emission of hydrogen gas, and are shown in blue. The optically visible galaxy is shown in green and yellow (). Hibbard, NRAO/AUI/NSF).

expect it to be pulled deeper in. Moreover, it may interact with gas clouds in the merger remnant, dissipating orbital energy. In any case, this gas is very likely to find itself compressed and star formation triggered within in it. In the end, gas that might have lingered undisturbed in the outer disk of a galaxy for another age of the universe, is pulled out, distorted, and then throw back into the merger, with induced star formation all along the way. The inner parts of tails fall back into the parent disk most quickly, and some tails never become very long. Much of the reason for short tails has to do with the specific parameters of the collision. Why doesn't material at the end of tails escape the parent galaxy? The main reason is that the dark halo is so large that this material does not get enough of a momentum boost to escape it. This fact can be turned around in several different ways. Firstly, the persistence of tails can be taken as more evidence for large dark matter halos. If they were not as extensive as they are, the tail material would drift away, and we would see few tails. In truth, this is a quantitative question that requires a statistical analysis of the frequency of tails (and possibly the run of velocity along them) for firm conclusions. To my knowledge, no such detailed analysis has been carried out. Numerical modelers John Dubinski, J. Chris Mihos and Lars Hernquist did a related analysis in the mid-1990s. At that time, there were suggestions that dark matter halos might be even more massive and extensive than presently believed, e.g., halo to visible matter ratios of 30-to-l, rather than the more standard 10-to-l.

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This group produced models showing that in such cases stars and gas clouds could not be flung out to great distances in prograde encounters. All the dogs would have stubby or clipped tails! And sometimes they would have a "bow" on the end. The bow is what has come to be called a tidal dwarf galaxy or TDG. In several ways TDGs are the strangest galaxies in the universe. They are indeed very small galaxies, but that does not make them unusual. If the most widely accepted model of galaxy formation is correct, all galaxies started off as dwarfs or as collections of dwarfs. Most of the dwarf companions to the Milky Way may be remnants of such early dwarfs. Like the majority of galaxies, these dwarfs formed after primordial gas fell into a dark matter halo that formed still earlier. In the gravity well of these halos the gas was compressed and formed the first generation of stars. The known TDGs did not form at the beginning of the universe; they appear to be forming now. The evidence suggests that they are just about the only class of galaxy that is forming in the present-day universe. Moreover, they do not form at the bottom of a pre-existing gravity well, but rather out on the end of a tidal tail. That is, in material that has been thrown as far out of the central gravity well as the interacting galaxy can manage. The modern study of these objects began in the late 1970s with the discovery of one by Francois Schweizer in the Antennae system. (F. Zwicky conjectured the existence of such objects in the 1950s, but his work was not followed up.) Schweizer carried out an especially long photographic exposure on the 4m telescope in Chile to make the discovery. Schweizer described his discovery as "a patch of luminous material near the tip of the southern tail." He compared its luminosity to a dwarf companion of the Milky Way, but noted its much greater size, and did not argue for its coherence as a single object. He did point out that the optical colors measured in the tails were similar to those in the disks, a fact in support of the Toomres' tidal model for the origin of tails. A decade and a half later, I. Felix Mirabel, Horacio Dottori and Dieter Lutz obtained new optical data that supported Schweizer's contention that this structure had many of the characteristics of a dwarf galaxy. They also used Zwicky's term "tidal dwarf," and since their paper received much attention, the term was brought back into circulation. Moreover, their emission line observations revealed that the region contained a large number of massive young stars. This was not just a typical piece of a disk pulling itself together; it was a young galaxy in the sense that it appeared to be forming the bulk of its stars at the present time. In the last few years, new studies of the Antennae's southern dwarf have been carried out using HST images and other data. These images are beautiful (Figure 3.11), and tell us some interesting things at a glance. One is that this is not a very well organized structure; the words lumpy and chaotic come to mind. It appears that star formation has taken place independently in a number of different locations. Whether they will eventually pull together into a more coherent dwarf galaxy is unknown. The Antennae system is the closest major merger, and

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Figure 3.11 Hubble Space Telescope images of the star formation regions in the tidal dwarf galaxy at the end of the southern tail Antennae system. The inset shows a larger field containing the main image (from I. Saviane, et al., Astrophys. /., Vol. 678).

because of this proximity, at least some individual stars can be resolved in the HST images. In most colliding galaxies we are lucky if we can resolve many individual star clusters. Analysis of the HST data by Ivo Saviane, John E. Hibbard and R. Michael Rich has shown two distinct populations of stars. The fainter and more diffuse of the two is an older population of stars born before the collision and carried out into the tail like the gas clouds. The young population has an age range extending from 2 to 100 million years. This rather large age range is very interesting and we will discuss it further below. As interesting as the Antennae TDG is, I believe that its unique circumstances discouraged the search for and study of TDGs for a time. This intrinsically faint object was discovered in part because the system is so close. Others did not

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appear readily in images of the ends of similar tails. I suspect that the psychological effect of this was an impression that such objects were impossibly rare. Even if they were not, it was still clear that comparable objects would be very difficult to detect at the distances of most known tails. To continue out on a limb of conjecture, I wonder if this situation might have continued for some time if the modelers had not intervened. Unique or not, the Antennae dwarf required some explanation. Moreover, it became evident from the available imagery that star formation in tails was not entirely unique to the Antennae - a number of others were blue and clumpy. Thus, there was a general process to be understood, and the Antennae dwarf might simply be an extreme example of the process. Within a year or so of Mirabel et al.'s paper, two independent groups had published two modeling results of this process: Joshua Barnes and Lars Hernquist used their merger simulations to explore this problem, while Bruce G. Elmegreen, Michele Kaufman, and Magnus Thomasson extended their work on ocular galaxies. The two groups came to somewhat different conclusions about how star formation occurred in tidal tails. The details are not important here. What is more important is the common result that star clusters and tidal dwarfs could indeed form without any great difficulty in tails. The minor controversy stemming from the different mechanisms also generated interest in the phenomenon. Hence, in the mid-1990s, a number of observing groups studied examples of possible dwarf formation in tidal structures. Felix Mirabel and collaborators continued to work in this area too, and stimulated the interest of a young colleague, Pierre-Alain Due of the CEA-Saclay Institute in France. Due began his work with studies of the literally far-flung dwarf in the Arp 105 system. This beautiful system (Figure 3.12) has been described as a guitar or fiddle, with the prominent dwarf at the end of the "neck." (There are other intriguing young star knots at the opposite end, the base of the instrument.) This system has been extensively studied since that time, and has become another prototype, and at a more typical distance for interacting systems than the Antennae. P.-A. Due also became something of a prototype, that is, a leader in this subfield, often summarizing the status of our knowledge in many international conferences over the intervening years. About half a dozen years ago, he too began mentoring a young colleague, who has also become a leading light in the area, Frederic Bournaud. Together, and with other collaborators, they carried out several studies that have taught us some important lessons about TDGs. A point of contention in the earlier models concerned the role of dissipation in facilitating gravitational instabilities in the tails, which could draw material together in clumps within the tails. Bournaud and Due argued that while this was all very interesting, the really fascinating phenomenon is the development of large objects at the end of tails, as in the Antennae and Arp 105. Based on a large collection of models that they computed, they concluded that because of the tidal stretching it was not possible to form such large objects along the length of a tail. However, such objects could form at the end of tails contained within a large dark halo, because the stretching would be halted there. Basically, gas from

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Figure 3.12 False color optical image of Arp 105, the Guitar system, with the tidal dwarf galaxy at the top (from P.-A. Due and I.F. Mirabel, Astron. & Astrophys., Vol. 289).

the outer parts of a disk would push as far out in the halo as it could given its initial launch velocity, but then it would be stopped. However, it takes some time for that material to slow to a complete halt and then turn and fall back in. During that time, more gas clouds are flying out to about the same point, stopping the infall of the first clouds, and piling up. The massive TDGs form out of this pileup (e.g., masses of order a hundred million to a few billion times the mass of the Sun). Initially self-gravity is not very important in the pileup, but later when the pileup becomes big enough, self-gravity pulls it together. Dissipation is not needed to trigger gravitational fragmentation, though it inevitably plays a large role in dissipating the heat of the pileup process. Observations had indicated that a tidal tail either forms a large TDG at the

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Figure 3.13 The star-forming region called Ambartsumian's knot in the Arp 105, Guitar system is visible as a blue clump at the base of the galaxies (Sloan Digital Sky Survey (SDSS)).

end, or has many star clusters along its length, but not both. Bournaud and Due's models explained the former as the result of pileup in an extensive dark halo. They explained the latter as a possible outcome when stretching dominates over pileup in less extensive halos. Recent observations suggest that the dichotomy is not strict, and that there are intermediate cases with elements of both phenomena, but that does not contradict the model results. The pileup theory is attractive not only for explaining end-of-tail dwarfs, but because it can be extended to other situations as well. One example is material that falls along a bridge from one collision partner to its companion, and then swings around the companion and piles u p on the opposite side. My

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collaborators and I have been studying two examples of this phenomenon some very dusty knots in the Arp 285 system, and "Ambartsumian's knot" on the south end of the Guitar, Arp 105 (Figure 3.13). In both cases, Bournaud and Due's process of tidal dwarfs pulling themselves together seems not to have gone to completion, especially in Arp 285. Another environment where it seems possible to get a pileup is near the balance point in the bridge between two galaxies as they pull apart. The Arp 305 system appears to have a beautiful example (Figure 3.14), as well as interesting young clusters in many other parts of the system. "Dwarf 3 " in the very southern

Figure 3.14 Ultraviolet image of the Arp 305 system from the GALEX satellite telescope. GALEX ultraviolet images are very sensitive to young star clusters. Note the many blue clusters in the tail in the lower left, and the bright ones between the galaxies. Observations of hydrogen gas show that the latter are embedded in a gas bridge between the galaxies (Beverly ). Smith).

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NGC 6221/6215 system, studied by John Dickey and Barbel Koribalski in Australia, may be another example. All of the models for the formation of TDGs that we have considered suggest that they have one property that stands in stark contrast to dwarf galaxies formed in the early universe - they should contain very little dark matter. A number of the dwarf satellites of the Milky Way have been found to be relatively rich in dark matter (that is, per mass of ordinary matter). Evidently they are primordial dwarfs. For the TDGs the explanation is that they form in the outer parts of the dark halo of their parent galaxy, where the dark matter density is low. In any case, the characteristic velocities of dark matter particles in the parent halo are greater than the escape velocities of TDGs, so the dark matter particles could not usually be captured by the TDG. Because of this, it was very surprising when a few years ago the French group discovered tidal dwarfs in the NGC 5291 system with internal velocities that seem to suggest the presence of considerable masses of dark matter. At present this is a very mysterious result. It needs confirmation in the form of similar discoveries in other systems. One possible explanation suggested by the discoverers is that there is a second kind of dark matter, whose particles are concentrated around the parent disk, and have orbits more like disk stars than halo objects. In that case, it could also be flung out in a tidal tail and captured in a forming TDG. Others have suggested that the origin of disk dark matter might be in primordial dwarf galaxies captured and disrupted by the parent galaxy. In this case, the second kind of dark matter might consist of the same kind of material as that of the parent halo, but differing only in orbit structure. This is all very speculative at present, but it conjures the fairy-tale image of dark matter, like gold coins, stolen from dwarfs, only to be flung away in a violent swirl and be recaptured by a young dwarf forming within the swirl. This is a strange corner of the world of galaxies! With regard to TDGs in general, one wonders how strange they are or, more prosaically, how common are they? The early answer was very, very rare, but now we have more information. Frederic Bournaud and others have argued that it is possible that TDGs actually make up a significant part of the dwarf galaxy population. Since it appears that they can be made in a variety of ways in a range of types of collision, it is possible that they were made at some time around many galaxies. Then the question is one of survival. In most cases, including when they form at the ends of tails, they will fall back, at least to a very close distance to the parent galaxy or merger remnant, if not directly collide. Depending on their compactness, i.e., how well they have pulled themselves together (and whether they have much dark matter), they may be tidally disrupted. Like many questions about the structure and evolution of TDGs this question cannot be answered at present. Many more systems must be studied in detail before we can really understand these objects and their population character­ istics.

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3.8 Retrograde encounters: dancing backwards

The bulk of this chapter, up to the present section, has focused on the outcome of prograde encounters between galaxies. In this section, I will say much less about retrograde encounters, i.e., collisions where the companion flies by a primary in the opposite sense to its disk rotation. This coverage ratio echoes that of the technical literature, where retrogrades have been relatively ignored, but there is a reason for that. The reason is that retrograde collisions have a relatively small effect on the primary disk. In prograde collisions, a disk star rotates in the same sense as the companion, so although their angular velocities may not exactly match, they do fly together for a relatively long time. The result, as discussed above, is that the companion has a strong effect on disk stars and clouds. The opposite is true in retrograde encounters. Collisions with little effect do not excite much interest among astronomers. However, though retrogrades are not as dramatic as progrades, they can give rise to some interesting, even subtle effects. In contrast to progrades, retrograde disturbances tend to be one-sided. Common forms include a broad, fan-shaped distortion of the disk on one side. They frequently have a single spiral arm, and this is often embedded within the fan distortion. The Arp Atlas of Peculiar Galaxies provides a number of examples (e.g., objects 21-26). Arp 24 can be taken as a prototype of this morphology (Figure 3.15). In a few cases, there is observational evidence to suggest that the one-armed spiral is a leading arm. That is, it spirals out in the same sense as the disk rotates. As early as the 1970s there were suggestions in the literature that retrograde collisions would produce leading, one armed spirals. The Swedish astronomer Magnus Thomasson investigated the phenomenon in some detail in the late 1980s. Chanda Jog and Francoise Combes have also revisited the theory in a recent review article on "lop-sided" galaxies. Thomasson advocated a resonance between the stellar orbits and the wave's motion around the galaxy disk. From the point of view of an observer moving with the wave, where it appears unmoving, this means that in one orbit a star always comes back to the same position in the wave. If the wave has an above average density of stars (this is what defines it), and they keep coming to the wave in sync, then the wave will be persistent. The orbits of most stars in galaxy disks are precessing ellipses rather than simple circles. This means that they move closer and farther from the galactic center as well as around it, and the precessing part means that the ellipses do not close. The net effect is that the orbits look like rosettes in a non-rotating frame of reference. In the frame rotating at just the right speed they would look like closed ellipses. In the resonant case, this is also the rotation speed of the wave. If a star happened to be at its closest point to the galaxy center when it was in the wave, then in the resonant case it would always be at the closest point when it intersected the wave. In the non-rotating frame, a star goes around about 230 degrees (not 360, because of the precession), between two closest approaches to the center. If the wave goes around 130 degrees in the opposite direction in that same time, then

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Figure 3.15 Sloan Digital Sky Survey image of Arp 24 system (as used in C. Cao & H. \Nu,Astron.J.,Vo\. 133).

together they will have covered the 360 degrees they needed to cover to be in resonance. It is more complicated than this, however, because the rotation and precession effects vary with distance from the galaxy center. We will not pursue these complications. Jog and Combes discuss a kinematical effect (e.g., unforced motion) that yields a long-lived leading wave. I have not made a detailed comparison, but I suspect that it is basically the same phenomenon. There are several cool features about this phenomenon. The first is that the term "long-lived" means that it can persist for more than a few rotation periods. This means that it can still be visible long after the collision when the companion has either moved very far away, or merged. A number of the one-armed galaxies in the atlases are

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Figure 3.16 Sloan Digital Sky Survey image of M101 (at the top) and its smaller companion galaxy NGC 5474 (at the bottom). Note the "lop-sided" nature of the disk relative to the nucleus in both galaxies.

not obviously involved in a collision, but this result means that a collision still could have generated the arm. A second feature is the generalization of an effect my former student Pnina Luban noticed in some computer simulations. She found that persistent one-arms formed at later times after ring galaxy type collisions that were not entirely symmetric. We conclude that a variety of collisions, not just retrograde flybys can make the leading arms, albeit not as promptly. Sometimes, as in the cases of Arp 25 and 26, there are several spiral arms within the retrograde fan structure. Perhaps, some of these spirals were present before the collision. Arp 26 is more commonly referred to by its older name,

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Figure 3.1 7 Near-infrared image of M101 emphasizing its two-armed aspect (2-MASS Project, a joint project of the Univ. of Massachusetts and IPAC/Caltech, funded by NASA and NSF).

Messier 101. It is one of the closest large galaxies to the Milky Way, and according to the NASA Extragalactic Database, it is at least mentioned in more than a thousand journal papers. Its stars, star clusters and other properties have been studied in great detail over the decades. It is a member of a group with a number of much smaller dwarf companions. Because the group is so close, its members are spread across an unusually large sky area. Its lop-sided disk has long been thought to be the result of an interaction, but perhaps because of the lack of an obvious nearby companion, and the mild effects within the disk, this has not been discussed much in the literature. Even the closest retrograde gets little respect, at least as a collision product.

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Figure 3.18 Hubble Heritage image of the complex spiral arms of NGC 4622. Note that the inner arms wind in the opposite sense of the outer arms (NASA, ESA, and the Hubble Heritage Team (STScl/AURA), G. Byrd, R. Buta, T. Freeman).

The most of massive of its dwarf companions, NGC 5474, is not too distant, only about three M101 diameters away (Figure 3.16). Intriguingly, it too has a fan shape with a very off-center nucleus. Perhaps both M101 and its partner have been dancing backwards around each other, or perhaps not. The spiral arms of the two galaxies open in the opposite senses, counter-clockwise for M101, clockwise for NGC 5474. If the dominant arms in the fan are both leading, then

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the flyby collision could not have been retrograde as seen from both disks. They are not dancing backwards together after all, which is too bad because it was a pretty picture. Recall that we started this discussion of M101 by noting that it seemed to have more than one arm. In fact, it seems to have at least two and maybe three, but in any case the arm opposite the fan structure is quite substantial. Figure 3.17 shows an infrared view of the object that seems to emphasize the two-arm structure, plus extra "spurs." Jog and Combes' review of lop-sided galaxies tells us that not only are onearms seen relatively commonly in infrared observations of disks, but galaxies that seem to be a combination of one and two arms are also not so rare. On first hearing, this may seem like a rather rare statement itself. How can a galaxy have both one and two arms? There is a technical explanation for this having to do with the combination of different fundamental modes, just like a typical sound consists of a combination of frequencies. We will skip that, and just invoke the image of a lop-sided, two armed spiral, which fits M101 quite well. This means that it was probably not produced in a typical retrograde flyby encounter. Other processes may be at work, including another encounter, or the effects of halo distortion by the companion dwarfs. Possibly the M101 disk was sloshing around in its halo as a result of an earlier dynamical event when NGC 5474 flew by. As per the discussion at the start of this section, modest tidal effects, like the lack of bridges and tails, make it hard to know. At the beginning of this section, I also promised some subtle effects, and in Figure 3.18 I present exhibit A. Are those not some of the most delicate and beautiful spiral arms? However, look carefully, and notice that the innermost one unwraps in a counter-clockwise direction, while the outer one is clockwise one of the rare galaxies with both leading and trailing spirals, mediated in cusp­ like joints. Gene Byrd and his collaborators at the University of Alabama have argued that it is the result of a close retrograde encounter, and possibly a minor merger. Remarkably, he was able to capture much of its structure in a numerical model. One wonders how much more we have to learn about the retrogrades, and their mixed up and backward dances.

Further Reading

Elmegreen, D., Galaxies and Galactic Structure, (New York, Prentice Hall), 1997. Schweizer, F., "Observational Evidence for Interactions and Mergers," in Galaxies: Interactions and Induced Star Formation: Saas-Fee Advanced Course 26, eds. D. Friedli, L. Martinet, and D. Pfenniger (Springer, New York) 1998. Toomre, A., and Toomre, J., "Galactic Bridges and Tails," Astrophysical Journal, Vol. 178, p. 623, 1972.

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Major mergers and their colossal consequences

4.1 Ellipticals f r o m mergers?

Most research scientists who have been in the business for some time have attended many national or international meetings. There is a great deal of sameness about the conference formats, the lecture halls, and the hotels. There are differences; some meetings may be very large gatherings, and others small workshops. The venue may have spectacular natural beauty, a unique cultural ambience, or be in a lively urban locale. More often, given the location of college campuses, it is less exciting. The presentations are usually a mix of updates on continuing projects of interest to the participants, and perhaps some new twists and discoveries that will be evaluated in many corridor conversations. However, most of it is not revolutionary stuff, and would be of little interest to anybody outside those particular corridors and lecture halls. However, all sciences have legends of truly revolutionary gatherings. Physicists would certainly include the early Solvay conferences where Bohr, Einstein and their colleagues met to try to understand the new quantum mechanics. In the field of galaxy evolution, many think that the conference entitled "The Evolution of Galaxies and Stellar Populations," which was held at Yale University in May 1977, was similarly important for that (much smaller) field. What made the Yale Galaxies Conference, as it became known, so successful? Certainly, the time was ripe for many of the new ideas presented there, but many of those ideas were presented at other meetings at about the same time. Like the Solvay meetings, one of the secrets of the Yale conference was that most of the speakers were leaders pursuing new ideas and techniques, and it was rare to gather so many in one place to discuss and argue those ideas. That did not happen by accident. The chief organizer and driving force behind the meeting was the young Yale professor Beatrice Tinsley. Her energy and very extrovert personality were also legendary, and allowed her to gather the speakers and assemble the program. I personally look back on this as a golden age, since I was a first year graduate student beginning research work with Tinsley. The chance to hear those talks, and meet some of those speakers remains a career highlight for me even these many years later. However, I think even a more objective observer would agree with the previous paragraph, that talks at that meeting outlined new or reinvigorated ideas that have guided research programs to the present day. I hope that someday historians will examine that founding time in the field of galaxy evolution more carefully. I would love to delve into the content of and C. Struck, Galaxy Collisions: Forging New Worlds from Cosmic Crashes, Springer Praxis Books, DOI 10.1007/978-0-387-85371-04, © Springer Science+Business Media, LLC 2011

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discuss a number of the papers presented there, but that would lead us astray. It is really one paper that I had in mind in bringing up the Yale conference, and it provides an excellent example of the importance of the meeting. The talk was by Alar Toomre, and was entitled simply "Mergers and Some Consequences," and it was the most important of his several revolutionary works in this field (and the inspiration for this chapter title). Toomre's primary "consequence" was that a significant fraction of all elliptical galaxies could be formed out of the "wreckage" (his term) of the merger of two spiral galaxies. At this time the leading theory of elliptical galaxy formation was what we would now call monolithic collapse of a huge gas cloud with little angular momentum. The latter condition implies no rotating disk would form, with no centrifugal force to restrain a very large compression in the gravitational collapse, presumably resulting in very efficient star formation and gas consumption. A merger model was shocking, in part because no other credible competing model had been put on the table before, and because few thought another one was needed. There were other reasons why it was shocking. The first thought that probably occurred to most of Toomre's audience was that such a collision must be quite unlikely. That is, it did not seem probable that two very similar spiral galaxies would find their way to a collision and merger often enough to account for the ellipticals, one of the most common types of galaxy. As we have seen in earlier chapters, galaxies come in many shapes and sizes and are generally spread quite far apart, so how often could you get two near twins together? Of course, this is a variant of the original argument against any kind of galaxy collision, that they were just too far apart. The answer to the original paradox was that galaxies are born into gravitationally bound groups and clusters, so they are almost destined from birth to collide with their group-mates. One piece of evidence on Toomre's side was the fact that most elliptical galaxies are found in large galaxy clusters. This is the environment where galaxies are packed most densely, and thus, the environment where most collisions must have occurred in the early ages of the universe. Nonetheless, this implied that cluster galaxies had to form as spirals initially, and later merge to form ellipticals. This is a more complicated picture than the direct monolithic collapse model, and so, required evidence for the two-step process. Toomre, however, had a different statistical argument. He listed 11 systems from The Arp Atlas of Peculiar Galaxies that seemed to be examples of the kind of merger he was talking about (Figure 4.1). He noted that this was a small, but not insignificant fraction of substantial and relatively nearby galaxies. Also the characteristic timescale for the merger was likely to be much shorter than the age of the universe, so if the number of presently merging systems was typical, there would have been a respectable cumulative number. Beyond that, there are a number of reasons to believe that collisions and mergers would be much more common earlier in the life of the universe, including the fact that the universe was much smaller and galaxies were closer together in the distant past. Thus, the currently observed rate could well be just the tail of a big dog. Incidentally,

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Figure 4.1 The Toomre merger sequence is a set of Arp Atlas galaxies selected by Alar Toomre to illustrate a possible evolutionary sequence from well separated galaxies in the upper left to well merged objects in the lower right, with partially merged cases in between (from Level 5 on the NASA Extragalactic Database).

Toomre's statistical estimate, though very simple, was the first computation of what is now called the merger rate. It has been estimated many times in a variety of different ways in the intervening years. His sample of 11 galaxies has also acquired a life of its own. Now known as the Toomre sequence, it has been studied in detail with the Hubble Space Telescope, and in a variety of other wavebands. Others in Toomre's audience asked themselves, if two comparable spirals come together from some great distance, surely they must have some relative angular momentum, and do not collide head-on. Surely, they must suffer a nearly head-on collision in order to merge promptly, rather than just quietly orbit each other at a relatively large distance? Toomre had an answer to this as well. He used a numerical model to demonstrate that galaxy collisions were just much stickier than most of us had thought. The process behind the stickiness is called "dynamical friction," and we will consider it shortly. Toomre's numerical results suggested to him that its effects had been greatly underestimated in galaxy collisions. Toomre's computer model was quite simple even by the standards of the day, so his conclusion was questionable. However, with this idea he again set a research question that would be pursued for decades. A third qualm concerned whether the merger remnant derived from two spiral

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galaxies would look anything like an elliptical. The shape would have to be dramatically changed, interstellar gas consumed or dispersed, tidal structures like bridges and tails dispersed, and details of the stellar velocities drastically altered. In the extensive question session after Toomre's talk, Jeremiah Ostriker, one of the deans of modern theoretical astrophysics, expressed it well: Most of the objections one might have to making elliptical galaxies from disks are based on the systematic properties of ellipticals - would galaxies made from disks really mimic ellipticals? I think there is no difficulty if you want only to make a moderate fraction of ellipticals your way; in plots of core radius versus luminosity, etc., certainly 25 percent of the points are not very close to the normal line. But there are problems if you want to make most ellipticals this way. This debate was opening the door for many more research projects!

4.2 A sticky aside on dynamical f r i c t i o n

Dynamical friction - it sounds quite dramatic, like squealing tires in a movie car chase. It actually is quite dramatic, but like everything else in the world of galaxies it takes a few hundred million year to play out. The idea was originally proposed by astrophysicist Subrahmanyan Chandrasekhar and mathematician and pioneering computer scientist John von Neumann. Although the mathe­ matics of this idea is not so simple, their basic idea can be described quite simply. Suppose that a massive particle is moving through a sea of much less massive objects. For example, the particle might be a dense star cluster or small companion galaxy moving through the halo or bulge of a large galaxy. The low mass objects could be individual stars in the large galaxy, or dark halo particles. Let us use the star cluster moving through galactic stars as a specific example. The star cluster's orbit is determined by the gravity of all of the galaxy's mass interior to that orbit, and assuming the cluster is small, most of that material is not much affected, in turn, by the cluster. However, the stars closest to the cluster are drawn towards it, since its gravity is significant in the local region. Yet it takes some time for those stars to accelerate up to a significant speed towards the cluster. The cluster's orbital speed is relatively large, so by the time the stars converge on its location, it has moved on. The converging stars simply pass through the cluster's former position, but for a short time they create an excess stellar density at that point. Such points are now behind the orbiting cluster, so this density excess is like the wake behind a boat, and is called a gravitational wake (Figure 4.2). The gravitational wake exerts its own gravitational pull, which pulls the cluster back in its orbit. The result is analogous to an orbiting spacecraft firing a rocket in the forward direction. This would slow it down and make it fall inward, and so too with the star cluster. The effect is even more similar to friction, because in the case of the rocket, the forward burn might only serve to put it on

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Figure 4 . 2 A schematic illustrating Chandrasekhar's wake theory of dynamical friction. The large, multi-pointed object represents a star cluster traveling through a field of stars in the direction of the arrow attached to it. Its gravity pulls stars around it and into a relatively high-density wake as illustrated. The mass concentration in the wake pulls back against the motion of the cluster (figure by the author).

an elliptical orbit, while the pull of the gravitational wake is continuous, and generates an in-spiral, not an elliptical orbit. The cluster or the companion galaxy is dragged down to the nucleus of the large galaxy. For better or worse, Ostriker called this process "galactic cannibalism." Chandrasekhar and von Neumann developed a formula to describe the deceleration of the star cluster due to the gravitational wake, and how long it would take the cluster to be pulled in. The formula shows that these quantities depend on the mass of the cluster, the local density of the stars and the relative velocity of the cluster and the ambient stars. Their work was published more than two decades before Toomre's mergers-from-ellipticals talk, and applied to problems like the star cluster one. However, what Toomre was describing amounted to a considerable extension of the mild gravitational wake picture of dynamical friction. His numerical model did not show a slow in-spiral between the two colliding galaxies (which were modeled as simple bulges or ellipticals rather than multi-component galaxies). They merged very rapidly. This core result of Toomre's paper was in itself quite startling. It slowly gained credibility as it was replicated by better computer models in the following years, notably (and independently) by Simon D.M. White and Jens Villumsen a few years later. In fact, White presented some preliminary results in the discussion period at the end of Toomre's talk. In contrast to the gravitational wake picture for a star cluster, there is no simple physical picture that describes the effects of dynamical friction in collisions between comparable galaxies. The best I can do for a rough description is to suggest that each galaxy effectively becomes the gravitational wake for the other as they both orbit the center of mass. That visualization may be conceptually helpful, but it is not sufficiently precise to lead to a reasonably accurate approximation of the merger time.

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The case of the wake of the star cluster is a relatively small disturbance to the structure of the galaxy. It is the kind of thing that physicists and mathematicians call a small-amplitude or linear perturbation. As we have already seen, the collision between two comparably sized galaxies leads to huge structural distortions. It is called a nonlinear perturbation, and as such can usually only be quantitatively described with the aid of detailed computer models. Thus, Toomre's result largely ended the possibility that pen and paper calculations, even ones as complicated as Chandrasekhar and von Neumann's, could tell us much about the physics of galaxy collisions. There was already a strong hint of that conclusion in Toomre and Toomre's earlier models of galaxy collisions, mentioned in previous chapters (and those models did not include dynamical friction). History has proven that progress in this field would depend on advances in computer technology, though many of us did not appreciate that in the early days. I will have much more to say about numerical models in later sections. Fortunately, almost nothing has advanced as quickly in the last few decades as computer technology.

4.3 B r i n g i n g a little data t o t h e subject

Toomre's talk generated a great deal of discussion, but I think it is fair to say that almost nobody realized the magnitude of the revolution his paper ignited. In effect, the professor from M.I.T. had carried off a Boston Tea Party.... in New Haven. Most extragalactic astronomers probably inclined toward Ostriker's point of view. A few modelers were quite happy to test his results on the nonlinear effects of dynamical friction. However, in a time of rapid advancement in the understanding of galaxies and their evolution, not too many observers rushed to the telescope in search of observations that would confirm or refute Toomre's ideas. F. Schweizer of the Carnegie Institution was an exception; he hitched his career to Toomre's star idea and pioneered work in the area. Schweizer focused on finding observational evidence for mergers. In his own research he used sensitive optical imaging and spectroscopy. In the modern terminology, most of the evidence he found concerned "major mergers." He summarized his findings in a paper for a conference in 1983. These findings included evidence of galaxies with multiple cores and asymmetric outer envelopes (especially in large ellipticals in galaxy clusters). Like tidal tails these features could not persist long enough to be remnants of galaxy formation. He also cited discoveries of disk galaxies where the gas in the core rotated in the opposite sense to gas in the rest of the disk. This was easiest to understand if the counter-rotating gas was acquired from another galaxy, e.g., in a merger. Of course, the most spectacular evidence was the tidal tails, ripples, and shells, like those that Toomre had cited in his examples. Schweizer's deep imagery (and shortly thereafter similar work by David Malin and others) showed fainter and more extensive tidal ripples or tails around a number of candidate systems (including NGC 7252 shown in an image taken later in Figure 4.3). This was the

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Figure 4.3 An enhanced view of the merger remnant NGC 7252. This object terminates the Toomre sequence, and was first studied in detail by F. Schweizer (from the NASA Extragalactic Database and A. Sandage & ). Bedke, The Carnegie Atlas of Galaxies).

case even in some systems where a short exposure would show only a rather normal looking elliptical galaxy. The faint ripples were presumably the tenuous remnants of a long and completed merger. The evidence that the normal, bright, central part of a merger remnant could apparently evolve to a normal Hubble type, as predicted by Toomre, was impressive. The fact that the structure of the tidal features suggested that it could do it within a time as short as a couple of Gyr was rather surprising. In some ways this proved to be true at a quantitative as well as a qualitative level. For example, the variation of the surface brightness, or brightness per unit area on the sky, with distance from the galaxy center was found to be essentially the same in the major merger remnants as in ellipticals. Specifically, the surface brightness profile in ellipticals was often fitted with a particular exponential function developed by Gerard de Vaucouleurs. That function also fitted the inner regions of the merger remnants.

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4.4 Relaxing in t h e merger: quickly and violently!

Rapid merging was explained with an extension of an old theoretical idea dynamical friction. The rapid relaxation to a long-lived form was also explained with a variation of an old theoretical idea. Its name is a wonderful oxymoron violent relaxation. The idea was developed by Donald Lynden-Bell, on the basis of statistical arguments, in 1967. The motivation for Lynden-Bell's theory was related to the later merger application. He was seeking an answer to the question of how relaxation to a nearly equilibrium, or final steady, state was achieved rapidly in the assumed monolithic collapse that formed elliptical galaxies. As usual in the world of galaxies, "rapidly" is a relative term. Here it essentially means much less than the age of the universe. Thus, hundreds of millions of years, or even a few Gyr could be rapid. The point is that this timescale is much less than what we would expect if the relaxation occurred by processes similar to those that affect the approach to equilibrium in our everyday world. A classic example is thermal relaxation in a gas like air. How does the temperature settle to a new uniform average value in a room after one part of the room has been abruptly heated or cooled, and how long does it take? In the real world waves and circulations aid the process. However, ultimately collisions between molecules, two at a time, with the energy of the fast ones shared with the slow ones, drive the relaxation to equilibrium. The time between individual collisions is a minute fraction of a second, but no single molecule travels very far (e.g., less than a micrometer) between collisions, so it takes a great many collisions to share energy across the room. The molecular motion is described as a "random walk." When the garage in my home has been heated on a summer day, closed, and then opened on cool, windless evening, the net time is of the order of some minutes. If galaxies relaxed via encounters and energy exchanges between their stars, two at a time, then the process would take many ages of the universe. So how do we know that elliptical galaxies are relaxed? Aside from their varied elliptical shapes, they have a number of smooth and common structural features, suggesting that once in place these features do not change much. For example, their surface brightness profiles, the light emitted per unit area as a function of distance from the center, have a universal form. This form was the primary thing that Lynden-Bell was trying to explain with his theory of violent relaxation. There are also regularities in the kinematics, or the structure of the velocities within elliptical galaxies. One of the most basic of these is the fact that they do not rotate much, and thus centrifugal forces do not play a big role in determining their elliptical form. Detailed velocity data were difficult to measure for ellipticals, and were obtained long after Lynden-Bell's paper. In fact, such observations were among the exciting works presented at the Yale Galaxies Conference. Such is the nature of the problem, but what was Lynden-Bell's solution? The basic idea was summarized by a statement in the abstract of his paper - "The violently changing gravitational field of a newly formed galaxy is effective in changing the statistics of the stellar orbits." I cannot explain all of what he

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meant by the phrase "the statistics of the stellar orbits." This is a very technical corner of the field of galactic dynamics, with traditions and many connotations wrapped together in such technical phrases. However, one aspect of this statement is that so many orbits would be affected so strongly by the "violently changing gravitational field" that the collection of all orbits would look very different from what it did before. Imagine some bags of ready-mix concrete and a good supply of water. Subject these ingredients to the "violence" of a cement mixer, and after the proper time pour the mix into the sidewalk mold. Wait a while, and the ingredients will "relax," or dry in this case, and never lapse back to their former state. Indeed, the mixing up of orbits is a key aspect of violent relaxation. Unlike the concrete, this mixing amounts to more than just one dollop of wet mixture changing places with a dry one. The proto-galactic orbits not only switch properties with each other, but acquire property values beyond the original range. One of Lynden-Bell's main points was that the gravitational shaking must be violent enough to really shake things up, in order to get quickly to the final state. Think of a salt shaker that has been sitting unused for a long time. Just giving it a little push will not loosen the salt. Even turning it upside down and giving it a gentle shake is not enough to get any salt. I bang it on the table to loosen big chunks and then shake it hard to break up those pieces. Lynden-Bell taught us that such is the world of galaxies too. He showed that if the galaxies had started off arranged in a way not too different from their final state, it would take them a very long time to reach that final state. For example, to get to the final universal surface brightness profile in a relatively short time, it will not do to start with a profile that is just a little steeper, or a little shallower than the final. In that case, there will be no violent gravitational fluctuations to drive the mixing of orbital parameters, and you will have to wait beyond the age of the universe for the two-body collisions to do the job. It is better to start from a much more chaotic brightness (or mass) distribution. An haiku koan of violent relaxation goes like this: Start close, and you will never get there; go far and you will arrive soon. In this section I have been liberal with the analogies and miserly with pictures. In this regard I follow the master of violent relaxation. Lynden-Bell's original paper had 58 numbered equations, many more unnumbered equations, and just two schematic figures (that had nothing directly to do with galaxies). It is an abstract topic, and difficult to illustrate. Even for the experts, Lynden-Bell's original paper raised many questions, and many different aspects of violent relaxation have been explored since that time. One of these questions was why or how orbit mixing would in fact result in such rapid relaxation. The late H. Kandrup championed the view that families of chaotic orbits, which can be present even in relaxed galaxies, play an essential

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role. Others have tried to exploit the similarity to a phenomenon called Landau damping, which is known to play a role in the relaxation of plasma systems. Incidentally, we considered a kind of orbit mixing, called phase mixing, as the means of effectively ending the ringing of colliding ring galaxies in Chapter 2. Violent relaxation is a more rapid process, and cannot be explained as a kind of phase mixing. There are different ways to derive equations that approximate the action of violent relaxation. Recently, Itai Arad and Peter H. Johanson compared the two best known of these approximations (including Lynden-Bell's) in detail to numerical models. They found that violent relaxation was too violent (or more technically "too nonlinear") to be well-represented by these approximations. This is very unfortunate because the approximations could form the basis of conceptual pictures of the operation of violent relaxation. The numerical result seems to indicate that these processes may be too complex for simple, yet reasonably accurate, conceptual pictures. Like Lynden-Bell's original paper, most of this theory has been developed in the context of strong gravitational collapse, which was the dominant theory of galaxy formation at that time. When Toomre proposed that elliptical galaxies formed from mergers he encountered the same problem as Lynden-Bell; he had to transform an initial state that looks nothing like an elliptical galaxy, i.e., two comparable spiral galaxies, into an elliptical in a relatively short time. Given that violent relaxation was invented to solve this problem, and as per the quoted haiku koan worked best with very different initial and final states, it was natural for Toomre to invoke it as the means to turn his merger remnants into elliptical galaxies. Three decades of numerical modeling have confirmed the correctness of his conclusions on the general operation of violent relaxation in mergers. The rapid relaxation of the models was as much a surprise to the early modelers as the efficient action of dynamical friction. The fact that this relaxation also produced surface brightness profiles with universal forms like those of real ellipticals, rather than depending on the details of the initial conditions, was a satisfying confirmation of a predictions of the theory of violent relaxation. Astronomers continue to pursue ever more detailed aspects of the theory, modeling, and observations of this process, or set of processes, and our understanding of it is advancing. For example, Monica Valluri and collaborators recently concluded, on the basis of in-depth analysis of high resolution numerical models, that "mixing occurs in a steplike fashion following pericenter passages (closest approaches) of the halos." Violent relaxation occurs in bursts. Moreover, they also found that "the mixing process that drives the system to equilibrium is primarily a response to energy and angular m o m e n t u m redistribution that occurs due to impulsive tidal shocking and dynamical friction rather than a result of chaotic mixing in a changing potential." If these conclusions are correct it implies that violent relaxation may operate rather differently in mergers than in gravitational collapse. Of course, mergers are a form of gravitational collapse, so the difference is in the structure of the

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initial state, and the conclusion is that the different processes that make up violent relaxation operate to different degrees in different cases. We still have some ways to go to make such vague statements more precise, and understand all aspects of the galaxy merger process. We cannot rest on our laurels quite yet.

4.5 Starbursts, super-starbursts and mergers

The Hubble Space Telescope is far and away the most famous NASA space science mission. With its production of hundreds of beautiful images, and with space walking astronauts to replace its instrument packages, this is understandable. Though it had a much lower profile, the 1984 Infrared Astronomical Satellite (IRAS) was also a spectacular success. With its data astronomers discovered the most luminous galaxies in the universe, a new type of interstellar cloud around the Milky Way (called infrared cirrus), and some of the first evidence for dusty disks related to planets around other stars. IRAS was a small telescope (diameter = 0.57m), and it was only in operation for about 10 months. Its fame is mostly restricted to professional astronomers. In contrast to Hubble, IRAS was small enough and sufficiently self-contained to be launched by an unmanned rocket in 1983, and designed to be expendable, not serviceable. Also, it only produced a few iconic images, and these were false color images, representing invisible emissions. This is because IRAS was designed to observe in the far-infrared, that is heat radiation, not optical light. Given the technology of the time its resolution was very limited; it did not really resolve any but quite nearby galaxies, for example. In order to be sensitive to cosmic sources, as opposed to its own heat radiation, it had to be cooled to a temperature of less than a few degrees above absolute zero. It was launched with a full tank of super-cold liquid helium, which kept it cool by evaporation, but ran out after 10 months. However, this was planned, and provided sufficient time for IRAS to survey the whole sky in several wavebands (e.g., from 12 to 100 micrometers) that had never been accessible before. This survey was carefully processed and archived, and is still in use today. What about those super-bright galaxies that IRAS discovered? They soon came to be called "ultra-luminous infrared galaxies," or ULIRGs (though there were several variant acronyms). With time the class was restricted to galaxies that emit a trillion (1012) times as much energy per unit time as the Sun. Others, which emitted a mere hundred billion times as much energy as the Sun were only LIRGs, not ultras. Immediately the questions arose - what were these enormously bright objects, and what provides them with their incredible power? The answer to the first question came relatively quickly. The ULIRGs were matched with optical (and radio) sources, and in most cases these were recognized as major mergers between gas (and dust) rich galaxies (Figure 4.4). Indeed, the newly minted superlatives flew about in conferences of the late 80s: ultra-luminous, super-starburst, major merger, etc. As an aside, I should note that the "quickly" answered question occupied the efforts of dozens of astronomers,

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Figure 4 . 4 Montage of four famous merger remnants and ultra-luminous infrared galaxies (ULIRGs). These are: (a) Arp 244 (The "Antennae"), (b) Arp 226 (NGC 7252, the "Atoms for Peace" galaxy), (c) IRAS 19254-7245 (The "Super-Antennae"), and (d) Arp 220 (IC 4553/54). Contours show the surface density of neutral hydrogen gas superposed on optical images (greyscale). The insets show K band (2.2 Im) images of the central regions as grayscale, with white contours representing molecular gas (CO) intensities (from Sanders & Mirabel, 1996, Annual Review of Astron. & Astrophys., Vol. 34).

including many on the IRAS mission team, for several years. The technical literature preserves a record of the struggle to match sources in the different wavebands, which was made more difficult because many of those sources were more distant than expected. Similarly, the IRAS objects were followed up with

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numerous spectroscopic surveys to determine their redshift, and thus estimate their distance. The distance to the objects was needed to determine their luminosity, and confirm their membership in the ULIRG class, for example. The class was discovered with relatively nearby examples of known distance. The importance of the class could not be known until many more members were confirmed by the distance and luminosity determinations. This is not the first time in this book where we have skated "quickly" over a topic of considerable depth in its own right. It took longer to answer the second question: what is the energy source? In fact, in many cases the question is still being investigated. This is because there were two very viable candidate power sources; either millions of hot, massive, young stars produced in a "super-starburst," or an energetic "active galactic nucleus" (AGN). One might think it would be easy to tell the difference between active galactic nuclei, which are famous for prodigious outputs of high energy radiation (X-rays and gamma-rays) from a very small source volume, and a large number of hot, ultraviolet emitting, but still relatively normal, stars contained within a larger, though still small, volume. Unfortunately, the radiation observed by IRAS, and numerous subsequent studies, is not the radiation directly emitted by these sources. The energy sources located in the centers of the merger remnants were surrounded by many, very dense clouds of gas and dust. The dust absorbs almost all of the primary emission in bands extending from the near infrared to the optical, UV, and up to the X-ray. The bulk of this energy is subsequently reemitted in the far-infrared, where the dust grains acting like miniature antennae radiate efficiently. The descriptive phrase, "buried in dust," stuck. Dust is also a factor in ordinary galaxies. It is a fairly modest one when we look at typical galaxy disks. There are various ways to estimate this effect, but one of the more interesting is by observing two galaxies that are relatively near each other, but which have disks that overlap in projection. Most such systems are interacting. Astronomer (and fellow Springer author) William Keel and his collaborators have carried out a great deal of research on such objects. They quantitatively confirm some common visual impressions; the dust obscuration is very patchy with large variations, and it generally increases towards the center. Away from the central regions dust typically absorbs less than half of the light emitted from the disk (or the background disk in the case of the Keel et al. studies). Generally, the most obscured parts of galaxies are their nuclei. Nonetheless, the nuclei are not usually entirely obscured from view in the optical, and this is even more true for infrared observations. Interactions and mergers on the other hand can push large masses of gas and dust into the central regions of galaxies. We will consider the question of how they do that shortly. But first, let us consider an example starburst or two. One of the closest and best studied is the Cigar galaxy, M82 (Figure 4.5). The nickname derives from the fact that the disk of this galaxy is seen edge-on and it has no evident bulge, so it looks like a cigar. The strong galactic wind driven by

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Figure 4.5 Hubble Heritage Project image of the Cigar galaxy M82. In this mosaic image published in honor of the sixteenth anniversary of HST, hydrogen alpha line emission is shown in red, and highlights the smoke (gas filaments) flowing out (in the galactic wind) of the cigar (NASA, ESA, and the Hubble Heritage Team (STScl/AURA),). Gallagher, M. Mountain, P. Puxley).

the intense central star formation looks like smoke coming out of the cigar, though why it was lit in the middle I do not know! It has been estimated that, on average, sources in the M82 nucleus are hidden by 27 magnitudes of extinction in the optical. According to the astronomical definition of magnitude, 5 magnitudes of extinction means that only about 1 percent of the light gets out. Thus, less than a few parts per hundred billion (1011) of the optical light of an average nuclear source in M82 escapes. However, this is not the whole story. Even at these very high levels, the obscuration is still patchy, and the evidence suggests that there are pathways through which the light can find its way out. The extinction along those pathways (as opposed to the light intensity) is reduced by many orders of magnitude. (Of course, the bulk of the light energy eventually finds its way out in the little obscured far-infrared wavelengths.) M82 is classified as a starburst galaxy, with a high rate of star formation and hundreds of newborn star clusters in its core. The high-energy photo-emissions, strong stellar winds, and supernovae explosions produced by the young stars in

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these clusters drive the galactic wind. With the interstellar gas being driven by a number of strong, local sources, one can envision that the starburst region is extremely turbulent, with large inhomogeneities. The turbulence is supersonic, so strong compressions will be produced by shock waves. Between the shock compressed regions and other lumps there must be relatively clear pathways out for some light beams. Have I forgotten to mention that M82 is a colliding galaxy? Indeed, M81 and M82 are the closest large galaxies undergoing a strong interaction. A number of smaller galaxies of the M81 group are involved as well. This was certainly not clear to early observers with modest telescopes, like Charles Messier, who cataloged them in the northern sky. The two galaxies are well separated and at optical wavelengths the tidal structures are very faint. This is not the case in the radio band, however. In observations of the 21 cm line of hydrogen, their enmeshment is most fully revealed, as we will see in Chapter 8. A discussion of M82 does not really belong in this chapter, since it will be a long time before the mergers are completed in this system. The system does not fit into one of the proto-typical classes discussed in the previous chapters either. It is too messy to fit there, though probably quite typical among interacting systems in that. Its complexity may well derive from having three or more galaxies, possibly involved in multiple, or prolonged encounters (discussed in Chapter 8). Yet, M82 does belong here because it is a prototypical starburst galaxy, and the starburst has most likely been triggered by the interaction. A major observational problem is that M82 is nearly edge-on. The starburst is buried by a strong concentration of gas and dust in the center, and we have to observe it through the disk. Most of the "clear pathways" mentioned above probably travel perpendicular to the disk. Infrared and radio observations are required to get a more complete view through this obscuration. However, this always true to some degree in starburst nuclei, because great concentrations of gas and dust are required to fuel starbursts. Which brings us to a key question how is that interstellar material concentrated in a relatively small volume around the nucleus? The problem is related to the phenomenon of wave formation in ring galaxies described earlier. In that case, the extra gravity of the overlapping galaxies pulled the stars toward the center, but not all the way to the center. As the stars move inward they reach a point where the increasing centrifugal force overcomes the total gravity, and they are stopped, and ready to rebound when the companion moves away. We also saw above that in prograde interactions some stars lose angular momentum and fall inward. However, they generally do not lose all their angular momentum, and are likewise stopped short of the center. In most cases, the so-called centrifugal barrier is difficult to overcome. It is not easy to get the gas cloud fuel squeezed down into the fireplace of the galaxy nucleus. Rather than being lost impulsively and partially in a relatively short interaction, angular momentum must be taken out continuously, or in stages, to move the gas into a small volume around the nucleus. It appears that galaxies can in fact accomplish the task in several stages. In the first strong interaction

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spiral or bar waves are often formed or greatly enhanced. These waves can be very effective at transferring angular momentum outward, so gas clouds in the inner disk lose it, while those in the outer disk gain. Bars are generally more effective in this process of angular momentum transfer, if only because the typical bar is a stronger wave than the typical spiral. The loss of angular momentum allows gas clouds to move into the central regions. There is now a good deal of evidence that strong bars can funnel large quantities of gas to the centers of galaxies, enough to feed central starbursts. There is even evidence that sometimes this occurs in stages. That is, the gas tunneled into the center may not directly initiate a starburst before its selfgravity triggers the formation of a smaller inner bar, which funnels gas still farther down. This bars-within-bars process is not generally observed to go on to produce a third or fourth bar, though the possibility is intriguing. Instead of multiple bars tunneling gas all the way to the center, it appears that often the gas is hung up some distance short, for example, at the radius of an inner Lindblad resonance of the type discussed in the previous chapter. The result can be a starburst ring, which has little in common with the rings discussed in Chapter 2. Figure 4.6 shows a beautiful example of a central ring in the galaxy NGC 1097. The observational studies undertaken to date (mostly two decades ago, with few recent ones) do indeed suggest that it is a bar-fed object, rich in star-forming molecular clouds. There is also evidence for an interaction with a nearby companion, including unique optical (but not radio) jets or plumes. Among its many noteworthy features, NGC 1097 has an active nucleus. In all, it is a very interesting object. As discussed above, prograde encounters can lead to the formation of bars (and spirals). Retrograde encounters can form one-armed spirals and other types of waves. Direct, ring-forming collisions can create transient central density enhancements. Thus, all types of collision can lead to increased central gas densities and enhanced rates of star formation - though it is generally somewhat trickier to raise the star formation rates up to the high levels that characterize starbursts. Over the past few decades, the study of star formation enhancements due to galaxy collisions before merger has also been a tricky one. We will consider this subject in more detail in a later chapter. The point here is that collisions can induce structural changes that drive some gas clouds into the central regions, which can enhance star formation even up to the level of starbursts. This seems to be the true in the case of M82. The evidence, as mentioned above, strongly suggests that M82 is interacting with the other galaxies of the M81 group. This evidence also shows that the interaction is complex. It is definitely not an example of one of the clean, symmetric, paradigm cases discussed above. This is probably one of the main reasons why it has received less attention from modelers than a number of other colliding systems. This is true despite the fact that it is so close. Nonetheless, even though we do not have especially detailed or accurate models of the M81 group, the observations do suggest the presence of a bar. This bar may have fed the central starburst, and may have been formed or enhanced by encounters with other group members.

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Figure 4.6 A mid-infrared image of the starburst ring deep in the center of the galaxy NGC 1097 from the Spitzer Space Telescope. As usual in Spitzer images the false color is set such that red represents hot dust, usually around luminous, young stars (8.0 micron emission), and blue represents old red stars (3.6 micron emission). This image does not show the optical jets (NASA, JPL-Caltech, SINGS Project Team).

Incidentally, we have not really defined the central subject of this section, starbursts, except by example. What exactly is a starburst and how does it differ from other modes or types of star formation? To understand the term better it is worth considering a bit of its history. The earliest use of the term "burst of star formation" that I know of was in studies of dwarf galaxies in the early 1970s by Leonard Searle, Wallace Sargent and William Bagnuolo (in author order from a well-known 1973 paper). The term and the idea were quickly adopted by Beatrice Tinsley in her well-known models of stellar populations in galaxies. The observation underlying the idea

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was that the star formation rates were so high in some dwarf galaxies that if they continued unabated then the gas cloud fuel would be used up in a small fraction of the age of the universe. If this star formation rate had been more or less constant through the life of the galaxy, then by chance we are just catching these galaxies as they are about to burn out. Given that a number of such galaxies are observed, this is implausible. More likely, in the words of Searle etal, these are "galaxies which undergo intermittent and unusually intense bursts of star formation." Around 1980, in studies of other non-dwarf galaxies experiencing intense star formation in their nuclei, Daniel Weedman shortened the term to starburst. The short version caught on very quickly, as did the realization that nuclear starbursts were just as prevalent as dwarf starbursts. Thus, the term starburst denotes any galaxy or region of a galaxy where the stars are being formed at an unusually high rate, and in particular, the burst is using its gaseous fuel at an unsustainable rate. Although originally defined in this way, the term is even more appropriate in the light of current understanding that the strong stellar winds and supernova explosions generated by massive young stars will generally drive the gaseous fuel away from young star clusters. That is, high rates of star formation generally tend to be burst-like not only because of their short fuel consumption timescale, but because they are self-terminating on an even shorter timescale. We began this busy section with a discussion of super-starbursts and ULIRGs. This led us to a consideration of the difficulties of observing buried starbursts in merger remnants, despite the fact that they are the most energetic starbursts. Dust enshrouded starbursts brought M82, the nearest example, to mind. Consideration of M82 raised the issue of how are such starbursts fueled, and the question of how pre-merger starbursts differ from merged ones? The latter issue was raised, but not considered in detail. In all, we have been taking a rather meandering overview of the phenomenology of starbursts, and previewing some topics that will be discussed more thoroughly later. Now it is time to bring the focus of this chapter back to mergers.

4.6 A n e w generation of merger studies

Let us return to the post-IRAS era, the late 1980s and early 1990s, after the discovery of ULIRGs and the recognition that these objects were mostly major mergers. As discussed above, this discovery inspired a great deal of observational follow-up in many wavebands, including beautiful imagery by the Hubble Space Telescope after its launch in the early 1990s. Data on the structure of merger remnants began to pour in. In astronomy, as in other sciences, observation/experiment and theory/ modeling are like two horses that pull the scientific wagon ahead. However, in astronomy these horses are not hitched too tightly together; they are loosely tethered on a shared line that can be pulled ahead on one side or the other in a way that no good stable worker would ever have allowed in the horse and buggy

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days. Sometimes theory gets ahead of observation, so that it is not possible to check the predictions of the former with the latter. Sometimes, perhaps as the result of a technological breakthrough or new space mission, unexpected observational discoveries are made, and theory or modeling must rush to provide explanations or understanding. This was the case in the study of merging galaxies at that time. The modeling work mentioned above had shown that the merging of galaxy halos could indeed occur rapidly on galaxy timescales. However, those models only gave rather broad clues to the details of the merger process, and how to understand the new observational results. On the other hand, computer technology was advancing as rapidly as observational technology, and there were important advances in software algorithms for galaxy dynamics. Thus, a new generation of models capable of capturing a number of merger complexities was possible, and soon appeared. The most influential of this new generation of merger simulations were those of Joshua Barnes and Lars Hernquist, who published a series of papers in the late 1980s to the mid-1990s, both individually and together. Barnes had also been involved in developing algorithms for the accurate and efficient calculation of gravitational forces between N-bodies, e.g., the stars and gas clouds of model galaxies, work which provided this team with an important advantage in producing better merger models. What the increased computer power and efficient software allowed Barnes and Hernquist to do was to include all three major components of a typical galaxy disk, bulge and halo - for both of the galaxies in their simulations. We have seen above how Toomre and Toomre and others had earlier modeled the tidal distortions of galaxy disks in different types of collision. These early models were simplified by using a simple fixed form for the combined gravity of all galaxy components. The way the gravitational potentials of each component changed through the collision was not accounted for in these models. Effects such as dynamical friction were also not included. Subsequent N-body models of merging elliptical galaxies used thousands of representative stars, each with the mass of thousands of real stars, to model more realistically the changing gravitational potential. As we have seen, this demonstrated the power of dynamical friction to affect mergers, but these calculations also pushed the limits of computer power in the 1970s and early 1980s. It was not feasible to model each galaxy component with an equal numbers of model stars, gas clouds, or dark matter particles until the hardware and software advances of another decade. The early models taught us a lot, and some of the approximations used then are still useful, so why was it so important to model each component more realistically? One answer to that question is the need to account for the exciting new observational advances mentioned above. The 1990s generation of models did indeed initiate sophisticated model/observation comparisons and an extensive dialog between observers and modelers that pushed developments in both areas, and continues to this day. However, a more direct answer to the

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Figure 4 . 7 Example of a multi-component merger model ). Barnes and L. Hernquist (1 996). Gas particles from the two disks are shown. Note both the formation of tidal tails, and the much more compact gas distribution in the merger remnant than in the original disks (from ). Barnes & L. Hernquist, Astrophys. /., Vol. 471).

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question comes from the primary results of the Hernquist and Barnes models (Figure 4.7). Let us consider a few specifics about these models. The two model galaxies were essentially identical. Each model galaxy consisted of a halo, disk, and bulge. The mass of the disk was about one fifth that of the halo (which was not nearly as extensive as real halos), and the bulge mass was a third of the disk's (i.e., a rather small bulge). The bulge and halo were modeled with collisionless particles. In some of these models, the disk contained particles representing both gas clouds and stars (about twice as much mass in stars as in gas clouds). About 45,000 particles were used in each model galaxy. The dissipative processes in the gas were treated in a very approximate way, e.g., the gas clouds were all assumed to have the same temperature at all times, but nonetheless such processes were included. Models including gas dynamical processes were inevitably simple at that time (and often only moderately more complex now). One reassuring if not too surprising result was that the various galaxy components were highly distorted in the collision and merger, just as in earlier models. In particular, the formation of long tidal tails was not disrupted by the more realistic and dynamic modeling of the halo. They formed much as in the early Toomre and Toomre models. A more surprising result was that the collision pried the various components of the galaxies apart to some degree. Specifically, the centers of halos were slightly separated from the disk plus bulge components. Dynamical friction worked somewhat differently on the different components on average. A gratifying outcome of these models was that the major components merged quite rapidly, as in previous models of the merger of elliptical galaxies (which after all are much like the merger of two halos). Thus, these models revealed the presence of robust tidal tails in the halo of the merged galaxies. This was in contrast to the limited earlier generations of models, which generally either produced tails, but did not include dynamical friction, or modeled friction, but could not resolve tails. The models including a gas disk component yielded an even more spectacular result. Gas and star disks were both extremely distorted by the strong tidal interaction in these collisions between equal mass galaxies. Despite the fact that conditions are much more extreme in this case, there are qualitative similarities with the tidal effects in prograde collisions discussed in Chapter 3. The basic, fundamental physical processes are the same, of course. Recall that the tidal forces in a brief (impulsive) prograde encounter spun up two opposite sectors of the target galaxy disk, and spun down the other two sectors. Material in the spun up sectors could be thrown outward to form bridges or tails. In the other two sectors, material could move inward to an ocular ring (and eventually spiral waves). The gas clouds in these sectors collide and dissipate energy, and so, would eventually settle into a more compact disk. This more compact disk would be more prone to gravitational instabilities, resulting in stronger spiral waves or the formation of a bar component. Analogous processes occur in the equal mass merger cases, though both

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galaxies may be regarded as primary. Moreover, the processes are more violent; strong waves and bars form rapidly in the merged disk or disks. (If the disks are in the same plane they will quickly become a single disk in the merger; if they originally had very different tilts, two tilted disks may exist for some time in the merger remnant.) As a consequence of the violence of the merger, the dichotomy between the material that is thrown out (stars and gas) and the dissipative gas that is compressed in the merger remnant is much greater than in a mild prograde encounter. As noted above, strong waves and bars are very effective in transporting angular momentum outward. Basically, the tidal tails carry away the rotational momentum of the gas disk, with the assistance of shock waves formed in the spirals and bars. The gas left behind is no longer supported against the galaxy's gravity by centrifugal force and falls into the central regions. This was probably the most important finding of this generation of models. Not only can significant quantities of gas be thrown out of galaxy disks (at least until it falls back much later), but large masses of gas can be concentrated in the central regions. When I first heard of this result, I was doubtful about its correctness. Having worked with numerical simulations for many years, I was well aware of their limitations, and I was skeptical about the accuracy of the calculations in the central regions of the merging galaxies, and the validity of the approximations made in the gas physics. It also seemed that the concentration of the gas was too large and occurred too quickly to be compatible with the observations. However, later and better resolved calculations have confirmed the basic correctness of the merger gas dynamics. My impression about the observations was also too conservative; very large masses of gas were found by radio telescope observations of young merger remnants. The fact that the models showed how major mergers could funnel gas into the central regions of the remnant was very exciting. This explained the superstarburst phenomena. Cold interstellar gas clouds are the fuel for the fires of star formation. The ability of bars to channel this fuel efficiently into the central regions created the environment conducive to the formation of hundreds of massive clusters.

4.7 Starbursts, w i n d s and nuclear activity

What happens after the super-starburst turns on? It is clear from the enormous energy output of ULIRGs that they cannot continue their profligate ways for long. The energy of the super-starburst generates heats the gas, and stellar winds and supernovae would push the gas clouds out of the central regions. Collectively, these processes are called "feedback effects." One of the primary mechanisms of feedback, galactic winds or superwinds, was also discovered in the late 1980s and early 1990s. Specifically, spectroscopic observations showed gas outflows at hundreds to thousands of kilometers per second from the centers of starburst galaxies. Observations of this type of outflow

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from M82 had been available for decades, but they were mysterious, and thought to be anomalous. For example, in the 1970s, M82 was often described as an "exploding galaxy," and there were not thought to be many of those around. Other galactic winds were more distant and harder to observe. (However, William G. Mathews had studied and modeled general galactic winds from the early 1970s.) The discovery of ULIRGs and the realization of inevitable feedback effects motivated new and successful searches for winds in relatively nearby starburst galaxies. In recent years, studies of galactic winds have helped answer many interesting questions from how dwarf galaxies evolve to how the intergalactic gas (between the galaxies and in galaxy clusters) became enriched in heavy elements produced in stars. However, we will not stray too far from our path to consider those topics. Feedback and winds play several important roles in determining the evolution of super-starbursts in major mergers. For example, in a very short time the young winds and supernovae must stir a great deal of gas turbulence in the merger core. It is likely that for a time this turbulence is more or less contained, i.e., confined to the core region. Still, it increases the local gas pressure, and probably the dissipation of momentum. Thus, some gas is tunneled down to even smaller radii, until it is within the grasp of the central black hole. A central black hole - where did that come from you might ask? I have to admit that this portion of the path through the forest of merger studies is not especially straight, narrow or unique. There are many turnoffs. Having just decided not to follow one to "galactic winds" we now encounter one to "central black holes." Again we will not make the turn, but it is worth reading a few words in the guidebook about it. To begin with, we have mentioned AGN (active galactic nuclei, or just active nuclei) above as compact sources of high energy output. AGN come in a variety of types, including quasars, blazars, Seyfert galaxies and radio galaxies. This zoo of AGN is yet another side trail we will not investigate. The salient points for us include the facts that AGN come in a range of types and span a huge range of luminosities, with all but those at the weakest end of the spectrum equaling or exceeding a significant fraction of the luminosity of all the stars and other emitters in the galaxy. Quasars, for example, usually outshine the galaxies containing them, in virtually all wavebands. However, despite their diversity the so-called central engines or power sources of AGN are thought to be, in all cases, accretion from a frictional gas disk onto a supermassive black hole. The term "supermassive" means with masses between about 10 6 and 10 9 times the mass of the Sun. They are believed to be ubiquitous components of galaxy nuclei (in part as a result of Hubble Space Telescope studies of motions in galaxy cores). Our Milky Way galaxy has a relatively modest one, at about 3 x 10 6 times the mass of the Sun. They are also believed to have formed early in the lives of galaxies. One piece of evidence in support of this belief is that the most distant objects observed are quasars, and the travel time for the light we receive from these quasars is a large fraction of the age of the universe. Like a strong starburst the accretion disk that feeds the AGN engine needs fuel.

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In a major merger the central black hole is the third one to get fed. First, there is the compression of the central disk leading to gravitational instability and bar formation. Second, there is the super-starburst, and the turbulence generated by it, which almost certainly results in feeding the monster. I hedge this last statement a little because the actual AGN feeding is very difficult to observe directly since it is buried deep within the dusty merger core, and occurs on a very small scale. We know little about the details. Nonetheless, there is a strong circumstantial association between starbursts and AGN. As noted above, a rather substantial literature has built up in the last couple of decades around techniques to distinguish between these two emission sources in deeply hidden galactic nuclei. The result is a number of useful techniques applicable to different observations of various types of source. In 1988, a team of Caltech/IRAS mission scientists (David B. Sanders etal.) proposed a very influential evolutionary scenario for merger ULIRGs, based on an extensive analysis of the available observations. Their scenario included the feeding processes just described. This is followed by a second stage in which the cumulative feedback effects, not only from the starburst, but also from the intense radiative output from the AGN, become strong enough to drive the gas and dust from the nucleus in a strong wind. The active nucleus is then revealed to direct observation, and they believed that it would have a sufficient fuel supply to persist for some time. Thus, they suggested that this was, in fact, the birth process of a quasar. The similarity to the much smaller scale birth of a single massive star is striking. This team did not invent the theory of "merger to gas tunneling to superstarburst to AGN and gas expulsion" from scratch. It was more a matter of assembling the pieces of the puzzle. Some of the pieces of the puzzle had been around for a long time. For example, Toomre and Toomre in their 1972 paper had suggested that galaxy collisions could lead to enhancement of both star formation and nuclear activity. Some of the pieces, like the AGN-starburst connection, were very au courant with many researchers investigating aspects of the topic. Indeed, this has continued to be true to the present day, and there have been many refinements of the Sanders et al. picture. Nonetheless, their paper has been very influential. At the present time it has been cited more than 1100 times in the astronomical literature, not far behind Toomre and Toomre's seminal paper, which has more than 1600 citations (according to the NASA Astrophysical Data System). A possible example of the mergers-to-quasars scenario is found in the quite nearby galaxy NGC 5128. Several different views of this galaxy, derived from ground-based and satellite observatories are shown in Figure 4.8. The optical picture in the lower right corner looks like an elliptical galaxy over which someone has drawn a rather strange moustache. The moustache is thought to be the unmerged remnant of a companion galaxy, essentially a polar ring, like those discussed at the end of Chapter 2. The radio and X-ray observations reveal another large-scale component of the galaxy, a huge radio jet, which originates at the active nucleus. Although we do not know all the details, it seems very

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Figure 4 . 8 Multi-wavelength views of the relatively nearby merger remnant NGC 51 28, showing the elliptical galaxy, the accreted dust lane and the nuclear jet (NASA, CXC, R. Kraft, etal., Harvard-Smithsonian Center for Astrophysics, NSF, NRAO/AUI/NSF, M. Hardcastle, etal.; European Southern Observatory, M. Rejkuba, etal.).

likely that some of the interstellar gas from the disrupted companion found its way to the nucleus to feed the activity of a pre-existent supermassive black hole. Since the jets are more or less perpendicular to our line-of-sight, it is likely that the AGN is hidden from view by its accretion disk (in addition to the polar ring), so we get only an indirect view of its activity. Another aspect of the Sanders etal. picture is very interesting. They evidently envisioned the merger, starburst, and gas dispersal phases as relatively short, and the AGN activity phase as longer. If this were correct, the model could avoid the obvious criticism that the host galaxies of quasars looked like more-or-less ordinary elliptical galaxies, not messy remnants of the merger of gas-rich progenitors. On the other hand, there is a good deal of evidence that the duty-cycle (specifically the on-time) of nuclear black holes is a small fraction of the age of the universe, so the lifetimes of mature quasars are not that long. However, there are ways around the apparent contradiction. The early quasar host observations were biased to the local universe and the current epoch, where quasars are relatively rare. With the instrumentation of the 1970s it was very hard to observe

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Figure 4 . 9 Hubble Space Telescope images of quasar host galaxies from 1997 (NASA/ STScl/AURA), ). Bahcall, M. Disney).

quasar host galaxies, because they were so faint relative to the bright quasars themselves. Later observations of distant quasars hosts, e.g., in the late 1990s using the Hubble Space Telescope, have shown some pretty messed up quasar hosts. Some of these have good morphological evidence for interactions or mergers (Figure 4.9). However, studies that are more recent show that the small number of objects, and the way they were selected, probably gave an incomplete picture and false optimism about the mergers to AGN picture. On the other hand, a number of studies since the late 1990s have shown that the nuclei of many AGN hosts have "post-starburst" spectra, i.e., aging starbursts, as would be expected in the Sanders et al. model. Thus, the basic conclusions of the Sanders et al. model seemed to hold up, though it really needed a catchy name. Although it is not all that catchy, I will continue to call it the mergers-to-quasars model. And there are still lots of loose ends, e.g., what about those nearby quasars that are in ellipticals; where did they come from? Perhaps in the present universe, meals for the nuclear black hole monsters have gotten rather slim and far between. Nonetheless, one can envision (with the help of recent cosmological simulations of the continuing build-up of galaxies) that these large galaxies continue to capture and occasionally ingest a much smaller galaxy. The result would leave much less evidence than a major merger, but still provide fuel for the fires of a black hole accretion disk formed long ago in more violent times.

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4.8 G r o w i n g quasars

Then there is the question of what it means for a quasar to form in a major merger? Certainly, a major merger is a great way to feed the central black hole maybe even feed it a good fraction of its original mass, and thus, build it up substantially. However, the presumption of the model was that there was already something there to feed. Actually, for a merger of two nearly equal galaxies there may be two "somethings" there to feed. Merger models have advanced to the stage that the orbital evolution of those two massive black holes has been studied in several recent papers. Ultimately they will spiral together to make a much bigger black hole regardless of how much each got to eat on the way down, but exactly how this works and how long it takes are complicated topics. A blanket of ignorance also extends over the subject of the true origin of the central black holes. While I will not attempt to answer the origins question, it is worth taking a little time to consider some of the issues about the in-spiral and coalescence of the two supermassive black holes in a major merger. Retracing our earlier discussions of dynamical friction is a good place to start. Even if the merger occurs at high redshift and the black holes are far from fully developed, they will be much more massive than the stars in the core of the merger remnant. There will be many of the latter, and they will likely be in a very dense configuration. Thus, we can envision our massive, point-like black holes moving through a dense sea of low-mass stars, which would seem to be an excellent situation for the application of the Chandrasekhar theory. We would expect that the answer from this theory would be a rapid in-spiral of the two black holes. Black holes are quite literally holes in our universe. Physicists believe that the space-time around their centers is strongly warped. A consequence of this is that they are surrounded by a spheroidal surface, the event horizon, such that no signal can come out to our universe from within that surface. Jakob Bekenstein and Stephen Hawking have shown that when two black holes collide they will coalesce into a single black hole, and the horizon surface of the new black hole will be larger than the sum of those of its precursors. Can we then conclude that in a merger, the two nuclear black holes quickly spiral together to form a new bigger one? Almost, except that we need to handle the word "quickly" a bit carefully. Chandrasekhar's theory would make one expect black hole coalescence in the central regions on a timescale of a few orbital times. Orbital timescales in the galaxy core are relatively short compared to those farther out in galaxy disks, so this could be very short indeed. Unfortunately, there are complications that make the story much less straightforward. The first one concerns the question of how the two black holes reach the core of the merger remnant. This is relatively simple to deal with. The two black holes were originally contained within the nucleus and the bulges of each of the precursor galaxies. Large bulges do experience strong friction according to merger simulations, and quickly coalesce, bringing their cores into the region of the common core. If either precursor lacks a substantial bulge this

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may increase the time it takes its core to reach the remnant center, but probably not by a huge amount. Once the two black holes reach the remnant core, does the Chandrasekhar theory correctly predict their rapid coalescence? As we will see in the next chapter, Agris Kalnajs discovered an effect in cored nuclei which could give us some reason for doubt. In fact, this situation is even more complex. Once they get relatively close, for example within a few hundred parsecs of each other (which distance is still orders of magnitude larger than their horizon size), the gravitational forces from the two black holes are great enough to strongly affect the distribution of stars around them. In essence, they can scatter the nearby stars away before those stars can exert enough friction to draw the two holes together. Models show that this type of effect can greatly extend the lifetime of the black hole binary, even though some orbital energy may be lost in the scattering process. On the other hand, stars can also be scattered into the vicinity of the black hole binary by the complex dynamics of the relaxing merger remnant. Moreover, gas clouds or streams can also be tunneled into the vicinity of the binary. It is much harder for the black holes to scatter extended dissipative clouds, and more likely that the clouds will contribute substantially to the friction. However, the gas can be heated and blown away. It can also form new stars, introducing even more complexity to the problem. Thus, we see that, of all the complex processes involved, the question of the lifetime of the black hole binary can be very hard to work out, even with detailed computer models. In that case, we might turn to observation for some answers. One basic observational input is that a number of nearby merger remnants (and a few not so nearby ones) are observed, for example in the radio continuum or in molecular lines, to have double nuclei. Since it is now believed that most galactic nuclei host supermassive black holes, it is likely that these double nuclei do too, but the observations do not provide direct evidence of that. Recently, two groups, a Princeton group lead by Xin Liu, and a primarily California group led by Julia M. Comerford, have assembled moderately large samples of galaxies with double-peaked emission lines in the optical. These oxygen emission lines are indicative of active nuclei. The double peaks could be caused by distinct emission structures (clumps?) around a single nucleus, but in at least some cases, subsidiary evidence suggests an origin in distinct nuclei. Here the astronomers seem to be finding real cases of in-spiraling nuclear black holes. Examination of the statistics of such systems and studies of their detailed properties should help to answer questions about the in-spiral process in the near future. In fact, in a somewhat more distant future, we should be able to detect the final coalescence and merger of supermassive black holes. According to General Relativity theory, the strong, time-dependent warping of space resulting from such a merger sends out ripples in space-time known as gravitational waves. It has not yet proven possible to detect such waves directly, but a planned spacebased observatory (LISA, the Laser Interferometer Space Antenna) consisting of a set of several satellites should be able to do so. That will be an exciting detection!

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Meanwhile in the last decade there has been a great deal of work on the general question of black hole growth, pushed forward both by large surveys of galaxies and their properties like the Sloan Digital Sky Survey, and also by the exponentially increasing study of galaxies at high redshift. The latter studies are possible because of the capabilities of the latest generation of large ground-based telescopes, e.g., at the Keck, Gemini, and European VLT observatories. It would not be much of an exaggeration to say that the field of high redshift extragalactic astronomy or galaxy archaeology has been born in the last two decades. Among the major recent discoveries are scaling relations between the mass of central black holes in galaxies and properties of the parent galaxy bulges (e.g., black hole mass versus luminosity and the random velocity component of the bulge stars). These relations are observed both in the local universe, and in the more distant, high redshift universe, albeit with some modifications. The point here is that bulges and central black holes evidently grew together. Both galaxy components can grow as a result of mergers, so the scaling relations can be understood in an evolutionary picture where their growth is largely driven by mergers. This has been demonstrated by recent numerical models. This picture is also in accord with the dominant hierarchical build-up picture of galaxy formation, which we will consider shortly. In fact, we seem to have a merger of theories here: mergers-to-quasars plus hierachical build-up of galaxy components yields the build-up of quasars (actually central black holes) from sequential, hierarchical mergers. Call this hierarchical-mergers-to-quasars. This theory not only addresses the question of how to turn on a quasar, but also steadily builds the central black hole. It is a very pretty elaboration of the original merger-to-quasar theory, but is it correct? One evident consequence is that quasars would be more common in the distant past, because mergers were, but the quasars were maybe less luminous, because the central black holes were not so supermassive. A few very luminous quasars have been seen at very high redshifts, but we do not yet have the kind of comprehensive data needed to evaluate that conjecture, only a few preliminary probes. Another consequence has not changed much since the time of Sanders et al. - if the hierarchical mergers-to-quasars theory is correct we still expect to see left-over evidence for mergers around quasars and other AGN. Recent observations are more complicated. Studies based on the Sloan Digital Sky Survey find that relatively nearby AGN are located in the most massive galaxies, which would seem to agree with the idea of hierarchical build­ up of central black holes. Moreover, the old result that most of the nearby quasars are in mature, early-type galaxies has been modified. Many are found in early-type galaxies, but these are often somewhat peculiar for their class, at least in the sense of having significant young populations. Nearly an equal number of AGN are found in isolated disk galaxies, and a similar number in interacting galaxies. This is a rather a significant modification! Moreover, samples of interacting galaxies show that these systems are more likely to contain an AGN than galaxy pairs with no signs of interaction. Nonetheless, not all, or even a majority of AGN in the local universe show the signs of a strong interaction, or

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a recent major merger. The observations are probably not yet complete, nor sufficiently sensitive to tell us about the role of minor mergers in fueling nuclear activity. Studies at higher redshift are less sensitive, and so more prone to selection errors. However, AGN activity is most common at a redshift of about 2, when the universe was about one-third of its present age. It would certainly be interesting to study AGN systematics at the time of their peak, as well as their evolution since then. Several recent studies have been made with AGN samples containing hundreds of members (similar to recent studies of nearby AGN) and extending out to a redshift of about 1. Unfortunately, the situation seems complex, and it is likely that even these fairly large samples are not adequate to decipher what is going on. To begin with, AGN are found in different kinds of nuclear environments (e.g., with different amounts of obscuring dust) using observations in different wavebands (e.g., radio, infrared or X-ray). There are good indications that these (observationally) different types of AGN have different types of host galaxy. This makes sense, since different types of host have different amounts of gas and dust, different total masses, and can be at different evolutionary stages. All of these properties can possibly influence the nuclear environment. One of these studies, by Ryan C. Hickox et ah, has suggested a revised version of the Sanders et al. picture of AGN formation, which depends on the mass of the host galaxy (Figure 4.10). Major mergers are an important part of this revised picture, but they are not the only, or even dominant player. As a second example of the complexity of this topic, we note that there is now some evidence from these studies that AGN hosts are no more likely to have close companions than the isolated galaxies of a comparison sample. This rather contradicts the early Hubble findings cited above, but those results were based on very small samples, and there is a possibility of bias (what astronomers call "selection effects") in any such sample. The implication that strong interactions and mergers are no more common among AGN hosts than other galaxies, and that most AGN show no evidence for interactions, seems to undermine the idea that the former are the primary cause of the latter. However, there are several counter arguments. First, experience at low redshift tells us that merged galaxies do not necessarily have (other) close companions, and can appear relatively normal in moderate resolution observations after quite short relaxation times. Second, the theory does not require that all interactions trigger an AGN, and there may be many that don't. Third, there are different kinds of merger, involving more or less gas-rich progenitors, of different masses and evolutionary states, which may leave more or less obvious signatures after the merger is complete. I have led us more than knee deep into this messy subject, if not quite over our heads, for a couple of reasons. Firstly, to show that it is very complex, and that we still have a great deal to learn. Indeed the literature on starburst-AGN-merger connections is vast, and the nuances are well beyond the scope of this book. Secondly, to show that a great deal of observational evidence is being acquired in ambitious surveys using some of the most powerful new instruments (as well as

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Initial halo mass (and clustering bias) High

Medium

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Figure 4 . 1 0 Schematic merger-to-active galactic nucleus (AGN) evolutionary scenarios. This figure emphasizes the roles of environment, progenitor galaxy mass, and cosmic time in driving different forms of nuclear activity (from R.C. Hickox, et al., Astrophys. /., Vol. 696).

the good old HST). Thirdly, this area, more than most others, shows that while we know that major mergers play an important role in the big picture of galaxy evolution, and are responsible for many important processes in galaxies, we do not yet understand all the mechanisms. Nevertheless, we are pushing ahead. In fact, we have pushed way ahead with the observational side of this story, leaving the models in the dust again. So let us return to the Barnes and Hernquist models of the early 1990s for a moment. Their result of strong gas funneling resonated well with the Sanders et al. theory and analysis of ULIRG observations. However, their models did not include star formation, or feedback, and so, could not say anything about the consequences of the gas inflows.

4.9 Merger models with feedback In the subject of galaxy merger studies in general, and computer models of mergers in particular, the time period from the mid-1990s to the present is

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analogous to the industrial revolution in Europe. In the beginning of the period, we can identify a few influential modelers, analogous to James Watt and his steam engines, beginning a process that will soon accelerate rapidly. By the end of the period, there are so many people involved that we speak of industries and output statistics. In astronomy, compared to industry, this is hyperbole, but with papers relating to mergers coming out almost every day, it feels that way. Among the most well known works of the mid-1990s were a series of papers by J.C. (Chris) Mihos and Lars Hernquist. In many ways, their work continued the earlier work of Barnes and Hernquist, but in the new studies they were especially interested in incorporating the effects of star formation in these models. A number of simplifications were adopted to make this tractable with the computer resources of the time. These resources had increased greatly from 20 years earlier, when computer modeling of star clusters and the stars in galaxies had begun. Nonetheless, the challenges of including even rudimentary interstellar gas dynamics and massive star feedback effects are great, even by present standards. The fundamental approximation made by most researchers attempting this task at that time was to treat the gas as "isothermal," meaning all of it was at a single temperature at all times. On the face of it, this is a ridiculous approximation for the interstellar gas in galaxies like our Milky Way, where this gas is observed (in many wavebands) to have a huge range of temperatures, densities and pressures. By huge I mean from 10 degrees above absolute zero to millions of degrees Kelvin (or Fahrenheit for that matter). However, we need to look at this situation with a more discerning eye. While some of the interstellar gas is very hot, this is only a tiny fraction of the total. Most of the mass of this gas is in cold to warmish cloud phases. Although they still encompass a large range of temperatures, these cloud phases have some important properties in common, which they do not share with the hottest phases. For example, their modest temperatures correspond to sound speeds (e.g., within individual clouds) that are much less than their orbital speeds within the galaxy. That is, they are moving supersonically, and so, when they collide with clouds from another galaxy strong shock waves form, and result in a great deal of energy dissipation (as discussed in Chapter 2). In addition, because the random motions of the clouds are much smaller than their orbital speed, the gravity of the disk is generally able to confine them quite closely to the disk mid-plane. Finally, and perhaps most importantly, the cool gas is generally found in dense clouds, where except under extreme circumstances the gas is able to quickly radiate away any heat input, stay cool, and keep a large fraction of the interstellar gas cool. The "extreme circumstances" include the formation of a star cluster with a number of massive, hot stars in the cores of large clouds, and the very strong shock waves formed when two gas disks collide at relative speeds of a few hundred kilometers per second or more. However, even when the gas is heated to quite high temperatures in those circumstances, much of it can cool and find itself back in large clouds in relatively short order; that is, on timescales much

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shorter than those characterizing the global dynamics, which are typically hundreds of millions of years. Thus, to adopt the isothermal approximation is to say that only the clouds exist, and the warm-to-hot inter-cloud or diffuse cloud material is viewed as ignorable fluff. In meteorology this might be analogous to deciding to ignore all clouds that aren't thick enough to produce rain. If you are mainly interested in precipitation, it could be a useful approximation. Still, it can also be very important to know the temperature and humidity of the cloudless air, to know how storms will develop. My own research in this area has often focused on the temperature/humidity side, rather than the stick to the big clouds perspective. In the mid-1990s I was undertaking studies of galaxy collisions that involved a range of temperature phases, in part to see how the balance of phases was affected by galaxy collisions. Because of computation and other limitations, various simplifying approximations had to be used, and only a restricted range of colliding galaxy dynamics could be studied. The isothermal approximation was a good place to start for many more broadly based studies in this field. A second area where rather simple approximations had to be made was in the determination of when a gas cloud begins to collapse and starts to form a star cluster within. As we will discuss in a later chapter, a key idea is that clouds build up and then gravitationally collapse in regions where the gas density is sufficiently high so the net self-gravity is strong enough to overcome forces that resist collapse, like pressure or the relative stretching of different parts of the region. This theory was developed for isolated galaxy disks, but although conditions are more dynamic, it should still apply in colliding galaxies. The theory predicts a threshold density, above which gravity wins and star formation should result. This value of the threshold density is not a universal constant, it varies smoothly in the disks of isolated galaxies. In colliding galaxies its value probably varies much more, and truly must be evaluated locally. In the computer models of the 1990s, this theory was incorporated in various more or less simple ways. The simplest was to adopt a single value for the threshold density throughout the simulation. This is not quite as crude as it may sound. The strongest star formation is likely to occur in regions of very high compression, where the threshold is easily exceeded regardless of its precise value. Another way of determining the threshold was to compute the magnitude of the various forces in local regions, and wherever self-gravity dominates to turn on star formation. This is not as good as it may sound. The limited particle or mass resolution of these models translates into very limited resolution in these force calculations. The isothermal approximation, or even simple heating and cooling approximations, can give a rather inaccurate view of the thermal state of the gas, and introduce inaccuracies in calculations of pressure forces. The saving feature in all cases is that the biggest difference between the behavior of stars and gas clouds in galaxies, is strong dissipation in supersonic collisions between gas clouds. This effect could be reasonably well represented in many approximate treatments. A third area where big approximations had (and still has) to be made was in

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the treatment of feedback effects. What has been commonly done is a bit like approximating the history of a war as the explosion of a single huge bomb. Specifically, it is often assumed that when the conditions are right a star cluster forms, and it is then assumed to die immediately, with a prompt input of energy, and sometimes momentum or expansion velocity, to the surrounding gas. Those who study the details of star-forming regions within our galaxy know that the details are vastly more complex. Moreover, the star-forming regions in our galaxy are quite mild compared to many of those in colliding galaxies, where the details must be even more complex. Nonetheless, many of these complexities are confined within the young star cluster and its natal gas cloud. Therefore, in computer models of galaxies where such regions constitute little more than a dot on the screen, and the lifetime of the most energetic stars is a tick of the clock, ignoring the details makes sense. Moreover, in the 1990s astronomers did not have a very good idea of exactly how much energy and momentum were returned to the interstellar gas anyway. Our knowledge of those factors has greatly improved in the time since, but reasonable guesses could be made then, and the results compared to observation. Another rough and ready truth in this area is that although feedback effects can assume great importance in a few specific contexts, they are generally a secondary effect in galaxy dynamics. The early models were good enough to heat the gas in the volume around a starburst and thereby turn the burst off. This is probably the most important feedback effect, since a primary goal of these models was to study the history of star formation through the merger process. The careful reader might wonder what "feedback heating" would mean when the constant temperature, "isothermal gas" approximation was being used? I do not want to delve too deeply into the technical details of the computer models, but this question does raise an interesting point. In many of these models, elements of interstellar gas (like real interstellar clouds) are often treated as particles, just like representative stars or dark matter particles. The difference is that they experience pressure, and other dissipative forces. In this representation there are two kinds of temperature. The first is associated with each particle, and can be thought of as the gas temperature "within" the particle. If heating and cooling processes are calculated then they change this temperature. If the gas is isothermal, then this temperature is constant, and the same from one particle to the next. The second temperature harkens back to the basic definition of temperature as the random motion of molecules in a gas, which we discussed earlier. In this case, it is the random motion of the gas particles, representing the random motion of interstellar gas clouds. Even if the first temperature is kept "isothermal," the second temperature does not have to be. Specifically, in many models feedback was implemented by increasing the random velocity of particles close to those that turned-on star formation, as well as the star-forming particles themselves. This is a way of simulating how the heat and explosive inputs of young star clusters push around the gas clouds in their immediate vicinity. Though simple, it is a clever representation of a complex process. Incidentally, the formation of such a star cluster will convert some of the gas

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into stars. This "gas depletion" was also kept track of in some models. For example, gas particles that had experienced several rounds of star formation could be turned into star particles. This is another simple but effective treatment of a complex process. A little aside on the philosophy of science - it may not be true, but it certainly seems like most of the simple effects and processes in science were discovered long ago, and modern science constantly confronts increasing complexity. The time-honored way for scientists to try to deal with that problem is to try to break complex realities into more tractable parts or basic interactions. This basic reductionist approach does not always work, and it requires a hierarchy of physical effects. Either some are more important than others to various degrees, or they operate on very different scales, and so, can be separated to some degree of approximation. This was done rather successfully in the model approxima­ tions described above. What did we learn from this generation of models? Well, they taught us about how the total star formation rate varies over the course of a major merger. Perhaps I should say that they confirmed a number of things that were suspected from earlier work and put those results into a coherent framework. For example, like many symphonic or operatic works, the net star formation initially starts out slowly, but then builds in intensity (through early encounters), until it rises to a crescendo (in the merger), and backs down to a quiet conclusion (with gas consumption or dispersal). As discussed above, most aspects of this evolution had been observed. Many parts of the dynamics had been modeled on the computer previously, but this generation of models showed the consequences in star formation and feedback. For example, much of the early rise of star formation was confirmed to be due to strong waves or bars driven by the early encounters. As can be inferred from models like that shown in Figure 4.11 there is a great deal of variety in the star formation history, which largely reflects the variety in the nature of the early encounters, and the resulting waves. As already described, factors such as the closest approach distance in an encounter, the relative velocity at closest approach, and whether that encounter is prograde or retrograde, strongly affect induced waves. The wave structure and evolution determine the star formation rate in turn. The way the waves redistribute the gas, especially driving substantial masses into the core, triggering starbursts and forming compact distributions of young stars could all be observed through the course of a model. The details could and should be doubted, given the rough approximations used, but for the first time the whole process could be "observed," at least on the computer. New effects could also be studied. For example, Mihos and Hernquist found that it was much easier to induce nuclear starbursts in galaxies that did not have strong bulge components. This is because galaxies without such bulges could more easily form strong bars, and bars are very effective in tunneling gas into the central regions. There are complications and this would be a difficult effect to detect in the

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Time Figure 4 . 1 1 Sample star formation history from a 1 996 merger model by L. Hernquist and ). C. Mihos, Astrophys. /., Vol. 464. Note the discreet starbursts associated with specific dynamical events.

observations of the time. In studying merger remnants one would have to be sure that the bulge consisted of old stars in a relaxed distribution, to confirm that it was not formed in the merger itself, out of either young or old stars. Secondly, one would have to correct for trends like the generally lower gas fraction in early type disk galaxies with bulges. High quality observations of a large sample of galaxies would be needed. Only recently, with data obtained in large surveys like the Sloan Digital Sky Survey, has it become possible to test such predictions. To the best of my knowledge a definitive study on this question has not been published to date. On the other hand, there are some effects that could not be studied very well with the models of the 1990s, including galactic winds and the feedback produced by supersonic jets driven by AGN. In the latter case, the phenomenon involves such a small fraction of the gas, and the launching of the jet occurs on such a small scale (roughly a light year), that it could not be resolved in simulations of the dynamics of the galaxy as a whole. Though not as extreme, these resolution problems were also a source of the difficulty in observing winds in such models. The observations suggest that the gas mass in winds is roughly equal to that in stars formed in the nuclear burst that drives it from a region of roughly 1000 parsecs in size. Only about one percent of the galactic gas might typically be involved in the nuclear starburst. In models that commonly used 10,000-100,000 gas particles, less than 100-1000 particles might be in the wind, which is a very low number for a structure that would fairly quickly spread over a

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very large volume of space. The shock waves and coexistence of gas at many different temperatures could certainly not be resolved. For this reason, simplified models of the inner few thousand parsecs of galaxies have been used to study the wind formation process itself, not including the large-scale dynamics that fed the starburst. I have gone into a little detail about these models and their results, to give you some idea of what could be achieved with such models, what could not, and how they were rapidly developing to achieve still more as computer capabilities increased. The progress was accelerating so much that even professional astronomers had somewhat unrealistic expectations of what could be simulated with the computer models. Nonetheless, the progress has been even more explosive in the present decade. As one might expect, this progress certainly consists of larger scale computations, with more particles, and much finer resolution in space and time, which allows more detailed comparisons between models and observa­ tions. With these resources most of the same kind of modeling carried out in the 1990s is still done, but applied to more specific galaxy systems, or to the study of new kinds of physical processes in galaxy collisions. However, that is not the half of it. Even more impressive is the vast range of new or enhanced types of modeling. I will not attempt to provide an overview here. We will consider some specific examples in later chapters. I will only mention two specific areas. The first is feedback effects. It remains true that the details of how a young star cluster or an AGN drives these effects cannot be modeled very realistically even now. However, a variety of different approximations and phenomenological treatments has been explored in different applications and their limitations are much better understood. Moreover, they can be applied over a much larger range of scales, and their affect on different scales can be evaluated and compared with observation. The second area is the study of galaxy collisions occurring naturally within very large-scale simulations of cosmological structure formation. We will discuss this more in the next section and in a later chapter. For now, it is sufficient to point out that the discussion so far has been constrained by the rules we adopted at the beginning of Chapter 2. That is, we would mostly limit consideration to collisions between two galaxies, and implicitly, to collisions which occurred in isolation from the rest of the universe. Those are good approximations in many cases. On the other hand, we also discussed how galaxy collisions are not random events. They are the result of the formation of larger structures, like groups and clusters, which bring the collision partners together. We are saved from contradiction by the fact that the two colliding galaxies are often able to complete the merger process before another is brought onto the scene. At least we believe this is true in the present universe. If collisions were much more frequent in the early universe, then interactions between three or more galaxies may also have been much more common. Cosmological structure formation models are run from a time before galaxies formed, and simulate that process as well as the build-up of groups and clusters

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of galaxies, and the super-clusters connecting them. Once galaxies and groups have formed these models show them being pulled together into still larger structures. These simulations are also able to model the galaxy (and group and cluster) mergers involved in that process with moderate resolution. For a given set of cosmological assumptions, these models can show what kinds of galaxy collisions occur in different ages of the universe, and how frequently they occur. This is a huge step forward that astronomers have only begun to exploit. An inevitable side effect of the increased model complexity is that the teams that develop these models and analyze the results are growing ever larger, just as in other areas of science. The corollary is that it is very difficult for one or two individuals to do such work, and as the group size grows, so the number of groups decreases, since there are few institutions or even funding agencies that can support such efforts. Most of the leaders of the large groups are people who acquired their skills in very small groups. On the positive side, the large groups have the resources to be able to make the raw output of their model runs, and in some cases their software, available to the broad research community. Increasingly, the simulation codes and individual runs or groups of runs, are large enough to be given their own names.

4.10 Ellipticals and mergers again

The combination of ever improving models of major mergers between spiral galaxies, cosmological structure formation models, and observations out to high redshifts, where galaxy formation was still a very active process, provides a great deal of new input to the issue of how elliptical galaxies form. I do not want to give the impression that the problem is solved. As we will discuss in this section, there has been a great deal of ferment in this field in the last decade, and it is not entirely clear that it is settling down yet. About the time that Alar Toomre was working at M.I.T. on the idea of building elliptical galaxies out of the major merger of two spirals, William Press and Paul Schechter were developing a different conceptual framework for galaxy formation at M.I.T.'s rival institution, Caltech. The specific statistical formalism they developed is now called the Press-Schechter model, and the general idea is often referred to as "hierarchical structure formation." The basic idea is simpler than the title might suggest. It begins by supposing that the first gravitationally bound objects to form were much smaller than typical present-day galaxies. This is strongly affirmed in cold dark matter cosmologies, which we will discuss later. It then assumes that the dominant path to assembling galaxies is two-by-two merging. That is, every two of the basic objects merge to an object about twice as massive, then every two of these objects merge to form something with four times the mass of the original unit, and so it continues until the process yields galaxies with their current average mass. Of course, nobody takes this literally. The obvious outcome is that all galaxies would have the same mass when in fact they are observed to range

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over a factor of a million in mass. However, this can be partially accounted for by assuming that the process proceeds through many more steps in high-density environments, where there are lots of the initial building blocks, while in lowdensity environments, where there are relatively few building blocks, the process goes through many fewer steps. In reality, there are many more complications, but this is not the place to consider them. The point here is that this is a very different picture than Toomre's for the build-up of ellipticals, even though it involves many major mergers. If the basic building blocks are a million times less massive than mature ellipticals, then it require about 20 step-wise mergers to build one. (That is, 2 2 0 is about equal to a factor of a million.) In Toomre's picture one has to ask: where did the spirals come from? I believe, as discussed at the beginning of this chapter, that at that time most people had a monolithic collapse picture in mind for the formation of disk galaxies. Of course, they probably also had such a picture in mind for the formation of elliptical galaxies, as per Richard Larson's pioneering computer models in the early 1970s. However, when ellipticals were discovered to be slowly rotating, that theory became somewhat less plausible, and Toomre's alternative more attractive. We should be careful to note that hierarchical structure formation and Toomre's single (or perhaps final) merger picture are not necessarily contra­ dictory. We have not considered the timescale of the various stages of the hierarchical build-up. One could imagine that most of the steps of the build-up occur quite rapidly in the early universe. In this case, many disk galaxies could be left in relative isolation to complete a few final mergers over the long age of the universe in order to complete their "hierarchical" growth. Some of these final mergers could be major mergers between two near-twins, resulting in a Toomre elliptical. As I have described this scenario it sounds like a final merger might be a rare occurrence, but that is not necessarily the case. The cosmological structure formation models can provide some useful input on the question. Before exploring the results of those models, I need to discuss a useful tool for analyzing these simulations, and for discussing how difficult hierarchical galaxy growth from small building blocks really is. The tool is called "merger trees." Merger trees are the family trees of galaxies, but not much like human genealogical trees. To begin with, you have to limit them to direct ancestors - no aunts, uncles or cousins! The analogy is best between the hierarchical build-up for galaxies and the genealogy of an isolated ethnic group for humans. Each person comes from two rather similar parents, and all of the individuals in each generation are also rather similar. On the other hand, the analogy can break down in rather hilarious ways. In the galaxy world, essentially all the matter in galaxies was also present in the original building blocks. Thus, there are fewer individuals in each generation (no cousins again), and the individuals in each new generation are much larger. It would take a microscope to see your Neolithic ancestors if human genealogical trees worked that way.

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However, for galaxies the merger tree idea can be extended beyond the hierarchical case to include minor or micro mergers. For a biological analogy, this is like allowing some of the ancestors to be from different species, e.g., like allowing one of my ancestors to be an elephant married to a mouse! In principle, it can be extended to include steady, on-going accretion onto a galaxy, approximated as many micro mergers. The elephant could have been married to many mice over her relatively long lifespan. Ridiculous analogies aside, it is not hard to see how merger trees can be used to analyze the life history of galaxies in very large-scale simulations. At any time in the model, regions of high density (usually dark matter density) can be identified automatically. At all but the earliest times, these regions are mostly discrete galaxies. Specific galaxies can be identified at the end of the computer run, and one can follow back how specific galaxies and building blocks were incorporated to produce this final object. This is done automatically to produce huge catalogs of such histories and examine them statistically. One of the disappointing results of this statistical analysis is that illustrative merger trees, with images of the building blocks at each stage, are rarely published in the scientific literature, only the statistics. As an aside, I should note that merger trees were being produced before it was possible to carry out large-scale structure formation simulations. The PressSchechter model was from the beginning a statistical model, and has been developed significantly since that time. In simple terms, one can start with an initial distribution of building block masses (e.g., all the same), and invoke a random series of collisions from the evolving distribution to derive merger trees. This is the conceptual basis of "semi-analytic" models of galaxy formation, which we will consider shortly. To get back to the point - what is our current understanding of elliptical galaxy formation, how correct was Toomre's idea, and how important are galaxy mergers to the process? The last question is the easiest to deal with. Mergers almost certainly play an important role in the build-up of elliptical galaxies, and those mergers were almost certainly not exactly as envisioned by Toomre, nor in the simple hierarchical build-up model. The first question is too complex to be able to answer in this book; we will only hit a few high points concerning it. Toomre's idea remains very influential, so we will focus on what current research implies about its correctness. One thing that has been clear since about the time of Toomre's conjecture is that the stars in elliptical galaxies are very old, close to the age of the universe itself. Ellipticals have at most a small fraction of young or intermediate age stars. This basic fact poses a problem for any model that involves mergers that would produce a significant amount of star formation after the first few Gyr. Recent observations show that there are red, elliptical-like galaxies at high redshift. ("High redshift" is a rather loose term. Here I will use it for redshifts greater than about 2-3, or times of less than about one-quarter of the current age of the universe.) The usual view of hierarchical models is of a steady build-up of galaxies by successive mergers over the age of the universe, and in that view these

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facts can be rather inconvenient. A number of observational studies also indicate that these red galaxies are less luminous than present-day ellipticals, and so, presumably smaller. This does suggest continued build-up of ellipticals beyond very early times. A currently popular solution to this conundrum is to assume that from early times to the present, red galaxies continued to build up, but without much star formation. That is, they continued to experience so-called "dry mergers," involving very little interstellar gas. What was the nature of the build-up at earlier times? One strong hint is a class of galaxies called submillimeter galaxies. Despite the name, these are not microscopic galaxies. The name comes from the fact that they are very bright at submillimeter wavelengths, a waveband bridging more typical radio bands and the far-infrared band. As in the latter band, strong submillimeter emission is thought to originate in dust re-radiation of energy originally produced in starbursts or AGN. In fact, given a high source redshift, the dust emission may be in the far-infrared, but the wavelength of that radiation is further increased by the cosmological redshift on the journey to Earth. Thus, it is not entirely surprising that there is a good deal of evidence to suggest that submillimeter galaxies are the high redshift siblings of ULIRGs. The existence of high redshift ULIRGs, and imaging of some of them, suggests that like their nearby counterparts, these objects are produced in gas-rich major mergers. In some cases, the observations also suggest that more than two galaxies are involved. Given the high rates of star formation, and the long times to the present for those stars stars to age, these objects seem like probable elliptical progenitors. This is especially true if they are processed further by dry mergers along the way. Not all of them may turn into ellipticals - some may evolve to early type spirals, or evolve past the elliptical form into massive "central dominant" galaxies found at the centers of large clusters - but many may do so. Without large-scale simulations, we would be left with as yet poorly resolved observations of such objects, and limited statistics of various subclasses to try to understand this evolution, and the structure of typical merger trees. One of the largest simulations in recent years is the "Millennium Simulation" carried out by the Virgo Consortium, which has been the basis of a number of studies since its publication (Figure 4.12). The Millennium Simulation is based on the popular LCDM (cold dark matter plus dark energy) cosmological models, in which galaxy formation does begin with small building blocks (like dwarf galaxies). If galaxy formation is hierarchical, this model should show it. It does, sort of, and this is an improvement. Older models of the 1990s, with lower resolution, and based on a cosmological model without dark energy, and with too much cold dark matter, had problems with assembling ellipticals and forming their stars promptly. The new models, like the Millennium Simulation seem to do a much better job of this, though there are complications. Many aspects of the Millennium Simulation have been analyzed by two research groups. One is centered at the Max Planck Institute for Astrophysics in Garching, Germany, and includes some of the creators of the Millennium. The other is centered at Durham University in England.

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Figure 4 . 1 2 Images at different levels of magnification of the massive Millennium Simulation of large-scale structure formation. The images show the dark matter distribution at the present time. The bottom image shows the largest scale of the simulation, which includes a significant fraction of the volume of the universe (1 Gpc = 10 9 pc = 3,100,000,000 light years). The middle panel illustrates well the filamentary structure on supercluster scales. The top image shows a large galaxy cluster (from V. Springel, et al., Nature, Vol. 446).

First of all, they find that the Millennium Simulation does assemble a population of relatively large galaxies, which form most of their stars rapidly, thus matching the high redshift red galaxies. On the other hand, they find that most present day ellipticals had an extended, i.e., long, formation history, in accord with the hierarchical models. However, their star formation history is not so extended. At least in the galaxies that will become large ellipticals it is shut down fairly early on, and as noted above, the build-up continues in a dry

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fashion. One thorny problem these groups attacked was why star formation turns off when accretion, including gaseous accretion, continues. Several effects seem to combine to keep infalling gas from forming new stars, including environmental factors in groups and clusters, gas heating by feedback from AGN, and relatively long cooling times as the universe ages and the infalling gas is less dense than in the early days. These analyses provide a number of other interesting results. The star-forming timescale is longer on average in the progenitors of lower mass elliptical galaxies. This phenomenon is also found in other types of galaxies and is called "downsizing." The formation history of ellipticals depends on their local environment. The fact that ellipticals form more quickly in the dense environment of large clusters than in smaller groups is not a surprise. However, it is reassuring that the Millennium Simulation with its small cold dark matter initial building blocks is able to reproduce it. These analyses find that the build-up of ellipticals does not occur primarily via major mergers of gas-rich progenitors. Minor mergers, other types of accretion and internal evolution are more important. However, according to a recent study (which includes members of the Max Planck group) the number of gaseous, major mergers does seem to be enough to account for the observed submillimeter galaxies, and these too will eventually evolve to ellipticals or other early types. Thus, it appears that with major mergers and hierarchical build-up both in the mix, everybody wins. Even the monolithic collapse partisans have a small victory in the rapid build-up of ellipticals in dense environments; at least to the degree that many nearly simultaneous mergers are like a monolithic collapse. However, before declaring victory we had better read the fine print of the peace treaty. That fine print tells us that the analyses we have been discussing are not completely based on the N-body hydrodynamics of the Millennium Simulation. It is still impossible to include all the relevant physical processes (even in very approximate form) in such a large scale simulation. There is an important intermediate step called a semi-analytic model. With merger trees derived from the Millenium Simulation model, what happens during each merger, in terms of star formation and the various kinds of feedback, is determined by the semi-analytic model. That is, approximate rules for the amount of star formation, gas consumption, and feedback are applied probabilistically at each branch of the merger tree, to provide an estimate of the full evolution of the final product. The outcomes for many, many merger trees include the distribution of different galaxy types (sizes, luminosities, etc.) in different environments, and these can be compared with observation. The comparisons generally turn out quite well these days, and disagreements in detail can lead to improvements in the models. The point here, however, is that many of the complex physical processes are still represented with quite simple approximations, so when it comes to detailed predictions about different types of mergers and their effects, we should still be cautious. These models are not the same as detailed models of individual collisions and mergers. Nonetheless, it does seem possible that we are approaching a long awaited

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Figure 4 . 1 3 Hubble Space Telescope images of star-forming regions and young star clusters in the Antennae system (from B.A. Whitmore, et al., Astron. /., Vol. 118).

consensus view of galaxy formation and evolution, at least within the context of the present consensus cosmology and its relative proportions of ordinary matter, dark matter, and dark energy. Caution is wise, until many observational checks have been completed. After all, there was also a "consensus view" in the early

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1970s - monolithic collapse - before astronomers had much inkling of the presence of dark matter and dark energy. Oops! However, Toomre's ellipticalsfrom-spiral-mergers model shocked the complacency of the time about galaxy formation. Advancements since that time have been huge, and many were strongly influenced by Toomre's conjecture. It now seems likely that there are a number of paths to forming ellipticals. Lynden-Bell's work would predispose us to believe that many are violent, if not actually major mergers. Evidently, some ellipticals do form from major gas-rich mergers, like those in Toomre's sequence. In some cases, this conclusion is considerably strengthened by detailed studies by Schweizer and others. It seems fitting to end this chapter with some of the gorgeous imagery of the Antennae galaxies, a gas-rich major merger from the Toomre sequence (Figure 4.13). Powerful star formation is occurring in this system, and thousands of star clusters have been formed. Their characteristics have been elucidated by Schweizer, Bradley C. Whitmore and others. Their work provides evidence for multiple starbursts. A tidal dwarf galaxy (one of the first known) has formed at the end of one of the long tails. The evolutionary forces at work in such a merger are palpable.

Further Reading

Barnes, J. E., "Dynamics of Galaxy Interactions," in Galaxies: Interactions and Induced Star Formation: Saas-Fee Advanced Course 26, eds. D. Friedli, L. Martinet, and D. Pfenniger (Springer, New York) 1998. Parker, B., Colliding Galaxies: The Universe in Turmoil, (A journalistic account of the IRAS/major merger discoveries), (New York, Plenum) 1990. Sanders, D. B., & Mirabel, I. F., "Luminous Infrared Galaxies," Annual Review of Astronomy and Astrophysics, Vol. 34, p. 749, 1996. Toomre, A., "Mergers and Some Consequences," in The Evolution of Galaxies and Stellar Populations, eds. B. M. Tinsley and R. B. Larson (New Haven: Yale University Press), p. 401, 1977. Veilleux, S., "AGN Host Galaxies" (A series of four pedagogical lectures), at http://arxiv.org/archive/astro-ph (astroph 0807.3904), 2008.

5

W h a t happens to the little guy: minor mergers

5.1 Classifications and destructive processes

After the high drama of major mergers, maybe it is time to calm down a little, to find a slower, lower energy activity to observe in the world of galaxies. Minor and micro mergers fit that bill, and were mentioned without being defined in the last chapter. Unfortunately, although the term "minor merger" is used frequently in the literature, there is no standard or universally agreed upon definition. Micro merger is not even a standard term, but it is one that I find convenient. Although there may be no standard, rigorous definitions for the terms, there are reasons for using them that suggest approximate definitions. Major mergers between comparable partners largely deconstruct, and then reassemble the partners. The effects of tidal forces and torques depend on the mass ratio of the galaxies, and at any one time, the distance between them. The net dynamical friction depends on the galaxy masses (and other parameters), and so the inspiraling distance is also a function of the masses. Thus, the mass ratio is probably the single most important factor in determining the effects of the merger process. Mergers in which the companion is much less massive than the primary often result in just as much damage to the companion as in a major merger, but not to the primary galaxy. Specifically, major mergers between two disk galaxies are likely to do a great deal of damage to both disks, while in a minor merger the primary disk may survive. This distinction is one of the main reasons for having the term. The problem is how to define it quantitatively? When the companion is more than half the mass of the primary, that is when the mass ratio is within a factor of two of unity, numerical models show that generally the result will be what we expect from a major merger. If the companion mass is only about a tenth that of the primary it almost always will be a minor merger, in the sense that the damage to the primary will not be "major." The minor merger territory may extend up to a mass ratio of a third. The range of mass ratios from maybe a fifth to a half is something of a gray zone. The specific outcome will depend on other collision parameters, such as the relative compactness of the partners, more than in other cases. When the mass ratio is less than about 5-10 percent the damage to the primary maybe very little indeed. This statement needs some qualification. If the Milky Way had collided with a 5 percent companion within the last few hundred million years, then we would probably be able to detect damage to the Milky Way quite easily. However, at the distance at which we observe most colliding C. Struck, Galaxy Collisions: Forging New Worlds from Cosmic Crashes, Springer Praxis Books, DOI 10.1007/978-0-387-85371-05, © Springer Science+Business Media, LLC 2011

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galaxies, we can only see fairly extensive damage. Thus, we would probably miss the damage to the primary. This is definitely the case if the mass ratio is less than one percent. When small dwarf galaxies hit a large galaxy, it is a pinprick. Since minor mergers are not so negligible, it seems reasonable to call interactions with negligible effect on the primary, "micro." One of the interesting things about micro mergers is that most galaxies may be involved in one most of the time. We will judge the validity of this observation within our Local Group of galaxies in Chapter 7. The above classification is based on the amount of damage to the primary galaxy, but what about the companion? In all of the cases, major, minor and micro, things do not go well for the companion. A question that has long intrigued researchers is when, or under what circumstances, is the companion destroyed? This question was first addressed by Lyman Spitzer in the 1950s in the context of globular star clusters orbiting the Milky Way. This was the birth of the theory of tidal shocking. In essence, tidal shocking is the same process as that which makes ring galaxies. In this case the small satellite may pass through the disk, for example, rather than the center of the larger primary. Yet, the idea is the same; the material in the companion is abruptly compressed by the gravity of the primary, and promptly released when it flies out of that potential. Numerical models show that in most cases tidal shocking does not destroy the satellite in one pass. However, a bound orbit that passes through the disk or other component of the primary once is likely to do so multiple times. On each passage some material is removed and the rest is bound less tightly, so the satellite is eroded at an accelerating pace. Its best hope for survival is if its orbit is sufficiently large such that there are few such passages. Nature seems to have provided vivid examples of the aftermath of repetitive tidal shocking in some members of the class of galaxies known as "shell galaxies." Anglo-Australian astronomers David Allen, David Carter, and David Malin first discovered this class in the early 1980s. Shell galaxies are characterized by ripples or arcs of stars, and sometimes gas clouds surrounding them. This material is believed to be shed as a result of tidal shocking, and left moving slowly outward when the companion galaxy reverses its course and falls back toward the primary. The models, pioneered by Peter Quinn, an Australian collaborator of the observational team, showed that the distribution of this material is actually three-dimensional, like partial domes or shells around the primary. (Later models showed that there are ways of deriving shells from a primary disk as well.) Frequently, shells are found around elliptical galaxies. Usually there are only a few, though sometimes there can be many more. Figure 5.1 shows a spectacular example from an atlas of shell galaxies assembled by Carter and Malin. In order to get many shells there must be a number of close encounters and tidal shocks before merger. In order to get many encounters the companion must be much smaller than the primary, to reduce dynamical friction and delay the final merger. It helps if the primary is a large galaxy, like many ellipticals.

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Figure 5.1 Deep photographic image of the shell galaxy NGC 3923 (from D. Carter & D.F. Malin, Astrophys. /., Vol. 274).

If the companion is small and is disassembled in many tidal shocks, then the amount of matter lost in each is very small indeed. This is why the shells are very faint, and can often only be clearly seen in sensitive, long-exposure images, and when the average light of the bright primary is removed from the image. David Malin is a master astro-photographer (and image analyst) and used his skill to reveal some of them. This, in turn, is why shells were not discovered earlier. Often no companion is visible. However, the models suggest that in many cases this is because it is merged, not totally destroyed by the tidal shocks.

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In Chapter 3 we discussed mass transfer from one galaxy to another via a bridge, or from one star to another via Roche lobe overflow. This is another way to pull a companion apart. To better understand this, let us consider a descriptive extension of the Roche overflow process to the present context. Begin by imagining a figure-of-eight drawn around the two galaxies. Suppose one could grab the crossing point and twirl the eight around it. The two ovals of the spinning eight will each become a three-dimensional ovoid. Roche lobes are similar, yet different. They are ovoid surfaces determined by a balance of forces. For each galaxy these forces include the inward pull of gravity, the outward centrifugal force, and the tidal force of the other. On the Roche lobes all these forces balance. Because the forces do not have the proper symmetry these lobes will not be exact surfaces of revolution. Material outside the Roche lobes will experience stronger centrifugal or tidal forces and can be removed from its parent galaxy. In rapid flybys, the Roche lobe concept is not particularly helpful, and tidal shocking theory is more relevant. The Roche theory is most useful when the satellite orbit is nearly circular. Even then the classical Roche theory from stellar astronomy does not apply because it assumes that the mass of objects is concentrated at the center of each, which is not true of the dark halos around galaxies. Nonetheless, several qualitative features of the theory are useful for visualizing mass transfer between galaxies. One of these is that the Roche lobe of the smaller, less massive partner is smaller than that of the larger partner. Thus, the crossover point, where material flows from one to the other, is closer to the smaller partner. Roughly speaking, the location of this point relative to the sizes of each galaxy determines which loses the most mass by this process. Diffuse companions are doomed, while compact companions could even gain mass, if for example the disk of the primary is bigger than its effective Roche lobe. The role of companion compactness was well illustrated by the numerical models published by P. Seguin and Christophe Dupraz in 1994 and 1996. These can be viewed as an extension of the slightly earlier shell galaxy models. The model galaxies only had one component, which we can view as the halo. The gravitational potential of each galaxy was computed via the self-consistent, Nbody method, that is, as the sum of the gravity of all the particles representing the galaxy. The mass of the companions was either one-tenth or one-twentieth of the primary's mass, and so within our minor/micro boundary zone. These authors found that while diffuse companions could be essentially destroyed on a first pass, compact companions could survive several close encounters, and until the final merger. In such simulations it is difficult to properly resolve the structure of the small companion, so the results have a corresponding uncertainty, which did not make them a popular model to run. However, with the recent discovery of the Sagittarius dwarf galaxy colliding with the disk of the Milky Way, and the great increase in computer power, they are much more fashionable. (See Chapter 7 for a discussion of newly discovered Milky Way companions.) The implicit message of the last few paragraphs emphasizes a point made

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earlier; a number of factors besides mass ratio determine the destruction of companions. We have now noted the roles of diffuseness/compactness, sizes relative to closest approach distances, and the distribution of mass in different galactic components. We have learned a good deal about these factors from models, so we might wonder what the observations can teach us? The rough answer is that except for shell galaxies, and a few other special cases, not as much as we would hope. The problem is that most of the destructive processes act on a quite short timescale, e.g., comparable to an orbital timescale in a disk, so the odds of catching them in the act are not especially high. Nonetheless, we do see a number of bridges of various lengths, and presumably of various ages. A more serious problem is the final state, since one of total or near total destruction is obviously not visible. It is much easier to see the earlier stages of an attack, rather than the final coup-de-grace. This is in contrast to the remnants of major mergers. This is unfortunate because it would be nice to be able to dissect the operation of the various processes on the sky as well as in computer models. It appears that we will only be able to test those models in the nearest galaxy groups. Tidal shocking and Roche mass transfer directly affect the primary constitu­ ents of galaxies: stars and dark matter particles. Some specific hydrodynamic effects between the gas clouds are also of interest. We discussed direct disk-disk collisions and Taffy galaxies in Chapter 2. As an example of hydrodynamic effects, let us continue that discussion a little. In some of the models described there, e.g., when the disks are face-on to each other, almost all of the gas of the smaller companion is removed in the collision. (This assumes that surface density of the gas disk of the companion is comparable to or less than that of the primary disk.) A splash bridge is formed from a mixture of gas from both galaxies. This gas falls back on to both galaxies. In the companion it forms a new gas disk, which is more compact than the original, and oriented at a very different angle. Both effects are understandable results of the fact that the angular momentum of the accreting gas is very different from that of the original disk. The new orientation of the gas disk means that it would not be in the same plane as what remains of the stellar disk. Not many cases of galaxies with tilted gas and star disk have been in the observations (though a rare few have). However, this situation is probably not dynamically stable, and in most cases where it occurs the merger follows promptly. Interestingly, the compactness of the reformed disk might give it a much better chance of surviving the merger. In the world of the galaxies, this is yet another strange region. An equally strange circumstance has been observed in a number of systems. This is an inner disk that rotates in the opposite sense to the outer disk. It is hard to see how a galaxy could be born with such multi-dexterous rotation. It seems much more likely that either the inner or outer disk was accreted, or simply the result of a merger. Often the (presumably) counter-rotating disk is a stellar disk, though it may have started out as both. Gas cloud collisions would have eroded and destroyed the lesser. However, after merger, our compact companion disk could become the counter-rotating inner disk.

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Figure 5.2 Hubble Heritage image of the inner parts of the Black-Eye galaxy, M64 (aka the Evil Eye, or even, the Sleeping Beauty galaxy) (NASA, ESA, and the Hubble Heritage Team (STScl/AURA), S. Smartt, D. Richstone).

Although we do not have enough information to know exactly how it formed, the famous Black-Eye, or Evil-Eye galaxy, Messier 64, could be an example of this kind of process. It certainly looks like it (Figure 5.2). Observations have also shown that it has two counter-rotating gas disks. The Hubble image certainly speaks of the power of hydrodynamical processes in minor mergers.

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5.2 Global dynamical friction and anti-friction In the previous chapter we considered one of the most important forces in the world of galaxies - dynamical friction. In particular, we considered Chandrasekhar's classical theory, which can be visualized as a gravitational wake. As realized by Alar Toomre, this formalism is very helpful for understanding how two galaxies can be drawn together into a prompt merger. This relatively simple picture is especially useful for understanding major mergers. Unfortunately, it has been found to be insufficient in a number of other circumstances, including minor and micro mergers. One problem is that there are several technical mathematical difficulties in even applying the Chandrasekhar formula to real situations. Any specific discussion of these difficulties is beyond the scope of this book. They have been considered in many other sources, including most advanced textbooks on extragalactic astronomy where the interested reader can find more details. These difficulties can be sidestepped, if not altogether avoided in specific cases. Technical difficulties aside, there are greater problems with the classical theory. Sometimes its predictions are simply wrong. We have learned a great deal more about this in recent years, but the dynamicist Agris Kalnajs presented an example in 1971 and 1972. He found that in a perfectly flat and uniformly rotating disk of stars, a satellite experiences no dynamical friction, none at all. Kalnajs was able to prove this surprising result exactly, mathematically in this special case. There is no danger that the result is due to an under-resolved computer simulation. This result has been rediscovered recently, after disappearing from the citation indices for some years. The new application is a very interesting one. Many galaxies have a well-defined core with a nearly constant density of stars. On the other hand, current theories of galaxy formation (e.g., with cold dark matter halos), as realized in computer simulations, predict the existence of density profiles that rise sharply to the galaxy center, so-called "cuspy cores." On the other hand, simply "cored" (versus "cuspy") centers rotate approximately uniformly, and so, approximate Kalnajs' example. Numerical models of merging star clusters (or small satellites) show that friction is greatly reduced when they reach the primary core, as per Kalnajs. Interestingly, some models show that such "sinking satellites" would also scatter the cusp stars, smoothing a cuspy core into a more cored configuration, and thereby reducing the friction of the next sinking satellite. (Note, however, that there is some controversy about this point in the current literature.) What causes what we can now call the Kalnajs effect of damped frictional damping (or more generally friction that does not follow the Chandrasekhar formula)? We now know that two effects are responsible, and we will consider them in turn. The first are called collective effects, and the second have to do with resonant orbits. Ironically, although Chandresekhar's theory calls on an infinite, homo­ geneous sea of background particles, e.g., stars or dark matter particles, it is

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formulated in terms of two-body interactions. Particles in front of the satellite pull it forward via mutual gravitational attractions, while particles behind pull it backwards. It is the grand sum of the two-body interactions that determines the result, and because of the excess of particles in the wake, the result is friction. The wake itself, a large-scale aggregation of particles, is a collective effect. However, it is a limited one in the hypothetical infinite sea of particles. What is meant by the term "collective effects" in this context is generally a global response in a finite system (the primary galaxy) rather than a relatively local response in an infinite one. This view dates back to Kalnajs' work in this field, and to earlier work in other areas of physics. A prototypical example is an interaction-induced bar, as discussed earlier, or simply a pre-existing bar experiencing friction within the galactic disk with or without a satellite interaction. Such examples have been considered in a number of studies, beginning with the work of Scott Tremaine and Martin Weinberg in 1984. Specifically, Tremaine and Weinberg used a mathematical formalism (a perturbative expansion technique) that allowed them to calculate explicitly the effects of collective structures and their motions on the satellite as well as the forces from the satellite on these structures. Of course, there is a price to pay for this more complete treatment. The mathematics are much more complex, while still involving considerable approximations, and so, only really calculable in simplified or especially symmetric cases. Tremaine and Weinberg applied the formalism to the case of friction on a weak galactic bar, and confirmed earlier specialized results of Donald Lynden-Bell and Kalnajs. In subsequent papers Weinberg computed the wake structure and frictional drag of a satellite orbiting a spherical galaxy on a nearly circular orbit, and the response of the galaxy to the wake. He found that the orbital decay time could be increased by a factor of 2-3 as a result of these interactions, especially due to the effect of the bulk motion of the primary galaxy induced by the satellite and its wake. A few years later in France, Seguin and Dupraz compared a similar mathematical expansion approach to numerical models of the case of direct, head-on collisions between a satellite and a spherical primary galaxy. Because the target galaxy is initially spherical this is not a ring galaxy collision, and because the satellite was modeled as structureless, it is not a shell galaxy collision, though shells or ripples are produced in the primary galaxy. It is most like a collision between two unequal elliptical galaxies. The main finding of this work was that the friction was much more impulsive than the case of a satellite on a nearly circular orbit. It spiked quite strongly when the satellite passed through the primary galaxy. Not surprisingly, the investiga­ tors found that the Chandrasekhar formula did not do a good job of representing this, while the expansion approximations did. On the downside, the authors themselves called the complex mathematical calculations "tedious and rather painful." Having carried out some similar calculations in other applications, I suspect they were not exaggerating. In the late 1990s several theorists (including Scott Tremaine) presented rather more elegant techniques for these types of calculations, e.g., based on the

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mathematical "linear response theory." The upshot of these developments is a much more complete and flexible theory than Chandrasekhar's original, which model comparisons show can more accurately represent a much wider range of circumstances. The main downside is again the complexity of the calculations needed to use the new theory. Let us now turn to the second effect that modifies the Chandrasekhar friction - resonant orbits. First of all, what are resonant orbits? They are orbits that are "in sync" with the satellite orbiting in a very specific way as defined in classical celestial mechanics. More precisely, the orbital period of a star in resonance with the satellite is either nearly the same, or equal to a low order fraction multiplying the satellite period. Fractions are written in terms of integer numerators and denominators, and by the term "low order" I mean that neither of those numbers should be very large. For example, 3/2 (or 3 times around for the star for every twice around for the satellite), but not 37/153 (though this number is quite close to \ , which would be "low order"). These distinctions apply generally, but are somewhat irrelevant to the present application, where the most important resonances have ratios of 1:1, or perhaps 1:2. That is, we are only interested in the lowest order resonances. These could be stars on near circular orbits of a particular radius in a galaxy disk. However, the periods of disk stars will generally be much shorter than that of a satellite orbiting outside the disk, so one would not expect the 1:1 resonance in such cases. Bars and waves may propagate relatively slowly through disks, and so, might be more likely to resonate with satellites. Or a satellite may orbit within the disk, or at least its outermost extensions. Or the satellite may experience friction against dark halo particles with comparable orbital radii. For simplicity, I will continue to talk about friction against stars, but mean "generic" stars, including dark matter particles. Tremaine and Weinberg were among the first to consider the role of resonant orbits, and found them likely to be an important player in the story of dynamical friction in their 1984 paper. Recently, Shigeki Inoue of Tohuku University has published a thorough study of the role of resonant orbits in dynamical friction, in the case of a star cluster orbiting a spherical galaxy. He identified resonant stars in his numerical simulations as those experiencing a significant energy change, presumably as a result of interaction with the satellite. He found two distinguishable groups of such stars. Figure 5.3 from his paper shows a couple of sample orbits. These orbits look very odd; we are used to disk stars with nearly circular orbits, or perhaps halo stars with nearly radial (in and out) orbits. The loops shown in the figure appear very different. The explanation is that we are viewing these orbits in a frame of reference moving with the star cluster. It is like watching an epicyclic orbit while moving on the guiding center orbit at the mean orbital speed. For a little more familiarly, it is like riding a carousel with every external reference point darkened to invisibility. Then it would seem that the primary motion is up and down, not around and around. In the figure, the dominant orbital motion is also not shown. We can indulge our imagination a little at this point, and suppose that many

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X[PC] Figure 5.3 Two sample resonant orbits, of the type that counter dynamical friction, in the frame of reference of a star cluster, located at the origin in the graph (from S. Inoue, http://arxiv.org, 2009).

slightly different, but generally similar orbits were added to the figure. Inoue certainly found a significant family of such orbits in his models. The ensemble of orbits would swirl around the satellite center like a cloud of gnats or deerflies around the head of a midsummer, woodland hiker. In contrast to the trailing wake, this cloud is weighted in the forward direction, and can pull the satellite ahead. Of course, the satellite in turn modifies the stellar orbits, and changes their orbital energy, like the hiker swatting at the annoying insects. In many cases, the effect of the resonant orbits is not enough to offset the Chandrasekhar friction, but when a satellite has sunk into the constant density core, it is. Figure 5.4 provides an example from Inoue's models. The satellite sinks steadily until it nears the core, then the friction is reduced, and ultimately almost disappears. Far into the woods the hiker finds that the number of flies has increased, and there are just as many in front as behind. He pulls his hat down tighter, and largely gives up wasting the energy to swat them.

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Figure 5.4 Model results showing frictional orbital energy inputs to a star cluster orbiting in a galaxy halo. Non-resonant orbits input negative energy, creating friction at an average rate roughly similar to the prediction of the classical Chandrasekhar formula. Resonant orbits counter the friction by inputting positive orbital energy. This cancels the frictional inspiral (from S. Inoue, http://arxiv.org, 2009).

5.3 Cumulative effects

In the previous two sections we focused on what happens to the satellite in the process of a minor merger. In such mergers the satellite suffers the most, and the first section above considered some of the torture techniques. In the previous section, we learned how the clever companion can postpone merger for a longer time than in major mergers. Although the effect of minor mergers on the primary is relatively moderate, it is not negligible, and we will consider of few of the details in this section. To start with the most basic point - minor, especially micro, mergers have modest effects on the halo or bulge of the primary galaxy. Depending on the size and compactness of the satellite, the escape velocity from the outer edge of the satellite is generally comparable to or less than the typical orbital speeds of particles and stars in the halo and bulge. In the first place this tells us that the satellite cannot capture particles from those structures. But more importantly, it is a measure of how weak the gravitational attraction from the satellite is throughout the halo and bulge relative to the self-gravity of those structures.

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Bulge stars or halo particles, which happen to have orbits that bring them very close to the satellite, will be affected, but not the large-scale structure to a great degree. On the other hand, in retaliation for what it has suffered, the satellite attacks the weakest parts of the primary, its disk and core. Figure 5.5 from the 1996 models of Ian R. Walker, Chris Mihos and Lars Hernquist illustrates very well the damage a companion of mass one-tenth that of the primary can inflict on the latter's disk. In this model the satellite orbit was prograde and tilted only 30° relative to the disk plane. We would expect the most damage to the disk from a prograde, in-plane merger and this is close to that optimal case. The most obvious effect, visible in the upper half of the figure, is that the companion induces very strong spiral waves in the disk, throughout the merger process. By now, we are very familiar with the idea that strong waves drive star formation, and evolve galaxy disks by this means. The figure suggests that minor mergers can be especially effective drivers of that process, at least in cases like the one shown. The image sequence in the lower half of Figure 5.5 shows that the disk is greatly thickened by the merger process. The satellite and the waves it induces both contribute to scattering stars in the vertical direction, dynamically heating and thickening the disk. The net effect of the induced star formation and the dynamical heating is to evolve the disk, and transform it into something resembling an early Hubble type. (Unless, of course, it started out as an early Hubble type with a thick disk, in which case the minor merger will have a much smaller effect.) We can see in this a weaker version of Toomre's spirals-to-ellipticals process in major mergers. In that scenario, the progenitors experienced a complete make-over as a result of strongly induced star formation, violent dynamical heating, and subsequent feedback effects, like galactic winds. In minor mergers all the same effects can be produced, but at much lower intensities. In micro mergers we expect more localized effects, at most. So much for how minor mergers affect the disks, but how do they affect the cores? The first part of the answer to this question is that we do not really know. Minor mergers have been studied much less than major mergers, and for details like what happens in the core, there have been very few studies. However, based on what we have learned about the basic physical processes from other cases, we can make some reasonable conjectures. I cannot help but point out that this is an excellent example of how theoretical astrophysics works. We use physical principles to try to understand observed structures and velocities, and we try to improve initial models with improved computer models and better observations. As we build up confidence in the models and conceptual pictures behind them, we try to use them in new applications, as in the present case. So what do we think happens in the core at the end of a minor merger? Perhaps even more than in major mergers there are many different possibilities. At one extreme, we can envision a satellite so diffuse and loosely bound that it is tidally disrupted before it ever sinks to the primary core. As we will see when we

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Figure 5.5 Snapshots of a model of a near-prograde minor merger. The top two rows (left) show the face-on view, showing the development of spiral waves and bars. The bottom two rows (right) show the edge-on view, highlighting the increase in disk thickness. See text for more details (from I.R. Walker et al., Astrophys. /., Vol. 460).

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discuss interactions in the Local Group, a great stream of scattered debris may be left behind in this case, but the core is largely unaffected. At the opposite extreme, a dense, compact satellite may sink to the core with hardly any damage. However, this is only likely if its core is denser than that of the primary galaxy, otherwise it will experience disruption in its final approach to the core, according to the Roche theory. Such very compact dwarfs appear to be relatively rare in the world of galaxies, so this is an unusual case. However, when it does occur, then the satellite can essentially become the new core of the merged galaxies. A number of late-type galaxies are known to have very compact star clusters in their centers. One wonders how many of these might have been acquired, rather than formed in situ? Between these extremes, we can envision a broad range in the amount and state of disruption in the stars and gas from the satellite, which reach the central core. Details of how much of a bulge or disk component the satellite has, and the nature of its in-spiral are important. In a recent study based on cosmological growth simulations, Philip F. Hopkins and collaborators concluded that mergers are indeed the primary agent for building the bulges in relatively large galaxies. They also concluded that minor mergers were a significant player, and specifically responsible for building about 30 percent of the mass of a typical bulge. Though certainly not exhaustive, their models would average over most of the parameters mentioned in the previous paragraphs. The details of the minor merger process are especially important in determining the feeding of an active galactic nucleus in the primary. As in a major merger, some fraction of the satellite's gas might be delivered to the AGN accretion disk, though only after the great majority of its angular momentum have been removed. As the satellite is merged we can expect turbulent dynamics to be the rule in the core for a time, and this would facilitate the loss of angular momentum in some gas elements. We have already considered the example of counter-rotating and, possibly interpenetrating, disks above. Moreover, as in other cases, strong waves and bar-like distortions produced in the primary disk (Figure 5.5), will enhance angular momentum transport, and gas flows to the central regions. The magnitude of the gas transfer may be less than in major mergers, but can still be significant. To conclude this chapter, we can step back for a moment, take a wide view of the world of galaxies, and notice that minor mergers can be seen in many places. That is to say, while major mergers are quite rare, minor mergers are much more likely. We have also learned that they can take longer to complete than expected, and substantially longer than major mergers in many cases. The evidence suggests that major mergers were much more common in the distant past. Cosmological build-up simulations show this, and that it is also true for minor mergers. We are at an early stage of accumulating evidence from large scale surveys of galaxies and their groupings, and cosmological models are getting ever more sophisticated. Yet even now, there is evidence to suggest that minor mergers may contribute nearly as much to the buildup of many galaxies as major mergers do.

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Recently, there has been renewed interest in what is now called "cold accretion" in the buildup of galaxies. Cold accretion is the infall of relatively unformed material out of cosmological filaments and sheets. Generally it is viewed as gas that has been compressed somewhat in these structures, but not entirely accumulated into galaxies. It is not really known how much of it is actually in dwarf galaxies and other structures so small that they are still hard to recognize beyond our Local Group of galaxies. Cold accretion and micro mergers may be overlapping phenomena. As noted above, at this time the available evidence suggests that galaxies do not acquire most of their mass in major mergers. The cumulative and relative roles of minor mergers, micro mergers and cold accretion are not yet well understood.

Further Reading

Beyond the technical literature, which in the case of some of the topics of this chapter is very technical indeed, there are no general sources on the topic of minor mergers. The sources listed in the previous chapter, and the more comprehensive sources listed in the Resources section at the end of the book, contain helpful material on the topic.

6

The secret of fire: triggered star formation

Let us turn back to fundamentals now, to keep our focus, avoiding the distractions along the many byways of the world of galaxies that we have wandered through in the last few chapters. The first of the two most important effects of galaxy collisions is that they build bigger galaxies from the small ones that formed first. They do this because mergers are inevitable, unless the collision happens to occur at very high relative velocity, e.g., in a dense galaxy cluster. The second important effect is that collisions and mergers evidently drive high rates of star formation, and increased nuclear activity in at least some cases. A major part of the meaning of the term "galaxy evolution" is the process of turning interstellar gas into stars, and increasing the heavy element abundance of gas and stars as a result of nuclear processing in successive stellar generations. Thus, collisions and mergers affect galaxy evolution, sometimes by a large factor. Induced star formation is also responsible for the beautiful imagery of colliding galaxies, with young star clusters scattered like jewels in some cases. We have already considered the blazing fires in the ultra-luminous infrared galaxies (ULIRGs), and how these are caused by the deposition of large quantities of interstellar gas into the central regions. I have also described a number of examples of how waves driven by galaxy collisions compress the interstellar gas and induce vigorous local star formation. Thus, it is clear that the dynamical effects of galaxy collisions can cause the formation of new stars and star clusters. This conclusion immediately raises a number of additional questions and issues. For example, while we have considered a number of specific cases of induced star formation, and the general process of merging, we have not considered what general relations can be derived from the observations and theory. We have also not considered the physical forces behind these effects. It is time to look more closely at induced star formation, heedless of the risk of being blinded by the light of millions of new stars.

6.1 C o u n t i n g y o u n g stars

In the mid-1970s, Beatrice Tinsley and Richard Larson of Yale University asked some of the most fundamental questions about collisions and star formation, and attempted to answer them with the available data. Among the first of these questions is: while collisions and mergers drive new star formation in some galaxies, and at very high rates in a few, is that always or even usually the case? The "always" part of this question can be answered immediately with a no. Some C. Struck, Galaxy Collisions: Forging New Worlds from Cosmic Crashes, Springer Praxis Books, DOI 10.1007/978-0-387-85371-06, © Springer Science+Business Media, LLC 2011

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galaxy collisions do not seem to trigger much star formation. Of course, many of those collisions involve early-type galaxies, i.e., ellipticals or gas-poor spirals where there is no fuel for the fire. The more important questions are: what is the average star formation rate (SFR for short) in colliding galaxies; and how does that rate compare to comparable isolated galaxies? Only fairly recently have we been able to give quantitative answers to those questions using data obtained in large surveys of galaxy properties. Larson and Tinsley had no such data resources; by modern standards they had hardly any data at all. However, since Hubble's time observational astronomers had continued to catalog and classify relatively nearby galaxies. The goal of this work was essentially the same as that of Hubble, which was based on the belief that the study of class properties would eventually lead to a more fundamental understanding of galaxies and their evolution. Among the properties being measured for catalog objects were their colors. Astronomers attach a more precise meaning to the term "color," than in ordinary usage. Even before they talk about colors they define photometric bands and the fluxes or magnitudes in those bands. A photometric band is a range of wavelengths, typically ranging from a few to a few hundred nanometers (nm, a billionth or 10"9 part of a meter) in width, depending on whether it is a narrow or broad band. For example, a typical broad band in the blue might be centered at a wavelength of about 440 n m and extend for about 50 n m on either side of that value, including blue, violet, and a bit of ultraviolet light. These bands were originally defined by the filters that were placed in front of the collecting devices (e.g., photographic plates) on telescopes to exclude the other wavelengths, and allow only the band wavelengths to pass through. When the ratio of the measured fluxes in two bands was calculated, the observer had a quantitative measure of the galaxy "color." These astronomical colors are only indirectly related to the color we perceive with our eyes. With three photometric bands covering the spectrum from red to blue, a relatively true-to-life image can be assembled, analogous to the way television has used red, green and blue dots to produce realistic color for 50 years. However, astronomers use false color images far more freely. Sometimes they use color to encode other properties like brightness. Often they produce a color image from data in only two photometric bands, a procedure which can convey a lot of information, but does not produce a very realistic image. Equally often, one or more of the photometric bands will be from non-optical wavelength, such as the infrared or ultraviolet. In that case, the color codes information, rather than giving a view like that any real eye might see. False color images allow us to see with eyes we were never born with. In the 1970s, total colors had been measured for many galaxies in standard catalogs. Larson and Tinsley carried out a conceptually simple exercise; they compared the optical colors of galaxies like those in the Hubble Atlas of Galaxies to those in the Arp Atlas of Peculiar Galaxies. Recall that for his atlas, Hubble tried to assemble isolated galaxies that could serve as prototypes of individual subclasses, and he rejected "peculiars." Arp on the other hand, collected

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peculiars, many of which were colliding or merging galaxies. As eventually published, the Hubble Atlas does include some colliders (and peculiars), and Arp's objects are not all very peculiar, so the separation is not perfect. However, it seemed good enough to reveal any strong difference between colliders and noncolliders, which was not simply an effect of different Hubble types. There is a large range in color among the normal, Hubble types. If the colliding galaxy progenitors had the same range of Hubble types, and a different average color, then that color was likely the result of enhanced star formation in the collision. There are lots of complexities and caveats in this logic, but the bottom line is that Larson and Tinsley expected that the Arp galaxies would be bluer on average, due to additional hot, young, and blue stars being formed in them. That is not exactly what they found. The average color difference was not great, and hardly statistically significant. What they did find was that the Arp Atlas galaxies were distributed over a wider range of colors than the Hubble Atlas galaxies. It was surprising that some of the colliding systems had colors redder than old elliptical galaxies. Tinsley's computer models of color evolution in stellar populations suggested that those galaxies, whose colors had a strong contribution from a recent starburst, should generally look bluer and younger. However, much of the color dispersion could be attributed to a range of star formation histories before the burst, and a range in the fraction of light contributed by the burst. There were also other complications that were not understood at the time. For example, Larson and Tinsley allowed for the effects of moderate dust obscuration - so-called reddening. After the observations of the IRAS satellite it was very clear, as described above, that merger-induced starbursts could be covered by vastly more dust than could be accounted for by standard reddening corrections. The point here is not details of what was right or wrong about Larson and Tinsley's analysis (or for that matter, with a similar analysis using near-infrared data, carried out by Tinsley and the author a couple of years later, and others since that time). The point is the effect they found, and that their discovery excited a number of other astronomers, and prompted a new literature of studies on the topic. Indeed several others were already at work at the time, and many others would soon follow. An example of the former was John Huchra, who obtained new data on galaxies derived from catalogs of objects with strong emission lines. His conclusions paralleled those of Larson and Tinsley. For example, he found no evidence for a population of very young galaxies, just as Larson and Tinsley did not find that the interacting galaxies were all blue, and young or rejuvenated. Many of these early studies were based on broadband colors, as I have described. However, these colors, especially if restricted to optical wavelengths, can be quite ambiguous indicators of the star formation rate because they depend on a number of other factors. The worst of these are dust obscuration and reddening described above. There are several ways around these difficulties. One of the most common alternatives to determining SFRs with color is to use narrowband measurements, or images made with a narrowband filter. The most

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common of these filters are centered on the optical (so-called Balmer) lines of the hydrogen atom. For a number of reasons ranging from the quantum mechanical properties of the hydrogen atom, to the fact that hydrogen is the most common element in the universe, to the specific physical conditions in the emission region, they are often strong lines in the spectra of star-forming galaxies. To be more specific, we note that most of the optical light from star-forming regions comes from the surfaces of the young stars, but the Balmer line emission comes from hot gas surrounding the hottest young stars. A significant part of the radiation of these hot, young stars is in the ultraviolet band, and much is absorbed by interstellar gas within a few to a few hundred light years around the star. This energy goes into heating the gas, and ionizing or ripping electrons off the atoms. A hydrogen atom only has one electron, so it tries quite hard to grab it back, or "recombine." Technically, we say that its recombination time is short. The star, however, keeps churning out the ultraviolet photons, so the atom is reionized quickly too. The upshot of this competition is that within the so-called HII region around a massive, young star, the hydrogen is almost completely ionized. The HII region ends at a fairly sharp edge beyond which the ionization fraction goes down to quite low levels. This is the classical picture, but real HII regions involve a number of additional complications, and yet this picture captures the essence of the phenomenon. To put it even more simply, HII regions are the "sunscreen" between a galaxy and its star-forming regions. The Balmer emission lines are produced by the recombining hydrogen atoms in the HII regions. Only stars more massive than about three times the mass of the Sun produce enough ultraviolet emission to make a significant HII region. The most massive of these stars have lifetimes more than a 1000 times shorter than that of the Sun, so by comparison they are "young" all the way up to their abrupt deaths in supernova explosions. Thus, we come to the conclusion of this short aside, that the presence of strong Balmer lines in spectra indicate the presence of massive, young stars, a sure sign of recent star formation. Even better, the measured fluxes of the Balmer lines can be directly related to the amount of recent star formation via empirical relations or computer modeling of the spectra. This relation was considerably refined, and used to study and compare samples of isolated and interacting galaxies in a long-term project undertaken by Robert C. Kennicutt, Jr. and William C. Keel and collaborators in the 1980s. A number of researchers have continued this kind of work up to the present time. Comparison of total Balmer fluxes in isolated and interacting samples largely confirmed the earlier work on net colors, i.e., they indicated some enhancement of star formation in interacting systems. Unfortunately, the Balmer lines suffer from the same problems with reddening that optical colors do. However, in combination with broadband colors, Balmer measurements can constrain the amount of star formation, the reddening and the average abundance of heavy elements simultaneously. I used the word "constrain" in the previous sentence, because depending on the circumstances the accuracy of the estimates of these

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Figure 6.1 A red (continuum) image of the nearby spiral galaxy NGC 628 on the left is compared to a hydrogen alpha emission line image on the right. Note the clumpy distribution of the emission line image of young star clusters, and the concentration of the line emission in spiral waves (NASA Extragalactic Database, NASA Spitzer Science Center/SINGS Legacy Project).

quantities may vary a great deal. In the cores of galaxies, observational interpretations can be confused by active galactic nuclei, which are also sources of Balmer emission lines. Images of galaxies could also be made through Balmer line filters, especially filters allowing transmission of only wavelengths near that of the first line, called hydrogen alpha, or Ha. A sample Ha image, as they are called, is compared to a broadband image of a nearby galaxy in Figure 6.1. Such images reveal where within galaxies the star formation is occurring. The reader of the earlier chapters already knows the answer to that question - the star formation generally occurs in starburst nuclei and in strong waves in the disk of one or more of the collision partners. In part, this had been known since blue images, wherein the young star clusters are prominent, had first been compared to red images, dominated by old stars. The difference was that the measured Ha fluxes could provide a quantitative estimate of the amount of star formation in different regions, and in fact, such observations started to turn the field from a descriptive one, into a much more quantitative one. Narrowband emission line observations open one pathway to improving on the information provided by broadband colors; a second was to look over a broader, and eventually, a much broader wavelength range. This was initially done by extending broadband photometry to bands in the near-infrared and the near-ultraviolet. The advantage of these bands is that they do not require new detector technologies. The huge disadvantage is that the Earth's atmosphere is much more effective at absorbing radiation in these bands than in the optical. Thus, only the brightest sources could be looked at, and obtaining the data

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Figure 6.2 Aerial view of the VLA radio telescopes on their Y-shaped track, near Socorro, New Mexico (from the website of the National Radio Astronomy Observatory/ AUI/NSF).

required long observing times. In the X-ray, far-ultraviolet, mid-infrared, and farinfrared the atmosphere absorbs even a higher fraction of the incoming radiation. Data in these bands was quite rare in the 1970s and 1980s (except for the IRAS far-infrared data), and generally, had to await the construction and launch of sensitive satellite observatories in the 1990s and after. Radio waves, on the other hand, are transmitted by the atmosphere and by the late 1970s the radio observations had very good resolution and sensitivity. With the construction of the Very Large Array of radio telescopes (Figure 6.2), completed in 1980, a new and useful window on galaxies was opened. The VLA, as it is known, consists of 27 separate radio dishes (telescopes). The signals received by each dish can be combined via the techniques of "aperture synthesis," so that the VLA can function effectively as one giant radio telescope. Moreover, the individual telescope platforms are like railroad cars on a Y-shaped track. The whole array of dishes is expanded or contracted every few months between fixed configurations with different resolutions. Thus, the array is like a telescope mirror whose size can be changed to optimize it for different types of observation. (However, the total energy gathering area of the telescopes remains the same in all configurations.) The reason for not keeping the array at its highest resolution all the time is that there are tradeoffs with sensitivity and field-ofview. Increase the resolution and you lose in the other two areas, in general. High-resolution observations are used to study star-forming regions within the

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Figure 6.3 Artist's impression of the ALMA telescopes, which are currently under construction in Chile (from the website of the National Radio Astronomy Observatory/ AUI/NSF).

Milky Way, which are relatively near, bright, and not too large. Most observations of colliding galaxies use the lower resolution configurations because these systems are relatively distant, faint, and the partner galaxies can have relatively large separations. Despite its age, the VLA remains a premier facility for radio astronomy, helped in part by various upgrades to its instruments and computer processors over the decades. It will soon have competition from ALMA, the Atacama Large Millimeter Array (Figure 6.3), which is being constructed on the Chajnantor plain of the Chilean Andes. As its name implies ALMA will observe at millimeter wavelengths, which are much shorter than the centimeter and decameter wavelengths observed with the VLA. There are many molecular lines to be observed in the millimeter waveband, which are not present at the longer wavelengths, so ALMA will be observing different kinds of emission sources. Moreover, since it is in the southern hemisphere it will observe many different astronomical sources. Thus, ALMA and the VLA are expected to be very cooperative, if distant, sister arrays. Let us return to the VLA in the 1980s and 1990s to ask what it contributed to our understanding of star formation in colliding galaxies. The first answer is information rather like that from optical telescopes. Fluxes centered on standard frequencies are analogous to optical broadband fluxes. Flux differences are used to calculate spectral indices (basically the slope of the spectrum over the relevant

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region) and are analogous to optical colors. These "radio colors" can tell us whether their source is likely to be "thermal," e.g., from many individual HII regions, or "non-thermal," e.g., from charged particles accelerated in supernova remnants or an active galactic nucleus. As in the optical, narrowband imaging is very important. Actually, for our purposes, one radio narrow band is primary. This is the 21 cm band, which is a wavelength of atomic hydrogen emission. We have already discussed above the use of 21 cm emission as a means to map the gas distribution in colliding galaxies. However, large fractions of the gas in galaxies can be in molecular or ionized form, as well as in atomic form. Thus, sensitive 21 cm observations do not tell us everything about the interstellar gas. Yet, such observations have been able to tell us a great deal about the distribution of gas and of gas velocities in colliding galaxies, and how these quantities evolve. Even more importantly, they have given us a great deal of information about the correlation between gas concentrations and star formation, which we will discuss below. An interesting point about the broadband or so-called "continuum" radio emission is that while most of it is ultimately powered by energy from stars, it does not originate from the surface of stars. The stars power supernova explosions and the ionization of gas in HII regions, where the radio radiation originates. (Active galactic nuclei are also strong sources of radio continuum.) Most of the observed emission in bands from the far-infrared to the ultraviolet does originate at stellar surface, though it is often reprocessed after leaving that surface. An important property of radio and far-infrared measures of star formation is that they are largely free of the reddening which so bedevils optical and ultraviolet measures. This is also true of X-ray measures of star formation. So, every method of measuring star formation has its own problems. The best outcomes are obtained when astronomers can use many methods to study the same system. This is usually possible only for very close systems, and that includes few colliding galaxies. In recent years, it has been possible to compare IRAS far-infrared measures and mid-infrared observations from the Spitzer Space Telescope to ground-based optical observations of a significant number of isolated and colliding galaxies. The rough rule-of-thumb derived from these comparisons is that optical measures of star formation miss about half of it, which is hidden by dust clouds. The infrared observations show us what is hiding, and an interesting finding is that the hidden fraction is largely the same in most locations. That is, there are few completely hidden regions of star formation, and few that are fully revealed in the optical.

6.2 The spreading fire

What is the color of tidal bridges and tails? That question is answered in Figure 6.4; there is a range of colors (though the figure does not happen to include any especially red tails). More fundamental questions lurk behind that question. Does star formation continue in tidal structures, or is it effectively turned off,

The secret of fire: triggered star f o r m a t i o n 163 * R r p 84 SDSS

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Figure 6.4 Sloan Digital Sky Survey images of a sample of tidal bridges and tails. The unlabeled system in the lower left is Arp 240 (Beverly). Smith and the Sloan Digital Sky Survey).

because, for example, the interstellar gas is stretched out so thinly? Alternatively, do the large molecular gas clouds where stars are produced continue to form in tidal structures, especially where the large-scale forces compress or accumulate gas? If so, how efficient is this star formation; how much gas is used up, and how much is held in waiting until it falls back onto one of the galaxies or the merger remnant? Just how is gas redistributed in tidal bridges and tails? One thing that seems clear from the examples in Figure 6.4 is that the new star clusters are not as regularly distributed as in M51-type spirals, or Cartwheel type ring waves. Even excluding the "dry" tails, i.e., gas-free ones that cannot produce more stars, there seems to be greater variety of outcomes. This suggests that the answers to the above questions will not be entirely simple. Prior to the 1990s, the first tentative answers to such questions were mostly anecdotal. As discussed in the previous subsection, comparison samples of interacting and isolated galaxies were being assembled and observed in a variety

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of wavebands. In most cases, the goal was to estimate the total enhancement of star formation of the system, a goal which was commensurate with the limited resolution and sensitivity in many of these wavebands. At the same time, more detailed studies of a few colliding systems, especially the nearest ones, were being published. These systems provided the anecdotal insights, and some examples have been noted in previous chapters. However, the star formation properties of tidal tails had not been studied in any very large sample. I should emphasize that the characteristics of specific optical detector types played a very important role in both making this situation and ultimately changing it. In the 1970s, photoelectric photometers were used to measure broadband fluxes. These instruments were generally easier to work with and calibrate than photographic plates, though the latter were still used for some purposes. One of the most important of these was to record the output of spectrographs. The disadvantage of the photometers was that the aperture size of the instrument was fixed, and usually relatively small compared with the scale of nearby galaxies on the sky. As an arbitrary example, suppose we wanted to observe star-forming regions in a colliding galaxy, whose disk was more or less circular on the sky, and with a diameter of about one arcminute (1/60 of a degree), using a photometer aperture of 5 arcseconds (5/3600 of a degree). The area of the galaxy disk would be about 720 times that of viewing area of the photometer, so about that many photometric measurements would have to be made to completely survey the star formation across the disk (at that resolution). Each of those measurements would require a slight position change of the telescope. This is not necessarily true in principle, but at most moderate-sized telescopes it was true in practice. Thus, such measurements would have been very laborious, and were not carried out. It was more feasible to carry out projects like measuring the few to half-dozen most interesting star-forming sites in a particular colliding galaxy system, or to look at the nuclei of many colliding galaxies, since the star formation is often strongest there. By about the middle to late 1970s a new type of detector was being developed, or at least brought into astronomy - the charge-coupled device or CCD. James Westphal and James Gunn of Caltech were driving forces in getting NASA sponsorship to develop CCD detectors for the Space Telescope in particular, and astronomy in general. This is a fascinating story, but one that has been told in other places and need not be repeated here. Nonetheless, this was an unprecedented technology development program for astronomy, and resulted in a detector revolution. In the next decade or so, most imaging and photometry detectors would be built using CCDs. This happened very naturally because a CCD is a light sensitive, solid-state chip that consists of a grid of individual elements, with each element functioning like a photometer with a very small field of view, while the grid functions like a photographic plate. That is, if the grid has enough elements, their outputs can be assembled, like pixels on a TV screen to make a picture. Most of the galaxy images in this book have been produced in that way. The output of each CCD element is readily quantified, so this technology is inherently digital.

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In fact, digital cameras use CCDs, and this change in detector technology is the same as that which turned photography from film to digital cameras. The foresight of the Caltech team brought this change to astronomy relatively early, and the advantages were considerable. CCDs are more light sensitive than photographic plates, so they use the light gathered by the telescope more efficiently. They can not only do mass photometry much more quickly than a photoelectric photometer, but the instrument and its output can be calibrated more accurately. Most importantly, digital data is very easy to transport and store (though that was not quite the case in the early days). The upshot for galaxy observers can be expressed even more succinctly: with CCD cameras, fluxes, colors, and estimates of star formation could be measured across the visible faces of galaxies. Even better, the process was not so laborious that it could only be applied one galaxy at a time; it could be used to acquire data on a reasonable sample of galaxies. The Hubble Space Telescope was not launched in the early 1980s, as expected by the instrument builders of the 1970s; one of several unhappy aspects of its early history. However, the manufacturing run that produced its CCD chips for it produced many extras, and these were distributed to many observatories. One of the first people to use a CCD camera to study colliding galaxies was Howard Bushouse, who at the time was a graduate student at the University of Illinois working under Jay Gallagher. In one influential paper, he published images in red and yellow bands and also hydrogen alpha images of a sample of 38 strongly interacting galaxies. These images immediately revealed the distribution of hydrogen emission, and thus, of star formation in these systems. They confirmed the impression from blue photographs that it did not all lie in the nuclei of the galaxies, though it was frequently found within the galaxy disks, and often concentrated toward the central regions. A number of tidal tails showed detectable hydrogen alpha emission. Bushouse's work included data from the radio and infrared bands as well, and he used all his information to estimate global star formation enhancements in interacting versus isolated galaxies. His basic results were in accord with those of Larson and Tinsley, though much more quantitative, and based on more pure samples for both the colliders and the control objects (although in the former case, the galaxies were chosen to be in an especially violent or disruptive phase of interaction). Bushouse found the colliders had star formation rates that were enhanced by a factor of about 2.5 as determined from optical measurements, but up to as much as a factor of 6 if the infrared and other information were used (presumably to find optically hidden regions of star formation). Like Larson and Tinsley, he also found that the colliding systems have a very wide range of star formation rates, and more precisely, of hydrogen alpha emission fluxes. By fairy tale standards, Larson and Tinsley's house had been built of straw, but Bushouse added a foundation of stone. Bushouse's work was published during the IRAS/ULIRG mergers-build-galaxies gold rush, discussed in Chapter 4, so it found an audience well prepared to

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receive it positively. It firmed up the hypothesis that collisions enhanced star formation in general, and provided quantitative measures of the effect before merging, albeit with a modest sample. One of the few negatives of this story is that most researchers in the area accepted these results so readily that few were motivated to work on expanding the sample. Yet questions remained, and there were new possibilities for addressing additional issues. As Bushouse was publishing his work, my collaborator Jim Schombert, with my graduate student John Wallin and I, had begun work on a related project, which turned into an unintentional follow-up project. Like many others, we wanted to know how much the induced star formation was concentrated in the galaxy nuclei versus more extended regions. Beyond that, we were curious how much star formation occurred specifically in tidal bridges and tails, and how much it varied between different collisions? In other words, what are the answers to the questions asked at the beginning of this subsection? Schombert was working at Caltech at the time, and through his efforts we were able to get a good amount of observing time on the 60-inch telescope at Mt. Palomar, which had a new CCD camera. This allowed us to obtain images in several colors of 25 colliding galaxy systems. In brief, we found that the tidal structures were not extreme in their optical colors or star formation properties; they tended to be about the same color as the outer parts of the disk to which they were attached. There were large color variations within interacting systems; some parts of the system had very blue colors indicating relatively strong recent star formation, while other parts did not. This color dispersion tended to decrease with time. That is, systems whose appearance indicated that the collision had been underway for some time (e.g., with long tidal tails, or multiple shell-like structures), had less color variation than those that appeared to be at an early stage of the collision. These results, published in the 1990s, were in general agreement with what we would expect from the computer models. For example, the models showed that tidal tails are pulled like putty from material in the disk, initially without huge density changes, so continuity between the disk and the tail is natural. On the other hand, overall disk compressions and the development of waves in disks are often strongest after closest approach, in the relatively early stages of the collision. Since these compressions are thought to cause star formation, this result is consistent with the observed color variations. Actually, these observa­ tions held out the promise that this reasoning could be turned around; one could observe to what degree bulk or wave compression triggered star formation. Unfortunately, like Bushouse's observations, the small sample size, and the limited resolution of the observations, constrained the results that could be obtained. In the 1990s, it seemed very possible that larger samples could have been observed, and that the tentative answers provided by these early surveys to the questions above could have been tested, and more definitive answers provided. As you can tell from the tentative syntax of the previous sentence, it did not happen. In a quirk of the history of science, nobody carried the work forward.

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Bushouse pursued similar work on infrared observations of the nuclei of interacting galaxies for a few more years. John Wallin graduated and went on to other work. Jim Schombert and I also moved on to other projects. In my case, this included work on special types of colliding galaxies, like ring galaxies and ocular galaxies, as described in other parts of this book. In the early 1990s, I. Felix Mirabel (and somewhat later in collaboration with Pierre-Alain Due) published observational studies of a couple of systems with tidal tails like the Antennae galaxies, and with dwarf-galaxy-sized star formation regions. For these as well as the original, he emphasized the point that the stars in these condensations were younger than the dynamical age of the tail. Thus, they must have formed within the tail, rather than in the disk before the tail formed. There was also a flurry of modeling work on the particular question of how star clusters and tidal dwarfs form in long tails. This gave rise to a debate about the relative importance of coupled star-gas cloud gravitational instabil­ ities, versus compression and accumulation processes in the gas clouds alone. Given the quality of both models and observations, such questions could not be answered definitively at that time (see the discussion in Chapter 3). Even with this renewed excitement about tidal tails, it remained true that detailed studies of individual objects were being pursued rather than large-scale observational surveys. Modelers attempting to answer general questions had to work with limited observational input. It was also still true that the primary focus of the field continued to be on mergers and merger remnants, as described in Chapter 4. A whole new chapter was opened in this area with the use of the superb resolution of the newly launched Hubble Space Telescope to study individual young star clusters and groups of star clusters in mergers. We will look at those discoveries shortly. In summary, both as a result of limited survey data and focus on other topics, by the middle of the 1990s, after nearly two decades of work, the questions posed at the beginning of this subsection had not been answered. There were important hints and clues to the answers, but no very firm ground.

6.3 A t t a c k i n g t h e flames w i t h the latest tools

The ground has been built up into a much firmer foundation in the present decade. We now have some quite good answers to some of the questions posed in the last two subsections. This is mostly a result of much larger surveys of all kinds of galaxies, including colliding galaxies, which provide enough data to make the results statistically convincing. The first question concerns the level of enhanced star formation in premerger colliding galaxies relative to more isolated galaxies. Recent results from a variety of surveys, including the Sloan Digital Sky Survey, in the optical and other wavebands, confirm that star formation is indeed enhanced in interacting systems by a modest factor of about 2-3. As per Larson and Tinsley's conclusion, samples of colliding galaxies do generally have a wider dispersion

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in colors and other indicators of star formation than comparable isolated galaxies. It is now clear, as suspected by Larson and Tinsley, that this is not only a result of different star formation histories in these galaxies, e.g., starbursts of various strengths occurring at various times. It is also a strong function of extinction and reddening, which are highly variable, and perhaps the heavy element abundance (metallicity) of the newly formed stars. As discussed in the previous subsection, measured enhancements of star formation further depend on the type of sample that is selected for study and the control sample, so comparisons are complex. These problems bedeviled the earlier studies, but are neatly finessed in more recent large-scale surveys. Beginning with the work of Elizabeth Barton and collaborators in the early years of this decade on about 500 galaxy pairs or groups, the strategy of a number of studies has been to look at the value of star formation indicators as a function of galaxy separation. In this approach, widely separated pairs can be taken as a de facto control sample, instead of searching for truly isolated control objects. Barton and company did indeed find that the closer the pair galaxies, the stronger the star formation (per galaxy). Given the long-standing evidence that most of this enhanced star formation occurs in nuclear starbursts, Barton et al. further speculated that the reason for the correlation that they observed is that in some, but not all systems, gas was driven into the central regions by induced bars or other tidal effects. In fact, the anti-correlation between star formation rate and galaxy separation is due in large part to only about 5 percent of the sample that has strong induced star formation. This makes sense because as discussed in previous chapters, some types of collision (e.g., prograde, M51-type encounters) generate strong effects, while many other types (e.g., distant or retrograde collisions) have only small effects. It also neatly explains why earlier studies often yielded such ambiguous results. If 5 percent of the sample is responsible for the effect, and the sample size is only a few dozen galaxies, then typically only 0, 1 or 2 galaxies show the effect. In that case, it is pretty hard to argue for the statistical significance of the result! Barton and company also found a couple of other very interesting results. The first is that the greater the line-of-sight velocity difference (a measure of how fast one is moving relative to the other) the weaker the star formation rate. This apparently supports the basic dynamical idea that the slower the close passage of the two galaxies the stronger the gravitational forces they exert on each other, and the greater the chance of forming a strong bar or spiral and driving gas into the central regions. Of course, we should be cautious, because our spectroscopes only get to see the line-of-sight part of the relative velocity, not the true relative velocity. With various assumptions, we can estimate the magnitude of this complication, and in this case, it is not large enough to cancel the result. A final result of these studies was that color data indicated a correlation between the younger stellar populations and the galaxy separation. The implication, again in accord with dynamical principles, is that star formation is induced most strongly near the time of closest approach of the two galaxies, and the induced burst ages as the two galaxies separate. (Of course, this is the

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average effect; there can be a great deal of variation regarding when individual colliding galaxies turn on their star formation.) A number of these results have been confirmed by subsequent observational studies. For example, in 2003 and 2004 several pair studies were published with samples of thousands to tens of thousands of objects. These confirmed the anticorrelation between separation and star formation, and the fact that this relation was dominated by a relatively small number of close pairs with high star formation rates. The studies also confirmed the anti-correlation between line-ofsight velocity and star formation rate. My collaborator Beverly Smith led our team in a study of several dozen colliding galaxy systems with mid-infrared observations obtained by the Spitzer Space Telescope, and the results were published in 2007. We found about a factor of two enhancement compared to a sample of non-interacting galaxies, like the many previous optical studies. This is interesting because it shows that no significant star formation is hiding in thick dust clouds, since far and midinfrared radiation can penetrate those clouds. Like the optical studies, we found that star formation tended to be concentrated in the galaxy nuclei. We presumably were detecting the emission from young stars that were more buried in central gas concentrations, than those detected in the optical. Studies within the last few years confirm that the star formation enhancement and the separation relation are valid out to cosmological redshifts of nearly one. The light left galaxies at such redshifts when the universe was about half its present age, so these phenomena are not recent developments in the world of galaxies. Altogether, with the large samples of these studies we can be much more confident about these results; comparable galaxies definitely seem to light up when they get near each other, and even more so when they move slowly past each other. This is especially true for relatively immature galaxies (i.e., those with enough interstellar gas to form many more stars), and less so for those more mature galaxies (with little interstellar gas). Before leaving this topic, I cannot resist mentioning that this was another case where theory and modeling were ahead of the observations. This is true firstly in a general sense. That is, since the earliest models (e.g., Holmberg, the Toomres, etc.) it was clear that close, strong, prograde collisions would generate a much stronger response, in the form of induced spirals and bars, than retrograde or distant encounters. This in itself helps us understand the observational proximity effect and why it is dominated by a relatively small number of systems that respond much more strongly than most. As already mentioned, similar qualitative statements help us understand the velocity-star formation effect. John Wallin and B.V. Stuart published specific predictions in 1992. They used an early parallel processing super-computer, a Connection machine, to run 1000 models of a wide range of galaxy collisions, and estimate the amount of mass transferred from one galaxy to another. They did not attempt to estimate the amount of induced star formation, but mass transfer is certainly a good proxy for

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estimating the strength of the disturbance. The proximity effect could be quantified with their results, as well as the dependence on other collision parameters. I am proud of this work of my former student, and feel that their results were not as widely used in the field as they could have been. One can only speculate why, but I suspect one reason is that their models were relatively simple, consistent with their goal of producing many of them. They used the numerical technique that Toomre and Toomre had used two decades before, which computes only the gravitational effects of the average halos of the galaxies, at a time when computers were getting powerful enough to compute much more sophisticated merger models as described in Chapter 4. However, one could not produce a thousand models with such computer codes. At about the same time Gene Byrd and Sethanne Howard compromised by producing a fairly large number of models that included a more modern and complete treatment of the gravitational forces, but they only modeled collisions where the galaxies' disks were in the same plane as the relative orbit. This obviously restricted their ability to test the effects of some collision parameters. They also did not attempt to estimate the amount of induced star formation. Computers are much more powerful now, and new "grids of models" as they are called have been published to investigate a variety of questions. In fact, codes that are more intelligent are being developed that run a number of collisions with different parameters, compare the results at different time-steps to observational images or velocity data, pick the best models, and explore more similar models to refine the fit to the data. My collaborator Beverly Smith has led a team of astronomers and computer scientists at East Tennessee State University in developing this technique. Joshua Barnes and John Hibbard have also developed a procedure for automatically modeling specific colliding galaxies. Their techniques are somewhat different but the goals are similar. In the coming decade I expect more large grids of models to be compared in detail to relatively large sets of multi-wavelength observations of interacting systems. This work should tell us a great deal about when, and how (in terms of the dynamical processes) collisions induce star formation.

6.4. G l o w i n g embers: induced star cluster f o r m a t i o n

Because of the resolution and sensitivity limitations described earlier in this chapter, astronomers could not look at regions of induced star formation in much detail until relatively recently. With the launch of the Hubble Space Telescope, and the installation on it of successively upgraded cameras, our view has definitely become much sharper. Large ground-based telescopes, with adaptive optics systems able to partially filter out the distorting effects of atmospheric turbulence, are also very powerful tools for the study of galaxies near and far. Thus, for the last 15 years or more, astronomers have had the ability to study the detailed structure of star-forming regions in colliding galaxies, and ask how it differs from that in other galaxies?

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One thing we know from studies of star-forming regions in our own Milky Way galaxy, and its satellite galaxies, is that stars are not born alone. They are born in clusters of usually many thousands of stars, which form out of the collapse of giant gas clouds that are typically tens of times more massive. The galactic interstellar gas is concentrated in its disk, where the typical cloud is much less massive than this. Such clouds must be collected and compressed to form a giant, star-forming molecular cloud. This most commonly happens in spiral arms, whose density waves have sufficient self-gravity to change the orbits of typical clouds and gather them together. This process can be easier to study in other nearby galaxies, where we can see the whole disk and the arms. Nonetheless, we have still acquired a great deal of information about starforming regions in our galaxy, and can use what we observed in other galaxies to fill in the missing pieces. The population of old star clusters is very sparse in the disk of our galaxy. However, the 200 or so globular clusters that orbit in the halo have a few things to teach us. Firstly, they are very old, and probably born about the time the bulk of the galaxy was formed. Secondly, they are individually larger than the star clusters of the disk; they contain hundreds of thousands to millions of stars. The lesson here is that very massive star clusters can be born under the right conditions; clusters that are much more massive than those that have been born in the last several billion years (Gyr) in our galaxy. This bimodal type of cluster population is also seen in Andromeda, the Milky Way's sister galaxy in our Local Group of galaxies. Two such prominent examples might make one think that this is the way it is - stars form in big clusters when the universe is young and vigorous, and smaller clusters in mature disks. However, a careful look at another local group galaxy, the Large Magellanic Cloud (LMC), yields counter-examples. The beautiful Tarantula nebula (Figure 6.5) lies on the leading edge of the LMC, i.e., the forward edge relative to its orbit around the Milky Way. At the heart of the Tarantula is a dense star cluster with a catalog name R136a, and a mass of a little less than half a million times that of the Sun. Its observed characteristics look very much like what we would expect from a young globular cluster. (Recent observations also show a range of ages in the globular clusters of the Andromeda galaxy too, including a substantial population of not so very old globulars. As I will describe in more detail later, the Magellanic Clouds are modest sized galaxies that are orbiting and have interacted with the Milky Way. They are relatively immature galaxies in the sense that a large fraction of their stars have formed relatively recently, within about the last quarter of the age of the universe. Obviously, R136a is something different from what we usually see in the Milky Way and Andromeda. On the other hand, it does not contradict the notion that stars form in large clusters in young or immature galaxies, and supports the additional idea that a large disturbance to the galaxy's equilibrium may aid the formation of such large clusters. Actually, we did not have to look quite that far to find exceptions to the norm. Even within the Milky Way, within a distance of about 5 kpc we find the massive,

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Figure 6.5 Hubble Heritage image of the Tarantula nebula in the Large Magellanic Cloud and the R1 36a star cluster (NASA, ESA, and the Hubble Heritage Team (STScI/ AURA), F. Paresce, R. O'Connell).

4-5 million year old cluster Westerlund 1. This is a very young age for a star cluster. At younger ages, the cluster will still be buried in its natal gas cloud. Westerlund 1 appears to be the most massive star cluster in the galaxy, and is one of the few that are large enough to be possible progenitors of globular clusters. Westerlund 1 seems to be located near the edge of the bar component of our galaxy. This is a particularly dynamic location, which might help account for the cluster's formation, though this is speculation at present. It is also a rather dusty location, or at least there is a lot of dust between the cluster and our Solar System. This is one reason why this very interesting object was largely overlooked for more than 40 years after its discovery, and has only recently come to prominence.

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Are R136a and Westerlund 1 just a couple of extremely rare artifacts of processes that were more common in the time of galaxy formation? Or are they not so rare? Dense clumps or knots of young stars have been observed for a long time in locations like strong spiral density waves, active irregular galaxies, and colliding galaxies. Unfortunately, in these more distant galaxies it was for a long time impossible to resolve these clumps and determine what they were. Were they individual giant star clusters, were they collections of somewhat less large star clusters, or were they clusters of clusters of moderate-sized star clusters? In the 1990s, one of the greatest discoveries of the Hubble Space Telescope derived from its ability to resolve the young stellar clumps in many galaxies. The resolution of HST observations was sufficient to show that some of the young star clumps contained, or were in fact, very massive and luminous star clusters, which were called "super star clusters" (of course!), or SSCs. These objects were detected in the early years of Hubble's operation in two different types of galaxy. The first type was nearby starburst galaxies. These several galaxies do have the advantage of being very close relative to most others. However, the tradeoff is that they are generally dwarf irregular starbursts, and not representative of the galaxy population in general. The second kind of object containing super star clusters was the major mergers. Since major mergers are rare, they are generally quite distant. Most of the dwarf starbursts studied with HST in the 1990s are well within ten million parsecs. One of the closest major mergers is the Antennae system, at a distance of less than about twenty million parsecs. (There has been some debate about the distance in recent times, with estimates ranging from about 13 to 19 million parsecs.) The Antennae system was mentioned as a merger remnant at the end of Chapter 4 (Figure 4.4), and again above as an example of a tidal tail system. Because of its proximity, and the early study of it by F. Schweizer, it is a merger archetype. Thus, it has been studied in many wavebands, and has its own subsection in the astronomical literature. A query of NGC 4038 (one of the two Antennae galaxies) on the Astrophysical Data System yields over a hundred refereed journal papers. Part of the reason it is of such great interest is that it is in the "final throes of merging," to quote Alar Toomre. (It is one of the galaxies in the Toomre sequence discussed above.) It has a very dramatic look, and it is close enough that we can both appreciate and dissect it with high-resolution observations. How do we know that it is in the final throes of merging? The proximity of the two galaxies, their modest relative velocities (almost negligible), and the evidence of substantial orbital energy dissipation (to form the tails) provide strong observational constraints. With those constraints, numerical models fill in the picture. That picture is a close passage about 600 million years ago, modest separation while the tails formed, return, and future merger in about 200 million years, according to a recent model. This general picture has not changed since the first model of Toomre and Toomre in 1972, but current versions "sweat the details" to get good fits to a wider range of observations. The two galaxies are members of a group of galaxies, so their encounter may be a more or less

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accidental collision between group members (that is, a result of the random or accidental formation conditions of the group). In 1995, astronomers Bradley Whitmore and Francois Schweizer (who had been studying the system for nearly two decades) published Hubble Space Telescope observations of the Antennae. In this paper, they announced the discovery of more than 700 blue emission knots, whose size and luminosity suggested that they were massive individual star clusters (and only part of the Antennae system was observed). Some of them were barely resolved by HST, the average inferred size being about 18 pc. Many were not resolved, but in those cases their sizes had to be quite small, so it was most probable that they were single, gravitationally bound clusters. The knots seen from the ground had been resolved (supposedly), but in many cases they were found to consist of groups of clusters. That is, another level of sub-structure was resolved with HST. Now, in this and a couple of other merger systems the characteristics of the star clusters could be studied for the first time. One very interesting characteristic was that the clusters had a very wide range of masses, specifically over a factor of 100 in optical brightness. This is similar to the population of open clusters in the Milky Way, and the clusters in the Magellanic Clouds. It is not like the ensemble of globular clusters, which have a much narrower range of masses and luminosities. Yet, the biggest of the Antennae star clusters had masses (and sizes) like those of the globular clusters. The Antennae (and its sister merger remnants) seem to have a population of open clusters like the Milky Way, but also a subpopulation that could be identified as young globular clusters. Whitmore and Schweizer listed several possible explanations for this situation. Firstly, maybe the massive clusters would never evolve into globulars, and should not be identified with them. Perhaps they should be viewed simply as a more massive open cluster population that did not form in the Milky Way, and we should look elsewhere for the progenitors of the globulars. Maybe conditions were substantially different when and where the globulars formed. (This was actually their second possible explanation). Thirdly, perhaps this is the universal way that star clusters form, but that the lower mass clusters "dissolve" with time, leaving globular clusters, with a narrow massive distribution. Dissolve may not be the best word to describe the process, as we will see shortly, but the key idea is that the cluster mass distribution may evolve. These authors and additional collaborators revisited the system with new HST observations in 1999. The new observations were nearly an order of magnitude more sensitive, and affirmed that the distribution of cluster masses is very wide. With the concurrent development of improved stellar population models, this second study focused more on the ages of the clusters. In terms of age, they found several different subpopulations. The youngest clusters are less than 10 million years old. They are concentrated near the densest, dustiest part of the system. This is the so-called overlap region, where the two galaxy disks may be grinding against each other as they pass. A mid-infrared study of about the same time, using data from the European ISO satellite found that in the densest part of

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this region much of the star formation is completely hidden from the optical band. The Whitmore et al. study found that in another region, in the north­ eastern parts of the system, many star clusters had an age of about 100 million years. A third population of star clusters was found with ages of about 500 million years, and probably formed in response to a first encounter between the two galaxies. Finally, a few true, old globular clusters, formed long before the interaction began, were found scattered around the system. Overall, the authors of this paper identified 14,000 compact objects in the HST images, and estimated that between 800 and 8000 were actual star clusters, as opposed to single very bright stars or foreground or background objects. The star fields they analyzed are simply gorgeous (Figure 3.11). Some of the more intensely star-forming regions have been called "localized starbursts," e.g., like modest starburst galaxies within a starburst merger system. Similar results on cluster mass distributions and ages were obtained at about the same time for a couple of other merger remnants observed with HST, namely NGC 3921 and NGC 7252, albeit with more than an order of magnitude fewer clusters. The reason for the reduced number of star clusters is that these objects are several times more distant than the Antennae, so that only the brightest clusters could be seen. There is no reason to believe that they simply formed many fewer. Brad Whitmore summarized early versions of these results in a presentation to the International Astronomical Union at Kyoto, Japan in 1998. They represented a very big step forward for the field. They showed that while stars are formed in clusters, like those seen in the Local Group of galaxies, there are important differences in the cluster populations. As hoped, these differences were suggestive of the possibility that star cluster formation in major mergers might be similar to that in the formation of all galaxies. Thus, such mergers may give a partial view of what galaxy formation was like more than 10 billion years ago in most galaxies. These HST studies also seemed to support another important conjecture - that not only do galaxy interactions induce star formation on average, but most of that star formation is triggered by the strongest disturbances occurring at close passages and final merger. (Note that the SDSS and other large survey evidence of a proximity effect in interaction induced star formation, discussed above, would come a few years later.) Admittedly, there were only a few examples by this date, but they clearly had stellar populations with multiple ages, indicating continuing star formation through the merging process. At the risk of damping the euphoria surrounding these discoveries, I should note that a recent follow-up study by a European group did pull back on some of the detailed conclusions of Whitmore and company. They found that a number of the putative clusters might be single stars (or small groups of stars), and that when they were removed the distribution of cluster masses might not be quite as broad as originally thought. On the other hand, spectroscopic studies in the last decade have provided evidence in support of the idea that some of the young clusters are massive enough and sufficiently compact that their gravity can hold them together when their natal gas cloud is blown away. Granted this and their

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other characteristics, it appears that they truly could evolve to become globular clusters like those in the Milky Way in the future. This is probably not true of all of them. There are generally many more young clusters in the Antennae than intermediate age or old ones. A few years ago Michael Fall, Rupali Chandar and Whitmore (Chandar being the younger addition to the established team), argued that clusters were disappearing continuously with age in the Antennae. We will discuss this process further in the next subsection. For now, I just want to note the fact that for the Antennae this is a contentious claim, and it has been questioned. The problem is that it assumes something about the rate of star cluster formation since the onset of the interaction. If clusters have formed at anything like the current rate since the time of the first encounter than many have indeed disappeared. However, we cannot be sure about that assumption. On the contrary, it may be that cluster formation has ramped up considerably as the merger has progressed. It is difficult to tell. For one thing, it is hard to count all those clusters hidden in dusty environments, and there are other difficulties we will consider in the next subsection. We are left in a quandary for merger remnants like the Antennae. I will conclude this section by noting that star clusters with a wide range of masses can also form in tidal bridges and tails. Consider the environments where very massive or super star clusters are found. We know they are found in starburst nuclei and scattered in the Antennae system, and by extension other merger remnants. They are also found in the star-forming rings of colliding ring galaxies. My research collaborators and I found a couple in the ocular ring of the galaxy IC 2163 and more in the prominent spiral arms of its companion NGC 2207. They have been found in tidal tails and gas transferred from one galaxy to another (Figure 6.6). It is widely believed that the formation of massive star clusters requires an environment with high gas pressures. This could be achieved not only in the cores of galaxies, but also in shock waves in more far-flung locations. In 2003, astronomer Karen Knierman and collaborators published an interesting study of star clusters in tidal tails in half a dozen systems observed with the Hubble Space Telescope. A couple were rich in clusters, while others had very few. A curious result of this study was that those tails that had formed a tidal dwarf galaxy at their ends, had few clusters along the tail, while those with many clusters had no tidal dwarf. It remains to confirm this intriguing result in larger samples, not to mention provide a physical explanation for it. (It will be discussed later in this chapter.) On the former question, additional observational work is underway, and more information should be available in the near future. On the latter question, there is at least one more clue. Star formation in tails seems to take place only when the gas density exceeds a certain threshold value. (Actually, the threshold applies to the observed gas column density, which is the total gas density summed along a particular line-of-sight.) It may be that when tidal dwarf galaxies are formed, much of the gas is piled up at the end of tail, instead of being more evenly distributed along it. As discussed in Chapter 3, Frederic Bournaud and Pierre-Alain Due have produced models demonstrating the feasibility of this conjecture.

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Figure 6.6 Images of several clumpy tails and bridges. The clumpy structure is emphasized in the ultraviolet images in the left column, from the Galaxy Evolution Explorer satellite (GALEX), but is still visible in the optical images in the right column (from B.J. Smith, et al., Astron. /., Vol. 1 39, and the Sloan Digital Sky Survey).

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6.5 Fading embers: star cluster d i s r u p t i o n

As mentioned above in connection with the Antennae galaxies (and by implication with the disk of the Milky Way), most star clusters seem to die young. This fact explains another remarkable fact about the majority of star clusters observed in a number of interacting systems - they are less than or about 10-30 million years old. I call this remarkable because in most of these systems the interaction has been underway for about 100 million years or so. Taken at face value, this could suggest that the interacting galaxies decided to restrain themselves from inducing any star formation for about 90 percent of the duration of the interaction, and then turn on the new stars all at once in many different parts of the system. Of course, this is ridiculous, if a bit of fun to think about. However, we need to consider other possible causes for this "the-clustersare-all-blue" effect. An obvious, but important, possible cause is the fact that star clusters fade with age. As mentioned above, the most massive and brightest stars in a cluster have short lifetimes, specifically, less than 10 million years. For all but the nearest colliding systems, the sensitivities of most observations are not much greater than what is required to see the brightest star clusters, though we know from Local Group studies that the luminosities of clusters can range over several orders of magnitude. Thus, the dimming with age can indeed render whole populations of clusters invisible. However, they do not disappear with the flick of a switch. They may be gone by the time they reach an age of 100 million years, but we really ought to see more in the 20-50 million-year age range. This is a very rough and qualitative description of a statistical question that researchers treat very quantitatively and carefully, yet the point remains. So the question becomes do we have a murder mystery on our hands? Is somebody killing off the star clusters? Actually, since they disappear at such a young age relative to typical stellar lifetimes, this effect has been dubbed "infant mortality" in the cluster populations. This is a rather unfortunate term, but one that grabs the attention. Infant mortality has been detected in a number of other galaxies besides the Antennae. Well-studied systems include local starburst dwarf galaxies, the Small Magellanic Cloud, and the Whirlpool Galaxy (M51, described in Chapter 3). In the latter case, there has been a great deal of recent research. Most of the star clusters in M51 are formed in the core (though it is not a strong core starburst galaxy), or in the beautiful spiral arms. Within the last few years, astronomers Nate Bastian and Mark Gieles published detailed studies of HST images of the clusters. Remco A. Scheepmaker recently completed a Ph.D. dissertation on similar studies. The conclusion of these works is that there is a very high rate of infant mortality, about 80 percent in the first 10-20 million years of the life of the M51 clusters. High infant mortality is also seen in dwarf starbursts. In M51 we still have to worry that a time-changing star formation rate might confuse our evaluation of a time sensitive death rate. The dwarf galaxies could potentially be very helpful to the degree that their star formation occurs in a single burst.

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However, they are small and do not produce a very robust cluster population. Nonetheless, these different galaxies all do seem to indicate some action of the dread mortality. On the other hand, that is not case in the Small Magellanic Cloud. Several recent studies, including work by the M51 team, find that baby clusters thrive and prosper there, with a mortality rate of only about 10-30 percent, or less. Evidently, even some large star clusters can be disrupted in M51, but this is not the case in the SMC. Very recent results suggest that the same is true of the SMC's sibling, the Large Magellanic Cloud. The authors of these studies speak of the lower density environment of the Clouds as being more conducive to their stability. However, this descriptive phraseology does not fully explain why clusters are not disrupted there. Actually, it may be more productive to ask why clusters are disrupted, and then later ask when and where the putative mechanisms fail to execute. A number of such mechanisms have been suggested and studied. There seems to be some rare consensus in this fast changing field that one particular mechanism is likely responsible for the rapid infant mortality - rapid expulsion of the interstellar gas. As described above, star clusters are formed out of giant molecular gas clouds. Models and observations of star-forming regions in the Milky Way suggest that, except in the most extreme cases, in the collapse of these clouds only a small fraction of the gas is converted to stars. As the most massive stars age, their strong stellar winds and eventual supernova explosions heat and drive the gas away. Since the gas is still the bulk of the mass, unless the young star cluster is very massive or compactly arranged, it will not have enough self-gravity to hold itself together. All the stars will drift away; hence the term "dissolve." Unfortunately, this seems like a very hard process to observe. It would not take the stars a very long time to drift far enough apart, to be unrecognizable as a cluster, or simply indistinguishable from other background stellar populations. My graduate student Brad Peterson has studied the star cluster populations in the colliding galaxy pair Arp 284 using HST and other observations. This system is considerably more distant than M51, and the Antennae, so we can only hope to see the largest star clusters or clusters of clusters. The larger of the two galaxies is well known for its starburst nucleus. Indeed, because it is observed nearly face-on and has little dust obscuration it is sometimes cited as a prototypical starburst. (It is not, however, in the super-starburst class.) The smaller galaxy is in a poststarburst phase. It is likely that its central regions burst into life near the time of closest approach, but with little activity thereafter. In addition to this nuclear star formation, stars are forming in various locations across the face of the primary galaxy and in tidal tails, and even in a connecting bridge, which is somewhat more rare. The Arp 284 star clusters are like those in previously studied systems in having a wide range of masses, as far as we can tell, being able to see only the more massive end of the distribution. They are grouped in clusters of clusters and found in locations that may well have experienced dynamical compression over the course of the interaction. As in the other galaxies, the overwhelming

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majority of the clusters appear to have very young ages. Unfortunately, because of the greater distance to Arp 284, which effectively reduces observational sensitivity, disappearance through simple fading is much more effective than in more nearby systems. Therefore, conclusions about the age distribution are less reliable. However, some interesting hints about the age distribution are available. HST spectral observations from the 1990s of the primary nucleus suggested the presence of an intermediate age (of order 100 million years) population of stars in addition to an old population, and the very young starburst. Moreover, Peterson found a very interesting phenomenon in a few regions of the primary disk when he used different kinds of observations to estimate ages. HST based ages of clusters grouped in these regions were very young. Ground-based optical, Spitzer Space Telescope infrared, and GALEX space telescope ultraviolet observations do not have the resolution of HST. Such observations could only resolve larger areas containing the groups of clusters. When the net age was calculated using all the radiation emitted from this area in the different wavebands (by collaborator Mark Hancock), intermediate ages of greater than 100 million years were indicated. There are several possible explanations for this perplexing result. It is possible but not very likely, given the wide wavelength coverage of the observations, that this is the result of observing a combination of old stars from before the interaction and the newly formed stars. More likely is the possibility that the light of the young stars in the clusters is exceeded by the light of intermediate age stars surrounding those clusters. If this is the case, then we do not know if those intermediate age stars are located in faded clusters, or if they have drifted out of those clusters and are diffusing out through the galaxy disk. All we can be sure of is that they are not located in very massive clusters, which we could detect. Also if this interpretation is correct it suggests that, at least in these particular regions, star formation has been ongoing, if intermittent, for at least 100 million years. These regions are located at the base of a tidal tail, a comparably long-lived structure, and a location where material seems to be infalling from a bridge. Some similar regions have been observed in other galaxies. We would very much like to acquire more data on these systems to firm up these conclusions. HST is just about the only instrument with the required resolution and sensitivity. Obtaining observation time on HST is an incredibly competitive task, and very little of it has been granted for these kind of studies. The observations that Peterson used were from the HST archive, and originally used to study a couple of supernovae that exploded in Arp 284 (Figure 6.7). Ironically, observation time has not been obtained to observe directly the star clusters and their possible demise, but was granted to study a couple of the agents of that demise. Let us return now to the discussion of other agents of cluster demise. Once we go beyond gas expulsion, the effectiveness of other processes becomes less clear. However, the appearance of the next process on the list, the death of stars, is

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Figure 6.7 HST image of NGC 7714 in the Arp 284 system, with star-forming regions circled (from B.W. Peterson, 2009).

inevitable. We have already noted the early death of massive stars in supernova explosions. Less massive stars go more quietly. The bulk of them will throw off their outer envelopes in a stellar wind. Although the speed of this wind is much less than that of a typical supernova remnant, the gas is still lost from the cluster, which typically has a relatively low escape speed. It is estimated that over the course of half a billion years the cluster could lose close to a third of its mass from this process. This process does not have the same mortal consequences as the rapid expulsion of the bulk of the mass at early times. This is more like a chronic illness. As the mass diminishes, the cluster's gravity is reduced and it expands, but it does not fall apart. However, like any victim of a chronic illness, the cluster is weakened and rendered vulnerable to other conditions. Moreover, parasites are nibbling away at it. That is, the tidal force of the galaxies, or later the merger remnant, will detach any star that wanders too far out in a parasitic fashion.

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Other diseases lurking include close passages with giant gas clouds (like the one from which the cluster formed), or other waves or disturbances whose passage can induce a gravitational shock. That is, sudden accelerations of differing magnitudes across the cluster can act to pull it apart. The physical mechanisms of these processes are quite well understood, and in principle their action could be calculated in any given situation. However, in the strongly fluctuating dynamical environment of merging galaxies there are many different situations for different clusters. This makes it pretty difficult to model the medical histories of a population of clusters. It may be possible in simpler cases like ring galaxy collisions, and I am sure it will be attempted in the coming years. A great deal more observational input would be helpful, as usual. For now, we have to accept that we are not yet in a position to resolve all the mysteries about the life and times of interaction-induced star clusters.

6.6 How t o start t h e fire

If we have to wait for progress on the evolution of star clusters, then maybe we should back up for a moment, look around to see where we are, and then see if we can advance along another path. In this chapter, we have seen that the net star formation is enhanced in galaxy collisions, even before merger, though not by a huge amount. It takes place in the form of clusters as in the Milky Way, and Local Group comparisons are helpful. Much of this star formation takes place in the cores of galaxies, though some of it also occurs in disk waves, and tidal bridges and tails. Except for the generalities of gas transfer into galaxy nuclei, we have not yet considered the mechanisms that drive this star formation in specific locations at certain times. A glance at the varied locations of the blue star clusters in the Hubble Heritage collection image of Arp 87 (see book cover) is sufficient to reveal the complexity of this problem. Yet there is a very basic answer to this complex question - compress the gas. We have already assumed this as a basic driving force behind nuclear starbursts, and in shock waves. In considering the role of compression, we have to begin by asking more basic questions, like compression of what? Dense interstellar clouds occupy only a small part of the volume in galaxy disks, with most of the volume being filled with warmer, and even more tenuous small cloud and inter-cloud phases, as noted in Chapter 2. Figure 6.8 shows the famous Eagle nebula and its surroundings in the Milky Way. Captured in that image are various clouds and inter-cloud regions, conveying some idea of the huge range of states in the interstellar gas. Even quite large compressions would not necessarily change the state of this inter-cloud material in many cases. Specifically, stars are not generally produced in this material, and such compressions would not change that. Stars form in massive, dense clouds. However, the internal pressures in these clouds must be large to come close to balancing their self-gravity. (A technical detail: large clouds are not thought to be in a perfectly balanced, equilibrium state. Their lifetimes may be quite short and relatively unbalanced,

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Figure 6.8 Optical image of the Eagle nebula star-forming region in the Milky Way (T.A. Rector, NRAO/AUI/NSF, and B.A. Wolpa, NOAO/AURA/NSF).

but even granted that, their internal pressures are observed to be relatively high.) My point here is that moderate compressions do not just squeeze new stars out of the gas like water out of a wet cloth. The effects of such compressions are probably more subtle and indirect than that in most cases. However, even that is not always true, sometimes the star formation appears to be the very direct result of violent compressions. Let us consider some of those cases first. The Hubble Heritage image of the young ring galaxy Arp 148, also known as Mayall's object, seems to be not so much a case of squeezing drops out of a washcloth on a galaxy scale as of blasting the water out with a very large bullet (Figure 2.2). As far as I know, the observations used to construct this image have not yet been analyzed in detail. However, it appears that hundreds (if not

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thousands) of new blue clumps, each representing at least one young star cluster, have been produced by a very recent collision between the two galaxies. Some of these clusters appear to be strung on filaments between the two galaxies. Yet we should be cautious in interpreting this image. For example, we do not know that these clusters are produced by the direct collision of gas clouds in the two galaxy disks. Specifically, we do not know the ages of the clusters. They look very blue, which suggests that they are as young as those in other recently analyzed colliding galaxies, that is, less than about 10 million years. The observed line-of-sight velocity difference between the two galaxies is about 250 kilometers per second. It looks like they may be moving as much in the plane of the sky as in the direction out of it, so this velocity may be only a fraction of the total. If so, then the time since closest approach, when the two galaxy nuclei were probably nearly on top of each other, could be close to 10 million years, and the star clusters could have begun forming at the time of collision. On the other hand, the ring wave would have also started forming at that time. Blue clusters in the wave may be less than a few million years old, and may have started forming at the same time as the ring wave itself. In Section 2.5 we considered the "taffy" and "shovel" galaxy branches of the ring galaxy family. Arp 148 is sort of an extreme shovel system, where the companion hits the primary disk in an edge-on configuration. Recall that taffy systems result when two galaxy disks collide face on, and their interstellar gas is strewn between them like pulled taffy. Only a few examples are known, and in the first and best studied of them the two galaxies are nearly perpendicular to our line-of-sight and hard to study. Another candidate taffy system that my collaborators and I have begun to study is Arp 261, an ultraviolet image of which is shown in Figure 6.9. As this image suggests, the numerous star clusters in this system are very blue, probably very young, and some of them may result from direct collisions between the gas clouds in the two disks. Much more study will be needed to know for sure. Thus, we do not yet have enough information to know if direct cloud collisions produce star clusters in taffy/shovel systems, but we do know where to look, and have some data to analyze. The Antennae system provides another possible example of star formation induced by the direct collision of two disks. Mid- and far-infrared observations have revealed a great deal of star formation in the so-called Overlap Region, where the two disks may be grinding against each other. Unfortunately, the reason infrared observations are needed to see this activity is because the Overlap Region is hidden by dense dust clouds, and consequently quite hard to study. In addition to direct collisions between parts of gas disks and their individual clouds, there is another way to trigger the prompt and violent collapse of large gas clouds. This is by over-running them with a strong shock wave. The shock wave may be generated by tidal forces, and propagate through the inter-cloud medium. It can compress the ambient interstellar magnetic field as well as the gas. Strong local shock waves may result from supernova explosions, but tidal waves may drive larger shocks, on scales of more than several kpc. The observational evidence for such shocks comes from very thin, evidently

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Figure 6.9 Ultraviolet image of Arp 261 system from the GALEX satellite. Note the numerous blue clumps of young stars (Beverly ). Smith).

compressed dust filaments in some spirals and tails. The HST image of the NGC 2207/IC 2163 system has a number of beautiful examples (Figure 3.6). Though long, the width of these structures is comparable to the size of individual star clusters. That is thin compared with typical observational resolutions. Moreover, the shock structures are not as intrinsically bright as the young star clusters that may form in their wake, so they are much harder to study in detail. This is also true of more moderate shocks that may trigger star formation by assembling large clouds through compression effects, rather than simply crushing them. These would be very interesting processes to observe in detail. We may gain some insight into these processes by studying Local Group galaxies, though very strong shocks are rare. In trying to account for super-starbursts in major mergers, astronomers Chanda Jog and Philip Solomon explored the idea that the clouds falling into the central regions of the merger remnant might be crushed by the pressure, and

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dense clumps within them squeezed into star formation. Their idea was based on the realization that large amounts of gas would be tunneled into the core of the evolving merger remnant, and as a result the ambient gas pressure would be very high. It could be much higher than almost anywhere within a normal galaxy like the Milky Way, and even high enough to exceed the internal pressures of giant molecular clouds. The insides of these clouds are thought to be highly turbulent with many small-scale clumps that could be squeezed directly into proto-stars. Here again, observational resolution is too limited to allow adequate study of this process, especially through the dense dust shrouds in major merger remnants. In less violent mergers or interactions, gas will not be fed into the center as rapidly, and wave-driven shocks will not be so strong. Then gravitational forces have a chance to operate within an individual galaxy disk, merger remnant, or even in parts of tidal tails. We have seen in earlier chapters that large scale gravitational instabilities in disks can lead to the formation of bars or large, socalled "grand design" spirals. These can feed gas into central star-forming regions on moderate timescale (e.g., hundreds of millions of years rather than the tens of millions of years or less that might characterize a disk-disk collision). Within spiral waves or parts of tidal tails, the interstellar gas may become selfgravitating and the gravity may pull diffuse gases together into large clouds, which can then collapse and form star clusters. Given typical average interstellar gas densities in the disk of the Milky Way, gas must be pulled in to a central point from a radius of about 100 parsecs in order to gather enough to make a typical star cluster. When waves compress the gas, part of the gathering is accomplished therein; all that remains is to gather in gas along the wave. We can envision many small clumps, each trying to pull more material onto themselves. We can also envision many clumps getting close enough to pull together via their mutual gravity, or being part of larger clumps that are collapsing. Chaotic and turbulent are the words that come to mind to describe this process. Even cotton candy swirling around the glass enclosure until it accumulates on a paper stick is probably a more orderly process, though with some similarities. Numerical models of this process have been published, and despite the huge range of scales and forces involved the most recent models are capturing it ever more faithfully. What determines the size of the large gas clouds, and ultimately the star clusters, that formed this way? We know from the discussion above that the size does depend on the environment. For example, we do not see super star clusters formed in the weak, wispy spirals found in some isolated disk galaxies. A theory dating back to work of James Jeans in the early twentieth century provides some answers. It says that in gravitationally unstable systems there is a characteristic length (or mass) scale, now called the Jeans length (or Jeans mass), that specifies the scale of fragmentation within the system. The Jeans length depends on the density of the material, and its temperature or the typical random speeds with the system. The Jeans mass is larger in the compressed environment of large-scale waves because of the turbulence, but there is enough material to exceed that critical mass on the relevant scales. This allows very massive clouds to form, and helps us understand the birth of super star clusters.

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In summary, it appears that there are a variety of processes responsible for star cluster formation via cloud build-up or cloud crushing. These various processes also operate on a wide range of scales. The complexity of this is very unfortunate for those of us who would like a straightforward theory of interaction-induced star formation with nominal predictive powers. However, this complexity is not totally unexpected. One of the most fascinating aspects of galaxy collisions is how dynamical processes extending over scales as large as tens of thousands of light years, control processes on scales of light seconds (the sizes of stars), and vice versa. That so many different processes should play a role in the range between these scales should not be very surprising. What is daunting is the possibility that we may have to take account of the processes working on the different scales in order to understand induced star formation. This is like the notion that we would have to understand the forces behind every gust in a thundercloud from millimeter to kilometer scales in order to predict how much rain falls. Fortunately, that may not be the case in either situation.

6.7 Forecasting fires

In using the phrase "predictive powers" above, I do not mean a prediction of the kind that we can sit back and observe playing out, which is absurd given galaxy timescales. Rather, I mean that we understand the physical processes well enough that we can make models that tell us quite precisely where and when star formation occurs in collisions of a given type, and that yield quite accurate agreement with observations of specific systems. This is like determining where and when the rain falls, given information about the large-scale air masses and fronts. It is one of the primary goals of this field to do the "5 day forecast" in units of hundreds of millions of years. There are reasons to hope that this can be done. Foremost of these hopes is the possibility that relatively simple phenomenological laws might apply in a wide range of environments and over a wide range of scales. Several have been proposed and used in computer simulations. The most well known is the Schmidt Law, which states that the local star formation is a direct function of the local gas density, more or less. The Schmidt Law is old by the standards of this field, having been first proposed by Maartin Schmidt in the early 1960s. It has been something of a work in progress through that time, so that there are a number of different forms of the law. Schmidt's original form stated that the local star formation rate in a galaxy disk depended on the gas surface density squared. Since the cold gas disk is generally very thin, e.g., a monolayer in terms of giant molecular clouds, it is nearly equivalent to say that the star formation rate goes as the local (volume) gas density squared rather than the surface density squared. However, that is not literally true throughout galaxy disks, not even in the Milky Way. As mentioned in an earlier part of the book, the gas disks generally extend significantly beyond the stellar disks in galaxies. In these extended gas

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disks, the star formation rate is very low. It was once thought to be essentially zero, but recent sensitive ultraviolet observations by NASA's GALEX satellite have shown the presence of very low levels of star formation in a number of extended disks. That discovery is not too important here, where the point is that star formation in the extended disks is much less than would be expected from an extrapolation of the Schmidt Law applicable to the inner (star-forming) disk. In short, the simple Schmidt Law does not apply to the outer disk. Robert Kennicutt, an American astronomer who, at the time of writing, holds the prestigious Plumian Professorship at Cambridge University in England, clarified this situation with extensive mapping of the star formation properties of a sample of a few dozen disk galaxies in 1989. He found good evidence for a threshold Schmidt Law. That is, a Schmidt Law was obeyed above a certain threshold value of the surface density. There are a couple of caveats to Kennicutt's result. The first and simplest is that the star formation rate seemed to depend on a slightly lower power of the gas density than the classic density squared law. This was not a great surprise since a number of observation studies both before and after Kennicutt's found values of this exponent ranging, typically, from 1 to 2. Some very recent studies show that the exponent is generally well above 1.0 when the measured surface densities are those of interstellar components of moderate density, like atomic hydrogen or carbon monoxide molecules. However, when molecules that are only found in the dense interiors of large clouds are observed, the exponent is found to be close to 1.0. The latter result makes a good deal of sense, since it suggests that the star formation rate correlates directly with density (and presumably the amount) of the gas component from which stars are actually formed. The implications of the first result are more subtle, and not completely understood. The second Kennicutt caveat is that the value of the threshold density varies from galaxy to galaxy, though not in an arbitrary fashion. It varies according to a formula given by Alar Toomre, which derives from the local balance of forces, gravity against gas pressure, local turbulence and shear forces in the galaxy disks. These forces, especially the later, depend on structural parameters of the disk. The threshold density, and the physical arguments behind Toomre's formula, helps explain the two-part structure of galaxy disks. In the stellar disk the gas density is greater than the threshold value and stars are formed at a respectable rate. At larger radii in the disk the density is sub-threshold and there is very little star formation. Kennicutt also advocated another somewhat older idea, that galaxy disks are self-regulated. He noticed that in the star-forming parts of galaxy disks the gas density was usually quite close to the value of the local threshold density, and suggested that this was not likely to be an accident. If the gas density is pushed much above the threshold, then the star formation rate will rise quite rapidly, resulting in a lot of energy input into the interstellar gas via the ultraviolet emissions of hot young stars and their winds, and the supernova explosions that end their lives. The star-forming clouds will be disrupted and more. Portions of

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the gas will be heated to very high temperatures and the thermal expansion will drive it to very low densities. In such a state, the timescale to re-coalesce into clouds will be long. Moreover, other portions of the gas will be thrown out of the disk in "galactic fountains" originating from massive star clusters, or moved into other low density regions like the outer disk. The once vigorous star formation will be terminated, or self-regulated by these "feedback" effects. Gas will also be locked up in stellar remnants, never to return to the interstellar medium. Dense gas disks will also be more susceptible to large-scale gravitational instabilities and form bars or strong spiral waves, enhancing the star formation within the waves. These waves, however, will work to redistribute the gas. Some will be tunneled into the central regions (possibly triggering a core starburst). A part will carry mass and angular momentum to the outer disk. In sum, we get more feedback and self-regulation from the waves. However, the self-regulation may work most strongly in one direction. Suppose that, by some means, the gas density is reduced well below threshold across the galaxy disk, and star formation rates are negligible. In such a case, the disk has no internal mechanisms to increase the gas density, and turn the furnace back on. The existence of the socalled low-surface-brightness disks, suggests that such disks can become stuck in the dark for long ages. These galaxies typically have old stars in a central core, but a disk dominated by gas rather than stars, the opposite of the situation in more normal disk galaxies. There are at least two ways to restart such disks. The first is a strong kick from the outside, that is, a collision with a substantial partner. Even a high velocity encounter that does not result in merger can drive waves through the quiescent disks and relight the fires for a time. The second way to restart a low-surfacebrightness disk is simply to add more fuel. Cosmological simulations show material continues to rain down onto galaxies, groups and galaxy clusters from extended filaments and sheets connected to them. This rain decreases with time, making it hard to detect, but observational evidence for its existence is increasing. I intentionally use the rather vague term "material" in the sentence above. The material that falls onto galaxy clusters may consist of galaxy groups or even other clusters. The material that falls onto galaxies like the Milky Way or the Local Group consists of gas that has not condensed into galaxies or small dwarf galaxies (or perhaps occasionally larger galaxies like the Magellanic Clouds, though in such cases we would be discussing galaxy collisions rather than a more gentle accretion). In the case of typical galaxies, some fraction of this infall will come to the disk, and probably land more softly than in a typical merger. Eventually enough material may build up to strengthen the hand of disk self-gravity and light the fires of star formation. However, this scenario is still speculative, and at present there is little observational evidence for it. The timescale may be too long; it certainly is in the case of extreme low-surfacebrightness galaxies, which have persisted in a state of somnolence for the age of the universe. We can conclude that self-regulation from a diminished state of star formation is a somewhat chancy affair for disks. That issue resolved, let us

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return to the Schmidt Law. Rob Kennicutt and his collaborators have continued their investigations of it up to the present. In the late 1990s, he published results of a study of starburst galaxies, including major merger remnants. There he concluded that the Schmidt Law held as a global average for these galaxies. That is, their star formation rates correlated with a bulk average gas surface density computed, for example, from their total gas mass and a characteristic length scale. He noted, however, that besides the Schmidt Law, other suggested parameterizations could fit the data. Even now, it is a bit hard to know what to make of this global result. The impressive thing about the earlier Schmidt Law results is that, relative to Toomre's formula for the threshold density, it scaled correctly across disks, as well as from disk to disk. In the global starburst results, at least the former result is lost, since spatial variations within the galaxies are not considered. On the other hand, given the chaos within new merger remnants it is hard to see how one could do better. Without a coherent disk, it is not clear that observable surface densities should have any relation to local star formation rates. Yet it is generally difficult to measure local gas densities, which we might expect would determine the local star formation. Another aspect of the ambiguity is the question of how to determine threshold density. Toomre's formula depends on disk parameters, which may no longer be well defined. More recently, Kennicutt's group has returned to the study of isolated, nearby disk galaxies. Specifically, he was the leader of the SINGS (Spitzer Infrared Nearby Galaxies Survey) group, which obtained large amounts of Spitzer Space Telescope observation time of a sample of 75 nearby galaxies. One of the goals of this group was to compare Spitzer mid-infrared measures of star formation to optical (and other) measures. Since Spitzer observations could see through a great deal of dust obscuration, they could detect sites of star formation hidden from the optical observations and help obtain a complete census of star formation in the sample galaxies. Among other things, this would allow a more thorough re-evaluation of the Schmidt Law, or as it often called now, the Schmidt-Kennicutt Law. The SINGS team did find missing star formation. Given that stars begin their lives buried in gas and dust clouds this was inevitable. What they did not find were special secret hiding places with massive amounts of star formation. The hidden star formation was generally located in the vicinity of optically visible star formation sites. The amount of hidden star formation was comparable to that visible in the optical. In these ways star formation in galaxy disks contrasts greatly with super-starbursts in major merger remnants. The SINGS team went farther and obtained infrared spectral observations with the Spitzer Space Telescope. They used these and other observations to confirm that there were no significant amounts of star formation hiding from both Spitzer and optical observations. More precisely, they calibrated a combination of the Spitzer and optical Ha emissions that could account for all the star formation in their sample galaxies. Without digressing too far, it is worth noting that this opens the door for a variety of other investigations of star formation in galaxy disks. Follow-up

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studies (by other research groups) include high-resolution observations of the atomic hydrogen gas and molecular gas in SINGS galaxies. In a subset of the SINGS galaxies, these observations have confirmed the validity of the Schmidt Law over a range of scales from a bit less than a kiloparsec to tens of kiloparsecs (i.e., the whole disks). These and other studies have confirmed the especially simple form of the Schmidt Law mentioned above - the direct correlation between the star formation and the amount of molecular gas (or the molecular cloud density). Because of the beauty of this result, I would emphasize that it has been found to hold in a variety of different kinds of galaxies, and in both cluster and field environment. It very much appears that the question of how stars are formed in isolated galaxy disks is being solved. Will the same solutions apply to colliding galaxies? As already noted, there are some reasons to think not. In isolated galaxies, molecular clouds are built up with the aid of local gravitational instabilities and larger scale gravitational anomalies in the form of waves that gather the gas together. Feedback effects limit the gas accumulations and the amount of star formation. There is selfregulation and a degree of order and balance in the operation of the different forces that control star formation. The violent disturbances of galaxy collisions surely upset this balance of forces, but how does this affect the star formation? A number of possibilities have been considered. Before looking at a couple of examples, I would note that there is still room for speculative theory in this area. Observational constraints are not nearly as strong as for galaxies that are more isolated. The reason for this is simple, and has come up several times before. Because galaxy collisions are relatively short-lived phases, and are becoming less common as the universe ages, they are rare. This means there are few nearby collisions, and that the average distance of a sample of colliding galaxies is inevitably much greater than a comparably numerous sample of isolated galaxies. More distant means harder to observe with comparable resolution and sensitivity. Of course, we do know a few things about star formation in colliding systems, which were discussed in earlier sections of this chapter and earlier chapters. Large-scale surveys and the Spitzer study of several dozen systems by Beverly Smith, our collaborative team, and other teams mentioned above, found that star formation is slightly enhanced in colliding systems even before merger. Several observational studies have also found that a good part of this enhanced star formation is due to nuclear starbursts. The longest wavelength observations (farthest into the infrared) showed emission that was more concentrated to the central regions, presumably as a result of star formation buried in dense gas and dust. We also have a good many studies of star formation in tidal bridges and tails, and in waves like those in ring galaxies. The problem is that these are not as systematic and thorough as those for nearby disk galaxies. The observational limitations have only allowed us to answer specific kinds of questions. For example, we have some information about the net amount of star formation in these structures, and in the nearest cases information about the ages and masses

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of star clusters within them. Because of their intrinsic interest, special attention has been focused on the study of tidal dwarf galaxies. Using HST observations of tidal tails in merger remnants, Karen Knierman and collaborators have found a number of tentative systematic trends in these structures. One already mentioned above was that there seems to be a dichotomy between those tails that form numerous star clusters along their length, and those that form a large tidal dwarf galaxy near the end of the tail. Very recent work shows that the dichotomy is not too strict. There appear to be faint clusters in all tails, and the dichotomy is more between bright clusters and tidal dwarfs. This may give a clue to the cause. For example, age may be a significant factor. Bright clusters may form preferentially when tails are young and dense, while dwarfs may form later when material piles up at the end of a tail stretched to its greatest extent. A second systematic feature investigated by this group is the dependence of star formation in tails on gas properties. They find evidence for a threshold gas surface density for star formation with a universal, constant value in tails. This is not the same as in the Schmidt-Kennicutt Law, which has a threshold value that depends on the local conditions. This is potentially very interesting, but given the small sample size and the limited resolution, I would regard it as a tentative result. For structures like bridges and tails in colliding galaxies, the observational database contrasts starkly with studies like SINGS and its offshoots. We do not have high-resolution maps of star formation indicators in a wide range of wavebands, and comparable maps of different phases of the interstellar gas for dozens of systems. Thus, we cannot address the fundamental questions in the same kind of detail. However, several research groups continue to work very hard on upgrading this database. Continuing advancements in astronomical instru­ mentation and observing facilities on ground and space promise much. We may be still like early cartographers in the vast wilderness, but an industrial revolution is coming. (A concrete indication of the latter is the data already assembled by the Sloan Digital Sky Survey and that promised by its successors.) From a general point of view, the SINGS effort convincingly nailed down the earlier findings of Kennicutt and others. These findings were based in turn on observational and especially theoretical work dating back to the 1960s. I think it is fair to say that the theoretical understanding of star formation in colliding galaxies is both controversial and quite infirm. In brief, the Schmidt Law may not apply universally, and we do not have a ready replacement. This is implicit in the discussion of several mechanisms in different sections above, e.g., shock waves and self-gravity operating on very different scales. Moreover, the star formation law in colliding galaxies may vary with the environment. This is a speculation and there are qualitative arguments both for and against. One against is that as far as we can tell the mass spectrum of star clusters that form in these different environments is the same, though the mass of the largest clusters does vary. One argument for is that it seems to require some special conditions to form globular clusters, and their likely progenitors the super star clusters.

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Personally, I think these questions are some of the most important ones in the field of galaxy collisions. In puzzling over them, I take a good deal of inspiration from the semi-quantitative derivation of Toomre's formula for threshold star formation in disks. The essence of Toomre's argument is a comparison of the strength of shear and pressure forces (which tend to resist cloud compression) with gravity on a variety of scales. In a typical isolated disk, shear is the dominant force on large scales, and pressure, including turbulent pressure, can overcome gravity on the smallest scales. There may or may not be an intermediate range of scales where gravity dominates and can assemble starforming clouds. Alternatively, that may only be possible in strong waves, which compress the gas enough to open a gravitationally unstable range of lengths. Presumably, the gravitationally unstable scale lengths are less than the width of the wave, or else it will still be difficult to assemble large clouds. I think this last example points the way to understanding the issues of cloud assembly and star formation in colliding galaxies. What follows are some personal (but not especially original) thoughts on how we might extend that example. Begin, like Toomre, with the assumption that the gravitational force, which is a long-range force, is always present at some level on all scales. Its operations with regard to star formation can be subtle, however. When we say that shear forces resist cloud assembly in disks, we should remember that those shear forces are a symptom of the differential rotation in the disk, which in turn, results from a balance between centrifugal force (angular momentum of the disk material) and the large-scale gravity of the dark halo. In short, big gravity plus angular momentum make shear, which prevents small-scale gravity from making star-forming clouds. It is a bit like the way a huge electrical discharge in lightning can turn off your electric lights. In the disturbed disks of colliding galaxies, there is a variety of forces that both resist and help gravity in gathering clouds. The most obvious, and the most studied, are tidal forces. This is rather ironic because tidal forces are in fact a variation on the theme of gravity. As discussed in earlier chapters, tidal forces are differential gravity, the difference in the gravitational pull (e.g., from the partner galaxy) at one location, relative to other nearby locations. The important point is that tidal forces stretch along the line of connection to the perturber and compress in the other directions, as discussed in earlier chapters. The stretching can pull material apart, reducing its self-gravity, and the chances of accumulat­ ing material in large clouds. The compression can have the opposite effect, and so the result depends on the specific circumstances. The next forces to consider are shock waves, which we have already discussed in the previous chapter. While shock waves can produce very strong compres­ sions, they tend to be quite narrow. Thus, their effect is like that of a spiral wave in an isolated disk. In fact, large shock waves are often produced within other waves (e.g., spiral, ring, ocular waves). Shocks can be produced on very small scales or over a range of scale in supersonic turbulence, which can be viewed as our third force. Strong turbulence can be produced via numerous instabilities in large-scale

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gas flows, or as a result of feedback effects of star formation as discussed above. While large-scale shocks can help gather gas into large clouds, small-scale turbulent shocks can be more disruptive, and act against assembly by self-gravity. However, the theoretical astronomer Bruce Elmegreen has emphasized the other side of this coin. In some environments turbulence may merely prevent the formation of modest clouds, because gravity can only act on large scales where there is enough mass to allow it to overcome the turbulent pressure. Without the turbulence, small-to-moderate clouds might have formed first, and their feedback effects dispersed the gas before it could be gathered into larger structures. Thus, turbulent effects can work to favor the production of very large clouds, and very large star clusters (or dwarf galaxies). Shear effects also become dynamic and variable in galaxy collisions. In fact, a lot of effects that are often casually referred to as tidal dynamics are in fact the result of torques - the increase or decrease of rotational motions relative to, say, the initial disk rotation axis. We have talked about this before. These torques spin material out in tidal tails or spin it down into the central regions of a galaxy. They drive spiral waves and large-scale shocks. Unfortunately, the various forces are interrelated, making things more confusing. The final type of force to consider is feedback. Like the others, feedback can operate over a large range of scales, from local gas heating by the ultraviolet radiation of a single hot star, to strong galactic winds driven by a super-starburst in a major merger remnant. Although not always true, feedback effects tend to do their work after the battle between other forces is resolved, and so they are often more or less decoupled from the other forces (except gravity). In conclusion, it appears that by considering the action of all the forces we can apply Toomre-type arguments to understand how clouds build up, and where and when star formation occurs in galaxy collisions. In isolated disks, the Toomre criterion can be reduced to a single equation applicable across the whole of the disk. In colliding galaxies, the forces that determine that equation vary from place to place and with time, and so, the criterion has to be evaluated locally (over a range of scales) and instantaneously with knowledge of the strength of all the forces. This is a rather overwhelming task! Even with modern computing resources, it is hardly possible to model all the scales and all the processes involved, let alone to do so routinely in order to decipher many individual interacting systems. So modelers, as well as those who want an understandable conceptual picture, look for simplifications. Let me give a couple of examples. Several years ago, Josh Barnes argued that while most modelers were simulating star formation by some kind of Schmidt Law or related prescription, one based on energy dissipation behind shocks might be more accurate. He backed this up with a comparison of methods in models of one particular system. At the other end of the spectrum, Florent Renaud and his collaborators have recently presented work showing the advantages of a simple model in which star formation is induced wherever there is tidal compression. They have applied this idea in models of the Antennae galaxies, where they found a reasonable match to the locations and local history

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of star formation. This is somewhat surprising given that their model does not even consider gas dynamics or shocks. I confess that I am a modeler who has long used a threshold density for star formation in my models. Yet I am not very partisan about this. I strongly suspect that all the different approaches, though over-simplified, are capturing parts of a dynamic Toomre criterion. Probably we will not have to do too much better until observations provide a very accurate picture of the star formation history of some nearby colliding systems. Where does that leave us in terms of a hoped for basic conceptual picture with predictive power? Not too far along, I am afraid. There is a lot to recommend the simple generalization that wherever cold gas is compressed, and held together for the relatively brief time it takes to form stars in large clouds, they will form. ("Build it and they will come.") This is a simple idea to state, but we can appreciate the fact that all the complex forces involved make it hard to quantify, even on a modern computer.

6.8 How did it burn? In the last sub-section we left the grand problem of the ways and means of induced star formation in colliding galaxies unsolved. In this section we take a slightly different approach. Instead of considering the details of that problem, let us look at what we can learn about the statistics of star formation in collisions and mergers from the large surveys and from deep probes of star formation in high redshift galaxies. Some interesting questions include: 1) what fraction of all stars in the universe is made in collisions; 2) when are most of these stars made; and 3) what are the competing processes? The last question is the easiest to answer here. Star formation can be either internally or externally driven. In the former case the mechanisms are usually bars or spiral waves. In the latter case the two major drivers are collisions or external infall. The distinction between the two is somewhat arbitrary. In both cases it is a matter of continuing cosmological structure growth. In brief, either material falls onto the galaxy or the galaxy falls into another structure. If the infailing object is a galaxy substantial enough to disturb the galaxy under consideration, then we call it a collision; if not, then it is simply "external infall." Micro-mergers, collisions with dwarfs just barely able to perturb the primary galaxy, are in a gray area between. A particular kind of infall material is primordial gas that has never been incorporated into galaxies, or gas that is a mixture of the primordial and gas that has been expelled from galaxies in winds. The latter is enriched in elements heavier than hydrogen and helium, formed in and then ejected from stars. It may well include a large fraction of the normal matter (as opposed to dark matter) in the universe. The possible role of this gaseous infall was first emphasized in the 1970s, and subsequent perceptions of its importance have waxed and waned like many

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other 70s fashions. Currently it is back in fashion. A primary reason for its variable popularity is that it is incredibly diffuse and hard to detect. An important reason for its current vogue is that several types of observation (ranging from the Milky Way halo to very distant cosmological observations) do seem to have begun to detect it, though we will not pursue the details here. Simulations of cosmological structure growth also suggest its importance, though more so in the past. As the universe expands, infall rates decline. So too does the net star formation rate decline, which is a partial answer to the second question above. The technical term for the mean star formation rate averaged over a large number of galaxies at a given age of the universe is the "cosmic star formation rate (CSFR)." There has been a great deal of interest in the evolution of the CSFR in recent decades as our ability to study very distant, high redshift galaxies has increased. The firmest result of work in this area is that the CSFR increases out to a redshift of about 2-3. The universe was less than about a few Gyr old at this redshift, so the previous statement means that the CSFR has been decreasing over most of the age of the universe. There is now increasing evidence for a second result, which is that the CSFR peaked at about this time, and also decreases at higher redshifts, i.e., the observations extend out to a redshift of about 6 (Figure 6.10). At this time the universe was very young indeed, and it is likely that most present-day galaxies were only beginning their assembly.

Figure 6 . 1 0 Summary graph of the results of various studies of the cosmic star formation rate versus redshift, which appears to peak at redshifts of about 2-3 (from A.M. Hopkins, http://arxiv.org, 2006).

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A third important result in this area is called "downsizing," which in this field means that the galaxies with the highest rates of star formation tend to be getting smaller with time. A lot of strongly star-forming galaxies at present, including many of the nearest colliding galaxies, like those in the Arp Atlas of Peculiar Galaxies, have masses of about 3 x 10 10 solar masses. This is an order of magnitude less than typical late-type galaxies like the Milky Way. Out at redshifts of 2-3, where the CSFR peaks, the strongest star forming galaxies had masses more like the Milky Way. The largest galaxies seemed to have pulled themselves together and carried out the bulk of their star formation quickly. Of course, such galaxies are relatively rare. The CSFR peaked when the middle classes of the galaxy world got it together. Now much of the star formation occurs down in the apparently still vibrant lower classes. Nonetheless, most of the gas within galaxy disks has been turned into stars or left in stellar remnants, and star formation is a declining industry. Let us now turn to the first question above: what piece of this industry has been owned by the colliding galaxy conglomerate? In the euphoric times that followed the discovery of ULIRGs, and the confirmation that most were major merger remnants, the betting was that this fraction was very high. However, the statistical evidence was not that good. More extensive surveys were undertaken in the 1990s, and they were able to tell us more about the pair fraction and the merger rate, both locally and at somewhat higher redshifts. They were not able to say much about the fraction of the star formation due to all interacting systems. Quite recently a new generation of survey results have been published. These survey results typically use a combination of ground-based observations extending to very faint magnitudes, and space-based observations to do both that, and extend the wavelength range to help determine the star formation rate. Because of these complexities, they typically involve quite large teams of astronomers. Hundreds of galaxies are studied. I regard this as kind of an intermediate number. It is a lot better than the sample sizes of a few dozen available 20 years ago. However, for really firm conclusions, we would like even more. The surveys have also found that only a small fraction (about 10 percent) of the star formation in massive galaxies is stimulated by apparent major mergers. So much for merger madness and the euphoria of the late 1980s and early 1990s! However, there are a number of caveats. The first is that making these estimates still involves a number of technical difficulties, whose effects are hard to evaluate. Secondly, even these studies acknowledge that the galaxies in their samples with the strongest ongoing star formation are usually in merging and interacting systems. Two more caveats concern time/age and the role of minor mergers. Concerning the former, at present large surveys only extend out to redshifts of less than 1.0, and most sample galaxies have redshifts of 05-0.7. This is back nearly one-third of the age of the universe, yet the time of peak star formation is back much farther still. We know that at the present time a substantial fraction of the most massive galaxies are early types, even the field galaxies. In the current jargon, they are red and dead. They formed most of their stars and lost most of

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their gas long ago. We also have evidence that in recent times such galaxies are more commonly involved in dry mergers, versus the wet mergers at the earlier stages of their lives. Like humans, most galaxies tend to find companions near their own age or stage of maturity. At least one major survey (lead by F. Hammer of the Observatoire de Paris) finds that as many as 75 percent of intermediate mass spiral galaxies have experienced a major merger, and presumably a starburst, within about the last half of the age of the universe. Other studies find that interaction-driven star formation seems to be more important in intermediate mass galaxies than in high mass galaxies. This is in accord with the downsizing hypothesis discussed above. Indeed, it is a slightly different view of the same thing. Thus, it is not necessarily true that major mergers do not contribute a good deal to the star formation of massive galaxies. Those mergers just may have occurred too far in the past for us to be able to observe it at present. This is not entirely different from Toomre's original idea that major mergers might make ellipticals, even though it was a very rare occurrence in the present universe. The new surveys provide more quantitative estimates, however. They are also beginning to allow us to measure the role of minor mergers at high redshift, which are much more difficult to see at great distances. One very recent study (lead by Sharda Jogee of the University of Texas with two dozen collaborators) finds that even with minor mergers included, only about 30 percent of the star formation in the last third of the age of the universe is the direct result of collisions and mergers. On the other hand, they find that over the same timescale about 68 percent of high mass galaxies were involved in some kind of merger. About two-thirds of those were minor mergers. We see two key points about minor mergers in those statistics. Firstly, they are significantly more common than major mergers. Secondly, they generate considerably less "bang," at least in terms of star formation (see Chapter 5). Once again we conclude that we do not have answers to all the questions yet. The most disappointing aspect of the new surveys may simply be that, yes we live in an age when galaxy collisions are rare, and we have not yet been able to see back to the age of mergers. This reminds me of an analogous situation in the 1980s. With great effort observers were obtaining broadband colors of galaxies out to redshifts of about 0.5, and finding little or no evidence of any galaxy evolution in those colors. Deeper observations allowing us to push back to earlier times solved that conundrum. Galaxy evolution is abundantly obvious in current high redshift observations. I would like to conclude this section with one final caveat. The observational studies can only see the direct effects of mergers. However, the indirect or delayed consequences may have equally lasting effects. The specific types of collisions considered in earlier chapters taught us that collisions and mergers can rearrange galactic gas in major ways with profound consequences for the star formation history. Gas from tidal tails and bridges can rain back down on disks and help fuel star formation for several Gyr. Gas tunneled to the central regions can trigger starbursts and strong galactic winds. Some of the wind material

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probably also returns to the disk. Do these processes delay star formation and gas consumption processes that were underway before the merger? Or do galaxies, especially the low star-forming, low-surface-brightness galaxies, get hung up without external stimulation? Are they unable to concentrate their gas and continue moderate rates of star formation without such stimulation? If so, the questions become how much of the stimulation is due to cosmological infall and how much to collisions? How can we tell the difference? Hard questions for a young century! Clearly there is much yet to learn about the lives of galaxies.

Further reading

Kennicutt, R.C., Jr., "Induced Star Formation," in Galaxies: Interactions and Induced Star Formation: Saas-Fee Advanced Course 26, eds. D. Friedli, L. Martinet, and D. Pfenniger (Springer, New York) 1998. Larson, R.B., and Tinsley, B.M., "Star Formation Rates in Normal and Peculiar Galaxies," Astrophysical Journal, Vol. 219, p. 46, 1978. "Starbursts From 30 Doradus to Lyman Break Galaxies," in Astrophysics and Space Science Library, Vol. 329, eds. R. de Gris, and R.M. Gonzalez Delgado, (Springer, Dordrecht) 2005. (This book is a technical conference proceedings volume, but contains some articles that are not highly technical, and provides an introduction to the literature.)

7

Not Always in a Galaxy Far, Far Away

7.1 Our Local Group of galaxies

"The sky is falling! The sky is falling!" Chicken Little In the world of galaxies, Chicken Little is not necessarily an extreme personality, because the sky is always falling. We will examine the "always" part of this statement in the next chapter when we consider the role of galaxy collisions through cosmic history. In this chapter we consider our own "sky", that is, the Milky Way (MW in this chapter) and its immediate environs. Direct evidence that our Milky Way sky might be disturbed by the gravitational impact of another object is visible from both hemispheres of our planet. I am referring to the fact that the Andromeda galaxy is visible as a fuzzy patch in the fall evening sky in the northern hemisphere, and the Magellanic Clouds are much more spectacular in the south. Naked eye galaxies are suspiciously close galaxies. The Andromeda galaxy is a fairly average-sized galaxy, at a distance of a little over two million light years, and close to the limit of naked eye visibility. (Admittedly, given the huge range of galaxy sizes, average is a rather ambiguous term.) If Andromeda were three times farther away, then it would be very difficult to see without binoculars or a telescope, and there is a good chance that it would not be bound to the MW. However, it is bound to the Milky Way, and the two galaxies are the main anchors and the largest members of the so-called the Local Group of galaxies - that is, the bound group of galaxies that we live in. The Local Group consists of a few dozen galaxies all contained within a sphere with a radius of about three million light years around the center of mass (Figure 7.1). The MW and Andromeda are both spiral galaxies of roughly comparable size and mass, though the debate on their exact mass ratio continues. A third member of the Local Group is the Pinwheel galaxy, M33, in the constellation of Triangulum (Figure 7.2). It is about one-third the mass of the MW, and so, of intermediate size. However, M33 hangs out closer to the Andromeda galaxy; currently the separation between the two is roughly half a million light years. The next biggest member of the Local Group is the Large Magellanic Cloud (LMC), which is at least several times less massive than M33, and is a satellite of the MW. Then come a collection of half a dozen or so that are more than a factor of a few less massive than the LMC. This group includes the Small Magellanic Cloud (SMC), and the dwarf elliptical galaxy NGC 205, which is the brightest close companion to the Andromeda galaxy. The remaining

C. Struck, Galaxy Collisions: Forging New Worlds from Cosmic Crashes, Springer Praxis Books, DOI 10.1007/978-0-387-85371-07, © Springer Science+Business Media, LLC 2011

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Figure 7.1 A three-dimensional map of the locations of Local Group galaxies assembled by astronomer E. Grebel. Note the concentrations around the Milky Way and the Andromeda galaxy (M31) (from E.K. Grebel, http://arxiv.org, 2000).

members of the Local Group are dwarf galaxies that are much smaller than these last two. Most of the members of the Local Group are not only bound to the overall group, but are also bound and orbiting quite closely to one of the two largest members, the MW and Andromeda (Figure 7.1). We will learn more about Local Group binding shortly. Our Local Group is seen by astronomers as a very typical small group of galaxies. Many, perhaps most, galaxies live in large galaxy clusters that we will consider in a later chapter. However, many others live in small groups, with a few dominant galaxies like ours. There are good reasons to believe that those groups also have a comparable number of dwarf galaxy satellites, some of which may be interacting strongly with a primary galaxy. However, such small galaxies are very difficult to detect in groups outside our own, so the conjecture is very hard to check observationally. However, the structure of the closest other nearby groups appear similar to ours. For the closest ones we can detect dwarf galaxies. These include the so-called Sculptor group and groups around the galaxies M81, IC 342, M83, and Centaurus A. We will discuss the properties of a couple of these later. However, rather than bemoaning what we cannot observe in other groups, our time is better spent looking more carefully at the Local Group. The basic group geography described in the preceding paragraphs has been well known for some time. That does not make it uninteresting; many fascinating details have emerged as a result of new observations in the last couple of decades. Yet that is

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Figure 7.2 Images of several Local Group galaxies to scale (upper row: Andromeda, M33, lower row: LMC, NGC 205) (Digital Sky Survey, produced by the Space Telescope Science Institute, under a grant from NASA, and the website of W.C. Keel).

not half the story. A number of qualitatively new discoveries have been made about the old familiar Local Group in the last decade or so. We will delve into both the incremental and the revolutionary discoveries in the following sections. Many of these have to do with galaxy collisions very close to the home world.

7.2. Collisions here and n o w : t h e Magellanic Clouds

In particular, let us consider objects number 4 and 6 on the Local Group census, the LMC and SMC, a little more carefully. To begin with there are the names. Our history records millennia of mythology, proto-scientific and scientific conjecture about the nature of the MW. Over most of that time the LMC and SMC were unknown to most civilizations, which as a result of plate tectonics were located in the wrong hemisphere. Thus, they acquired a name from the age of European exploration. They did not play a role in the early, defining debates about the nature of the Milky Way. In a world of hemispheric equality, they should have. We can speculate that they might have helped us come to a better understanding of the

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basic nature of galaxies somewhat more quickly. Interestingly, they also did not immediately take center stage when the possibility of collisions between galaxies began to be considered in the mid-twentieth century. Their proximity makes them obvious collision candidates, at least in retrospect. Writing in 1936, Hubble stated: .. .a pair of satellites, so close that their outermost regions probably mingle with those of the primary body. The extragalactic character of the Clouds was not fully recognized until comparatively late. Because of their proximity and their unusual types - are highly resolved irregular nebulae - there was a tendency to regard the Clouds as possibly local aggregations in the galactic system. Although they were really the most accessible of the nebulae, they were passed over, in a sense, and the first definite conquests in extragalactic space were made among more distant systems. (From The Realm of the Nebulae) Viewed as colliding galaxies, the Clouds continued to be "passed over" for several more decades. For a view from the 1950s and '60s we can take a quick look at the text of the Hubble Atlas of Galaxies. The SMC is not included there, but the LMC is. The text describes a variety of morphological features, but does not raise the possibility of a collision. This view does seem typical of the time. At the risk of being unfair to my predecessors, astronomers did not initially see an arrow in the sky pointing to the Magellanic Clouds with a caption saying collision remnants this way. Amazingly, something like that arrow exists, in the form of a long stream of hydrogen gas, but it took a while to find it. The master colliding galaxies sleuth, Alar Toomre, was first on the trail. However, he seems to have picked up the scent by a rather indirect means. The disk of the MW is not perfectly flat; it is curved up in one direction on one side, and curved in the opposite direction on the other side. This phenomenon is not uncommon among galaxy disks, and is referred to as a "warp." In the late 1960s, Toomre and Chris Hunter were studying how rapid disturbances, like the close passage of another galaxy, could affect a disk, e.g., make warps and oscillations. Specifically, was it possible for a close passage of the Magellanic Clouds to produce the MW warp? In retrospect, that question could not be answered in a time before dark halos were discovered, and is still not completely answered, though it looks unlikely. However, Toomre asked another question: what effect would the putative collision have on the SMC? His answer was that it was quite likely that a tidal tail would be produced, and it might be observable in the 21 cm emission line of atomic hydrogen. This story is told in a 1973 paper by astronomers I. Felix Mirabel and Kenneth C. Turner, who used a radio telescope in Argentina to search for the tail. Unfortunately, they failed to detect it (though radio astronomers Peter Wannier and Girard Wrixon had detected high velocity hydrogen in the vicinity somewhat earlier). However, within another year, in a broader and more sensitive survey, an Australian team did discover a long tail of hydrogen gas extending out from the Magellanic Clouds. The abstract of the paper by Don H. Mathewson, M. N.

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Cleary, and J. D. Murray ends with the proud statement, "This filament, which follows very closely a great circle over its entire 180 degree arc across the sky, is given the name the Magellanic Stream." A gas bridge connecting the Magellanic Clouds was also discovered about this time. More recent observations and sophisticated models show that the Magellanic Stream is not a simple tidal tail by any means. However, its discovery made clear that it was very likely the Clouds were a collisional system. Figure 7.3 shows an artist's impression, which combines the optical and radio observations. Given their proximity, not to mention the possibility that they were interacting with our own Milky Way, this was a system well worth further study. The Clouds have been studied intensely since that time. The NASA Astrophysical Data System lists more than 10,000 articles with the name Magellanic Clouds in their abstract between 1975 and 2005. Books and scientific meetings have been devoted to the Clouds. This book is not, so we must narrow our focus. Many of the articles on the Clouds are about star clusters or stellar populations there, topics of great interest because for a long time the Clouds were the only external galaxies where we could resolve individual stars in clusters. The number of such galaxies is still small. Here we will neglect that large and fascinating literature. Instead, we will limit our discussion to observations and models that tell us about the orbit of the Clouds relative to the MW, about their collision history (and future), about the formation of their tidal structures, and hopefully about galaxy collisions in general. Besides star clusters, there are a number of features of the Clouds that can be observed at a level of detail that is impossible in any other interacting galaxy system. Two types of observation are especially helpful in deciphering its collision history: the distribution of HI gas in extended tidal structures, and the proper motion of the clouds. Let us start with the second of these. Proper motion is a term that would have come up very early in a discussion of nearby stars or star clusters in our galaxy, but it is rarely used in the world of galaxies. The motion of any celestial object is conventionally broken into two parts: the radial velocity, which is towards or away from us, and the proper motion, which is the motion across the sky. In many cases, the former is relatively easy to measure as red or blue Doppler shifts in spectral lines. All celestial objects are very distant. Therefore, even if they move at a high velocity, the distance they cover on human timescales is very small compared with their distance from us. This means that even if their motion is entirely across the sky, they will traverse a very small angle in any feasible observation time. Proper motions of nearby stars are traditionally cited in units of arcseconds per year, which gives some idea of how small the motion is. Only by careful positional calibrations against distant background objects, with negligible proper motions, can they be measured at all. Measuring the proper motion of stars in other galaxies, even the nearest ones, is a heroic undertaking. Nonetheless, it has been done for the Clouds, and we have estimates for the Andromeda galaxy, M31, and its companion M33. In the future, we will have accurate measurements of the velocity components for most of the galaxies in

Figure 7.3 Composite image of the Milky Way, the Magellanic Clouds, and the Magellanic Stream. Pink false-color image of radio observations (21 cm) of the stream is superimposed on an optical all-sky image, with the plane of the Milky Way visible as the bright horizontal band (David L. Nidever et al., NRAO/AUI/NSF & A. Mellinger, LAB Survey, Parkes Obs., Westerbork Obs., Arecibo Obs.).

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the Local Group, as well as accurate positions of their centers. Very high spatial resolution is required to make these measurements, and the Hubble Space Telescope has played a key role. Estimates of the proper motion of the Clouds became available in the early 1990s and guided new models of their orbits. In 2007, Nitya Kallivayalil published much more accurate values for both of the Clouds as part of his Harvard Ph.D. thesis. The magnitude of these values is about a couple of thousandths of an arcsecond (a milli-arcsecond) per year. Although these values are exceedingly small, they are larger than previous estimates, and imply that the Clouds are moving more rapidly than was thought. This has an important effect on the derived orbit of the Clouds relative to the MW, which we will consider shortly. The other critical observation for our understanding of the Clouds is the largescale distribution of HI gas around them. We have seen in previous chapters how the morphological structure and the motions of tidal waves, bridges, and tails can tell us a great deal about the collision between galaxies. Also, recall that because gas disks tend to extend to much larger radii than the stellar disks of galaxies, they are usually more strongly effected by the tidal forces in a collision, and thus, can tell us more about the collision. With the discovery of the Magellanic Stream, we had a powerful clue that the Clouds were colliding with the MW (as well as with themselves). However, the early observations were not very sensitive or detailed. As the Australians, in particular, have brought new radio telescope arrays on line with more sensitive instruments, their observations have given us broader, more detailed and sensitive views of the gas in the Clouds. This is especially true in the last decade, as illustrated by Figure 7.4. Several basic properties of the gas distribution are immediately evident in this figure. First of all, the hydrogen gas distribution is very extended compared with the distance between the two Clouds. The Clouds are contained within a common envelope of gas. A good part of this common envelope is in the gas bridge that connects them. The Magellanic Stream extends a great distance behind this envelope. It contains about 25 percent of all the gas in the system. Comparing the smallest resolved clumps in the Stream with its overall extent shows the impressive dynamic range in the recent observations. Comparing the Stream in this figure with the previous one, we see the latter not only has impressive detail, but additional structure as well - specifically, a branch of the tail on the right side and running at an angle to the main stream. Thus, the Stream seems to consist of two major sub-streams. Very recent observations from the Arecibo Observatory in Puerto Rico indicate that four separate sub-streams can be identified near the tip of Magellanic Stream. Note that these features can be identified not only as distinct structures in space, but also as having separate velocities along the lineof-sight (radial velocities). However, these velocity differences are relatively small. The overall mean velocity of the Stream is quite close to that of the Clouds. Generally, they are traveling through space together. It is frequently assumed that the Stream material is falling behind the Clouds in their common orbit, perhaps as a result of interaction with hot, tenuous gas in the MW halo. There is

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Figure 7.4 Detailed map of the 21 cm hydrogen gas emission from the Magellanic Clouds, Stream, and the Leading Arm. Note the filamentary and clumpy structure (from M. Putnam, et al., Astrophys. /., Vol. 586).

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evidence that gas is being pushed (by young star winds or supernovae), or swept (via interaction with halo gas) out of the bridge region, to replenish the base of the Stream. These considerations have been taken to imply that the Stream traces the orbit of the Clouds in the recent past. This is a very intriguing possibility. Generally, we do not have the orbit of a satellite galaxy actually drawn on the sky. Yet, at this time, the conjecture is far from proven. The HI observations revealed another predominantly gaseous structure in the system, the so-called Leading Arm, on the opposite side of the Clouds from the Stream. This forward stream is more clumpy than its larger sibling, and contains only about one-quarter as much gas (6 percent of the system total). As we have seen in earlier chapters, tidal interactions often generate two tails, or a bridge and a tail, on opposite sides of the parent galaxy. The existence and relative locations of the two streams seem to support the tidal explanation for both. One problem with this facile interpretation is that neither stream obviously originates in one of the two Clouds. Both streams seem to have connections to both Clouds, or at least to their common gas halo. This is especially hard to sort out in the case of the Leading Arm, which is really more a group of clumps, rather than a well-defined stream. Nonetheless, very detailed analyses of the radio data have been published in the literature. There is no firm consensus, but recent work indicates that much of the material of both streams may originate at a gas rich region in the Large Magellanic Cloud. There is also evidence for gas outflows, probably driven by the activity of young stars. This raises the intriguing possibility that the streams are fed by something like a galactic wind, or at least, the collective action of many smaller scale outflows. This is not the first process that has been considered as an alternative to tidal forces in forming the Magellanic Stream. Ram pressure stripping has long been on the scene as another possibility. The operative agent in this process is an incredibly tenuous, hot gas filling the MW halo, and exerting a pressure on the gas in satellite galaxies like the Clouds. The term "ram pressure" is justified because the relative velocities of galaxies are supersonic, at least from the point of view of cool to warm gas clouds in the galaxies, and so the interaction is effected via shock waves. One point in favor of the ram pressure idea is that gas in tidal tails does not usually trail behind a galaxy along its orbit, but rather juts out at angle relative to the orbit. Of course, as mentioned above, we are not entirely sure that the Stream does follow the orbit of the Clouds. A point against the ram pressure idea is that the putative hot halo gas is nearly impossible to observe, because its low density makes radiation from it incredibly faint in all wavebands. There are theoretical reasons suggesting it should exist. Gas collapsing through the halo to form the MW could be heated to high temperatures, and the low-density regions would take a time exceeding the age of the galaxy to cool. Hot gas could also be pumped into the halo by a galactic wind in periods of vigorous star formation. It would also take a very long time to cool. A more detailed aspect of this discussion concerns how far out the hot halo extends, and how close to the MW the Clouds have come. In the early days, it

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was thought possible for the Clouds to have come close enough to have impacted the outer gas disk of the MW. This might have formed the Stream by a different kind of ram pressure - the direct collision between parts of the gas disks. This is now thought unlikely. The questions of whether the Clouds came close enough to ride against a hot halo wind, and whether this is at least partially responsible for creating the Stream, and for pushing back and clumping the Leading Arm, remain without a definitive answer. This ambiguity leads us back to the question of the Clouds' orbits, and what the proper motions, radial velocity and distance observations tell us. With regard to the latter, the Clouds' were among the first external galaxies for which distance estimates were obtained. Indeed, as indicated by the above quote from Hubble, the Clouds were not generally recognized as external galaxies until their distance was estimated. Since the early days, many techniques have been used to estimate their distance with ever-increasing accuracy. At present, in some cases, we can make estimates of the distances between different structures within the Clouds along a single line-of-sight. Such details aside, the current distance estimate for the Clouds is about 50 kpc from the center of the MW. This is only about 160,000 light years away. The light we observe left the Clouds when humans were starting to use language. When the presently observed light left any other galaxy, the human species was not yet present on Earth. Returning to the topic, the radial velocity of the LMC is about 90 kilometers per second (km/s), and the tangential (across the sky) velocity derived from the new proper motion determinations is about 370 km/s. These values yield a net velocity of the LMC relative to the MW center of about 380 km/s. The fact that the tangential velocity of the LMC is so much greater than its radial velocity is consistent with a nearly circular orbit, but does not guarantee it. Note that the LMC is much more massive than the SMC, so at this point, it is convenient to focus on the former. What does this relative velocity tell us about the orbit of the Clouds? Unfortunately, it is only half the story. The other half is determined by the structure of the dark halo of the MW - how massive is it, and how is its matter distributed in space? The answers to these questions determine whether the halo's gravity is strong enough to bind the Clouds at their present mean position and velocity. Although the halo is dark, there are independent observational constraints on its structure, though most of these apply to much smaller distances from the MW center. One of the most powerful of these constraints is the rotation curve of the MW. This is a graph of the orbital speed of stars or gas clouds on circular orbits as a function of distance from the galactic center. We know that the stars and clouds in the galactic disk have approximately circular orbits, so we can use many observations of their line-of-sight velocities to determine the rotation curve in the disk out to the Sun's radius and somewhat beyond. This is less than or about a quarter of the distance to the Clouds, which would leave us still in the dark about the structure of the dark halo. However, the rotation curve turns out to be quite flat - the same constant velocity at all radii. This is also true of most

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external galaxies, and in some of those cases different kinds of observations allow us to deduce that the flat rotation curve extends well beyond the disk. These observations, and detailed numerical models of the cosmological growth of galaxy halos, strongly suggest that they have a nearly universal form - the same form from one galaxy to the next (albeit with some dependence of the halo structure on the galaxy's group or cluster environment). This last result allows us to extrapolate the structure of the MW halo, as determined within the disk, to the Clouds. It is literally and figuratively a huge extrapolation! It is likely that less than a few percent of the dark halo mass is within range of rotation curve measurements. However, there are other observational checks that we will not consider here. These provide some assurance that seeing only the tail still gives us a pretty good picture of the dog. It is clear now that estimating the orbit of the Clouds is a much more complicated problem than determining the orbit of a small planet around the Sun (with apologies to Kepler). This is all the more true if we wish to include the gravitational contributions of the MW bulge and disk. Secondary effects like these may be important because it has been clear since the first proper motion estimates that the Clouds are not strongly bound to the MW. This means that even with good determinations of the present position and relative velocity of the Clouds, it takes a detailed computer model, including the different components of the MW, to derive the orbit of the Clouds. Such modeling began with the work of Hunter and Toomre mentioned above, and has continued intermittently since that time. Before proper motion estimates were made, the early modelers were definitely working at a disadvantage. Once proper motion estimates started to become available, more modelers attacked the problem. Lance Gardiner and Masafumi Noguchi, building on early work by Japanese groups, produced a very influential set of models. An example of the type of orbits deduced by these groups is shown in Figure 7.5. The key feature of these orbits is that they are based on the assumption that the Clouds are bound to the MW, so they are basically precessing ellipses. Even the precessing aspect is not that important, because most of models of the Clouds do not attempt to follow the orbit for more than one period. There are a couple of reasons for this. The first is that the orbit is so large (hundreds of kpc) that it takes a considerable fraction of the age of the universe to traverse it. The second is that the farther back we try to trace the orbit the more uncertain our calculations become. In a word, we do not know where the Clouds came from. Unfortunately, it appears that we do not even know where they came from quite recently. In a very extensive review and modeling effort, Gurtina Besla and collaborators (including Nitya Kallivayalil and other members of the proper motion team) argue that both bound (though very large) and unbound (hyperbolic) relative orbits are consistent with the data (Figure 7.6). However, the hyperbolic orbits are more likely, given the larger values of the new proper motion determination. (The velocity figures and other information used above are derived from the Besla et al. article.) Although a hyperbolic orbit has been seen as one possibility for a long time, an elliptical orbit has been more

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Figure 7.5 Schematic of a model of the Magellanic system, based on the assumption that it is bound to the Milky Way (from Gardiner, et al., Mon. Not. Royal Astron. Soc, Vol. 266).

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Figure 7.6 Bound and unbound orbits of the Magellanic Clouds around the Milky Way (MW), according to some recent models. The three sets of orbits bracket the range of possible orbits for the measured proper motion, and the adopted dark halo distribution (from G. Besla, et al., Astrophys. /., Vol. 668).

commonly assumed, and this assumption has colored much of our view of the Clouds and their structure. In some ways, a hyperbolic orbit should not be surprising. Numerical simulations of the buildup of galaxies and larger scale structure show that most minor merger companions fall out of larger scale filaments or sheets. Although the collision partners may not be bound to each other, they are bound to the larger scale structure. Thus, moderately hyperbolic collisions are not unusual in the cosmic scheme of things. As Besla and collaborators point out, this at least helps us understand why the Clouds are so different from other MW satellites in their appearance, the abundance of hydrogen gas, and their very high relative star formation rate. On the other hand, traditional bound models have been able to account for many features of the Cloud system. This especially includes the Magellanic Stream, as well as the bridge and the Leading Arm. Whether the Stream is the result of ram pressure stripping, or whether all of these features are tidal, there is more time on a bound orbit to produce them as a result of interaction with the MW. On a hyperbolic orbit, the Clouds are a recent arrival moving at high speed

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through the MW halo, so the interaction is minimized. If some of these features are the result of interactions between the two Clouds themselves, then the orbit does not matter. However, it is hard to account for the Stream in this way, and as might be expected, Besla's team found they could not reproduce it in simulations with a hyperbolic orbit. They noted that processes beyond those considered in their models would be required, though they did not have any clear candidates. In the last decade, Kenji Bekki (and sometime collaborator Masashi Chiba) has taken up and considerably advanced bound models in the Gardiner and Noguchi tradition. Specifically, they have produced models that can reproduce the major structures of the LMC, including its central bar, thick disk, and stellar halo, as a result of interaction with the MW for a time extending over most of the age of the universe. They have also argued that the young and intermediate age stars and star clusters in the LMC are produced from metal-poor gas accreted from the SMC. Gardiner and Noguchi earlier suggested that the Stream originated from material stripped off the SMC, though this has since been contested in the literature. So are all these model explanations of the observed structure lost in the light of the new proper motion measurements, and the proposed hyperbolic model? Is the possibility that interaction with the Clouds has affected the structure of the MW disk, a la Hunter, Toomre and later authors, completely implausible? Maybe not, because recently Bekki has raised the possibility that the Clouds have a larger and more diffuse dark matter halo than previously thought. This would be a common, merged halo enclosing both Clouds. With this extra dark matter the gravitational attraction between the Clouds and the MW would be stronger, allowing for a bound orbit, and prolonged interaction, even given the high velocity of the Clouds. One objection to this scenario is that observations of the SMC show little evidence for dark matter in the body of the galaxy. Bekki argues that this is not a contradiction. In his revised scenario, most the SMC halo has been removed and merged with the LMC halo, to make the common halo. Consequently, we find ourselves in another thicket of technical arguments, and not yet able to conclusively answer the basic questions about the orbit and interaction effects of the closest colliding galaxy system of them all - our own! This is despite the huge observational and modeling efforts in the last decades. Now we find that the answers to these questions depend not only on accurate measurements of the orbital parameters, and the structural parameters of the MW halo, but also on the details of the Clouds' halo. This latter aspect is a bit of a surprise because given the small relative size of Cloud halos we might have thought ourselves entitled to ignore details of their structure. However, Bekki's solution is not a small change in the details; it demands a major increase in the mass of the halo of the Clouds. In some ways, the story of the Clouds is a very sobering one for those of us who study much more distant colliding galaxies. If we cannot even sort out the basic facts of the closest system, how can we hope to understand these systems? To sidestep that issue for a moment, I believe we may be converging on a good understanding of the Clouds. The suggestions of Bekki and others can be tested

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by observation. As we will see in the next section, there are other ways to discover the structure of the MW halo, and constrain the orbit of the Clouds. However, it is also true that we could not sort out the history of a system like the Clouds at distances beyond the Local Group. We must limit our work to simpler systems, those with comparably sized partners, and extensive collisional debris to provide clues to the encounter history.

7.3. Collisions now and then: contrails in the halo

On the plains in the Midwest region of the U.S. one can see a lot of sky. Because of its central location, a lot of airline flights pass over the area. On clear days in the winter when the high altitude winds are not too fast, a criss-cross pattern of jet contrails decorates the big sky. Beginning in the last decade, and especially in the last few years, an analogous pattern has been discovered in the halos of the Milky Way and the Andromeda galaxies. Of course, these galactic contrails are vastly larger (tens of kiloparsecs or more), and are not the result of jet exhausts! They are vast filaments or streams consisting of stars and gas clouds. These streams have very low surface brightnesses, which is the main reason that they have been detected only recently. Although they have generalized the name, these streams contain much less hydrogen gas than the Magellanic Stream, and so are not easily visible to radio telescopes either. In fact, they are generally much less massive overall than the Magellanic Stream. Detecting these streams required an instrument that could detect very faint features spread over a large part of the sky. This instrument is not a new kind of telescope, it is a new kind of astronomical collaboration and a new approach to astronomical observation. I am referring to the fact that many of the streams were discovered by the Sloan Digital Sky Survey (SDSS) or the Australian 2dF ("two degree field") survey projects and their successors. These are relatively large coordinated efforts. For example, the SDSS involves scientists from about two dozen academic and government institutions. (The telescopes used in these projects are not particularly large, as will be clear from Figure 7.7.) This is still a far cry from the true "big science" efforts in high energy physics and some areas of biology, which involve thousands of workers, but it is equally far from the archetypal observational astronomer working alone at a mountaintop observatory. How­ ever, the most novel aspect of these projects, and the real reason for the large teams, are the huge amounts of data obtained in such deep surveys of large fractions of the sky. (Even two square degrees of sky was large a decade ago, but SDSS and the successor projects have surveyed much more.) This is also a big change for extragalactic astronomy, where the tradition has been to slowly acquire data on one system at time, until there is enough data to discern a significant trend. This was the way Hubble discovered his famous distance-recession velocity law. The data in his first presentation was embarrass­ ingly sparse by modern standards. Less than 30 years ago the discovery of the

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Figure 7.7 Image of 2.5-meter Sloan Digital Sky Survey telescope at Apache Point Observatory in New Mexico (from the website of the SDSS).

relations between global properties of elliptical galaxies that have come to be called the "fundamental plane" involved observations of only a couple of dozen galaxies. Studies of these relations with modern surveys like the SDSS use observations of thousands of galaxies. Depending on the rarity of the object of interest, SDSS studies can include hundreds to millions of objects. In addition to big sky surveys like the SDSS and 2dF, there are also narrow beam surveys

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that look out to the farthest reaches of the universe, albeit in a very narrow cone. In the last couple of decades, extragalactic astronomy has gone from a very data poor science to quite a rich one. In terms of data collection, it is like an industrial revolution. In terms of large-scale statistical studies of populations of galaxies, it is like the computer/information age revolution. In fact, the data analysis required for these studies rests in part on the revolutionary increase in the power of computers in recent times. Stellar and galactic (MW) studies have also benefited from these surveys. The newly discovered streams are a prime example. Maybe, at this point, we should back up a bit and take the story of their discovery from the beginning. One basic point is that most of the streams are associated with dwarf galaxies. These are not the dwarf members of the Local Group mentioned at the beginning of this chapter. They are similar in many ways, but like the streams they are recently discovered objects. However, the old and new dwarfs have a number of things in common, as well as a few important differences. Given that, it is worthwhile to start the story with the old dwarfs. Most of these objects are dwarf spheroidals, so-called because even with their very low brightness and somewhat amorphous appearance they seem more like the spheroidal or elliptical Hubble types than the irregulars. In fact, they can look like little more than a rather small increase in the ambient star density (Figure 7.8). Generally, they are too faint to have been picked up by Messier, Herschel or other early catalogers of the nebulae. A couple were spotted by Shapley in the late 1930s, a few others by Albert G. Wilson from Mt. Wilson or Mt. Palomar observatories in the 1950s, and so many of the most well-studied ones had been discovered by the 1960s. There are less than a couple of dozen of them, about half a dozen bound to the MW, most bound to the Andromeda galaxy, and some in the outer fringes of the Local Group. The luminosity and size of these objects put them at the very low end of the spectrum of galaxies. However, they do exceed almost all of the globular clusters in these measures. Nonetheless, the question immediately arises: are they tiny galaxies or extra big globular clusters? The immediate answer is that they do not look very much like typical members of either group. Neither type of object typically has such a low density of stars. Laborious spectroscopic studies in the last few decades have yielded good evidence for dark matter halos around the dwarf spheroidals. Globular clusters do not have dark halos, so the old dwarfs are tiny galaxies. A classic example, the Draco dwarf, has a luminosity of 100,000 times that of the Sun, but with dark matter included, it has an estimated mass of up to 50,000,000 suns. Draco has one of the highest percentages of dark matter known among the galaxies. As we will see in Chapter 8, tiny galaxies are important because they may be related to the first building blocks of all galaxies. A few more dwarfs were discovered in the 1970s and 1980s, mostly in the less studied southern hemisphere. Their properties were not greatly different from other members of the class, so they did not attract a great deal of interest among astronomers. In 1994 a very interesting one, Sagittarius dwarf elliptical/

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Figure 7.8 Sloan Digital Sky Survey image of the Draco dwarf spheroidal galaxy. Note that all the bright stars are foreground objects in the Milky Way. Draco is visible as little more than an enhancement in the ambient density of stars.

spheroidal galaxy (SagDEG) was discovered by Roderigo A. Ibata, Michael J. Irwin and Gerard Gilmore. One of the first curious aspects of this object was that it was close, with a distance of only about 24 kpc. The title of the discovery paper was, "Sagittarius: the nearest dwarf galaxy." It is also quite large; that paper estimated about 10° across. How could we have missed it? The main reason is that it is located in the constellation Sagittarius, i.e., towards the center of the MW, where stars in the disk and bulge, and many gas clouds complicate or block most lines of sight. More precisely, SagDEG is nearly on the opposite side of the MW disk from the Sun, and about twice as far out from the center. Fortunately, it is some distance up from the mid-plane of the disk, or there certainly would be little hope of seeing it through the bulk of the

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disk. In fact, it was spotted in a study involving deep imagery of a few rare lines of sight near the MW disk with little obscuration from gas and dust clouds. These are the so-called "holes" or "windows" in the dust fog around the MW. This particular window is defined by the presence of the globular cluster M55. SagDEG lies behind and several times farther than this cluster. The field also contains the globular cluster M54. In fact, subsequent studies have shown that this cluster is likely the core of SagDEG. So ironically, Charles Messier did discover at least the core of one dwarf spheroidal, though the extended stellar distribution was far too faint for him to detect. In size and mass, SagDEG appears to lie between the Fornax galaxy, previously the largest known dwarf spheroidal, and the SMC. It has much in common with the former. Its stellar populations, in particular, are very similar to Fornax. Both galaxies show evidence of multiple generations of stars, and substantial intermediate age populations. The stars of both galaxies show a fairly wide range of metallicities (abundances of heavy elements), though these are generally much lower than those of the Sun. Like Fornax, SagDEG has globular clusters of its own - evidently formed within it and bound to it. Including M54, it has four globulars, and a fifth (Palomar 12) may have once been bound to it. Unlike the SMC, SagDEG has virtually no interstellar gas. However, it does have one important property in common with the SMC. Within a year or so of its discovery, SagDEG was found to have long extensions (e.g., out to about twice its size) that seem to be the result of the tidal forces of the MW. With time, and ever deeper observations, those extensions got longer, until they were recognized as a stream encircling the galaxy. By 2003, Steven R. Majewski and Kathryn V. Johnston, who have worked long on the study of SagDEG, and their collaborators had discovered that the associated stream extended more than once around the galaxy. Interestingly, they made this discovery not on the basis of slight overdensities of stars observed in the optical, as was the case in the discovery and later works. Instead, they used red giant stars identified in the 2MASS infrared survey (the Two Micron All Sky Survey, long directed by researchers at the University of Massachusetts). This is a rather traditional imaging survey of the sky like the famous Palomar optical survey. It was not as sensitive and the data were not processed as automatically as in more modern surveys like the SDSS. However, 2MASS did survey virtually the whole sky for the first time in three near-infrared wavelength bands. The Majewski et al. study also yielded another interesting result; the leading tidal arm apparently passes through the MW disk quite near the Sun's position. In contrast to the Magellanic Clouds, SagDEG's leading arm is long and curves through more than 180° from SagDEG to our side of the MW. The implication was that stream material is passing very near our Solar System. Interestingly, a few months later another research team reported that a group of stars including the bright star Arcturus had velocities and heavy element abundances indicative of stars captured from a dwarf galaxy. It is a very intriguing story - that some of the bright stars in our sky originated far from the MW, in another galaxy, which is now being torn apart on the other side of the MW. Unfortunately, although

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Figure 7.9 Sloan Digital Sky Survey image of the so-called "Field of Streams," i.e. streams of relatively high stellar density in the halo of the Milky Way (from V. Belokurov, et al., Astrophys. /., Vol. 658).

the Arcturus group may come from another galaxy, that galaxy is not SagDEG. A 2007 study by Heidi Jo Newberg etal., based on more extensive SDSS data, shows that the SagDEG arm does not intersect the MW disk anywhere near us. SDSS investigations have added a lot to our understanding of SagDEG and its associated streams - yes, streams. At a certain point, the stream appears to break into two parts (Figure 7.9). This has been interpreted as a superposition of various tidal arms. SDSS studies also have played a major role in the controvery over the Virgo Overdensity. As suggested by the name, this is a region in the constellation Virgo with a significant overdensity of stars - probably from a disrupted dwarf intruder. For a time, it seemed that this stellar accumulation might be the result of a bend in the SagDEG stream, but later results contradicted that. The somewhat convoluted history of these discoveries is in many ways parallel to that of the Magellanic Stream, albeit written in stars rather than HI gas. Indeed, while there is no gas in SagDEG itself, some of the same Australian researchers who study the Magellanic Stream have discovered a gaseous stream in the halo with a mean velocity and a velocity variation along it that suggest it was stripped from SagDEG. The authors suggest it was removed in SagDEG's last pass through the outer parts of the MW disk, about 200-300 million years ago. If this is correct, then its large number of intermediate age stars would represent the last round of star formation before the gaseous fuel was removed. As in the case of the Clouds, it has been somewhat difficult to pin down the precise orbit of SagDEG, though it is approximately polar relative to the MW disk. The various studies indicate that it has orbited the MW for at least a billion

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Figure 7 . 1 0 Map of red giant stars, derived from the 2MASS infrared sky survey, outlining the local of the stream associated with the Sagittarius dwarf galaxy. Note the obscuration from the galactic plane near latitude b = 0. Globular clusters that may also be associated with the dwarf are indicated by name and large blue circles (from M. Bellazzini, Memorie delta Soc. Astr. Italiana Suppl., Vol. 5).

years, and the disruptive tidal processes have removed more than half of its mass, including the globular cluster Palomar 12. The history of SagDEG is much shorter than that of the Clouds. The pace of astronomical discovery is very rapid these days. After SagDEG, and with data from the new sky surveys, a number of new companions and streams were discovered, mostly in the last few years (Figures 7.9 and 7.10). This story is briefly and humorously told by a listing of some of the journal paper titles, which are rather quirky in this field. Consider the following. 1995: 2003:

Sagittarius: The Nearest Dwarf Galaxy One Ring to Encompass them All: A Giant Stellar Structure that Surrounds the Galaxy.

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2003: 2003: 2005: 2005: 2006: 2006: 2006: 2006: 2006: 2007: 2007: 2007: 2007: 2008: 2008: 2010: 2010:

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A Dwarf Galaxy Remnant in Canis Major: the Fossil of an In-plane Accretion onto the Milky Way The Galaxy's Eating Habits The Distribution of Satellite Galaxies: The Great Pancake The Closest View of a Dwarf Galaxy: New Evidence on the Nature of the Canis Major Overdensity A New Milky Way Dwarf Satellite in Canes Venatici Detection of a 60° -long Dwarf Galaxy Debris Stream A Faint New Milky Way Satellite in Bootes A Curious Milky Way Satellite in Ursa Major Is Ursa Major II the Progenitor of the Orphan Stream? Cats and Dogs, Hair and a Hero: A Quintet of New Milky Way Companions An Orphan in the "Field of Streams" A Pair of Bootes: A New Milky Way Satellite Galactic Halo Stellar Structures in the Triangulum-Andromeda Region On the Newly Discovered Canes Venatici II dSph Galaxy On the Genealogy of the Orphan Stream Re-Assembling the Sagittarius Dwarf Galaxy Big Fish, Little Fish: Two New Ultra-Faint Satellites of the Milky Way

Admittedly, this is a small, and arbitrary selection of the many recent papers on these objects (see the NASA Astrophysical Data System for more information on these papers). It emphasizes discovery papers. Nonetheless, I think it correctly conveys a couple of important points. First, the pace of discovery has accelerated in recent years, especially with the availability of SDSS observations. Secondly, each of these objects has an interesting story to tell. As to the first point, since the discovery of SagDEG the number of known dwarf companions to the MW has roughly doubled. Some of the newly detected objects are intrinsically very faint. It seems possible that at least as many more are as faint, and even more that are significantly fainter remain to be discovered. Perhaps a few more will be discovered in SDSS data. However, to detect the much fainter ones will require new and yet more sensitive surveys. As to the second point, we have looked at the story of SagDEG in some detail. Most of the dwarf companion stories will require a lot more observation and analysis before they can be told. However, it worth briefly discussing at least one more of these stories, that of the Canis Major dwarf and the Monoceros ring. The ring was discovered in 2002, by two independent teams, with results presented at the January 2003 meeting of the American Astronomical Society. One team was a group of SDSS astronomers analyzing the millions of stars detected by the survey at that position. Another was a European and Australian collaboration using telescopes on the Canary Islands. It was clear from the beginning that the ring was very long, and those results have been extended by more recent work. It now appears that the ring or "low latitude stream" wraps several times around the MW. There is no danger of confusion with the SagDEG stream, since the latter is

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nearly polar, while the present stream is nearly in the plane of the MW disk (and rotates in the same sense as the MW disk). In 2004, a lump was found in the stream, the Canis Major dwarf. Here again the discovery was made on the basis of the distribution of red giant stars in the 2MASS survey. The mass of the Canis Major dwarf is comparable to that of SagDEG, and it also has several globular clusters orbiting it. It turned out to be even closer than SagDEG, at a distance of about 8 kpc. With a galactic longitude of 244° (where the galactic center has a longitude of 0°) it is on our side of the galaxy. The circumstantial evidence suggested that the low latitude stream was material tidally ripped off the dwarf over the course of several orbits around the MW. Except for the location of the dwarf, and the orientation of its stream relative to the MW disk, this story sounds quite similar to that of SagDEG. So, what happened to the notion of a unique story for every dwarf? Well, this one took several twists. The first was the suggestion, made shortly after its discovery, that the stellar overdensity that defines the low latitude stream, might be part of the galactic disk itself. Since the disk is known to be "warped" in its outer parts (see previous subsection), the fact that the stream is not exactly in the mid-plane of the galactic disk (about 8° out in the vicinity of the Canis Major dwarf), is not a problem for this theory. A variant of this theory has been proposed more recently. It is that the stream is a spiral arm in the warped outer disk. Because the motions and compositions of stars in the low latitude stream and the Canis Major dwarf are evidently not greatly different from those of the outer disk, it has proved difficult to resolve the debate between the tidal debris and warped disk theories. We will not delve into the technical arguments here. Some of them are suggestive, such as the evidence that Canis Major has a substantial intermediate age population, in contrast to much of the outer disk. However, if Canis Major were a disrupted dwarf galaxy, it would have been a lot easier to sort out if it had orbited in the opposite sense to the disk rotation! Nonetheless, unless it is a figment of the warped disk, its orbit is quite unusual. Most of the dwarf companions of the MW have more nearly polar orbits, like SagDEG. Actually, this is a very interesting point in itself. Over a period of many years astronomers had occasionally noticed that many of the dwarf satellites seem to be on or close to a single great circle around the MW. Plotting their positions on a projected great sphere quickly reveals this. As early as 1982, Donald Lynden-Bell, a pioneer in the study of spiral waves in galaxies (as well as the process of violent relaxation discussed previously), investigated this question, and the possibility that they might have a common origin as the tidal debris of a larger galaxy. Unfortunately, this distribution could be coincidental, especially at a time when the pattern consisted of only about half-a-dozen known objects. Even if many of their bulk properties were similar, the evidence was intriguing at best. Now that the number of objects that fit the pattern has nearly doubled, the question has arisen again in the literature. The fact that a number of old globular clusters with low abundances of heavy elements are also found near the same

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great circle adds more evidence. Moreover, preliminary proper motion estimates are becoming available for a few of these objects. Noam I. Libeskind and collaborators, in particular, have taken on the question recently. In a paper with the provocative title "The Distribution of Satellite Galaxies: The Great Pancake," they study cosmological models of the infall of satellites onto a parent galaxy. They find that a thin disk-like distribution of captured satellites is not an uncommon evolutionary outcome. The interpretation here is not quite the same as an origin in the disruption of a single massive companion. Rather, the proposal is that a number of satellites fall onto the primary, not from random directions, but rather out of a few large-scale filamentary structures where they formed and which connect to the primary halo. Other groups have also investigated such models, come to similar conclusions, and extended them, e.g., pointing out that groups of satellites may fall together rather than singly, accounting for the multiplicity of streams. Still others have pointed out that accreted clumps and streams may exist in the halo dark matter, partially independent of the distribution of visible objects. It seems likely that the archaeology of these structures and reconstruction of their formation have just begun. On a clear (northern) summer evening, when looking toward Sagittarius, one can imagine the nearly invisible debris streams running perpendicular to the Milky Way. We can hope that many more details about them will emerge in the coming years. Farther afield, a recent study of large loops around the galaxy NGC 5907 may provide a younger example of how satellite disruption can generate streams and clumps (Figure 7.11). To conclude this discussion, we can step back and ask what the story of the dwarf satellites tells us about galaxy collisions in general, and the formation of the halo of our galaxy in particular? The unsatisfying answer to the first question has already been alluded to above: there are probably many small-scale collisions occurring in more distant galaxies that we will not be able to detect in the foreseeable future. However, the search for great circles in the distribution of their dwarfs and globular clusters might be worthwhile. As to the halo of our galaxy, the globular clusters and dwarf spheroidals were once very confidently seen as relics of the monolithic (halo) collapse that led to the formation of the galaxy. If they were rare, well perhaps it is not surprising that few inhomogeneities would have been able to pull together to form distinct structures, given the rapidity of the collapse. In the last couple of decades, with the rise of the cold dark matter theory and the consequent picture of the hierarchical buildup of galaxies from ubiquitous building blocks not much bigger than globular clusters, the dominant view and the questions changed. For some time the dwarfs were seen as leftover building blocks that were not incorporated into larger structures. However, numerical models suggested that there should be many more of them; why are there so few? More recently, it has been realized that the models really predict there should be many leftover, low mass dark halos, but whether small galaxies could have formed within in them is another question entirely. In fact, in the hostile radiation environment created by active nuclei and central starbursts in the nearby primary galaxy, it may have been hard

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Figure 7.11 Giant stellar loops around the edge-on disk galaxy NGC 5907 may provide a view of what the streams around the Milky Way and Andromeda galaxies would look like from a distance (from Martinez-Delgado, et al. & R. Jay Gabany, Astrophys. /., Vol. 689).

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for such galaxies to form. Numerical models of these processes do affirm this general picture. Now we have evidence from the "great pancake" that the dwarfs may have been accreted after the galaxy and its dark halo had formed. At least some of them may be tidal dwarfs formed in the tails of more massive captured galaxies. The putative progenitors would have been more massive than the dwarfs, but less massive than most of the companions discussed in previous chapters. That is, they would have been of such low mass that they were largely disrupted before they could merge with the MW. Both the lower initial mass and the disruption would reduce the effects of dynamical friction, allowing them to continue orbiting the MW and be further disrupted. In the case of the Magellanic Clouds we may be seeing history repeating itself. For those of us who grew up in the profession with the monolithic collapse model, it is rather shocking to realize that most of the dwarfs and many of the globulars are aliens to our galactic world. Perhaps this is too melodramatic; a better analogy might be to young people raised on the farm who migrate to the city. But just as we start to acclimate to this new paradigm, a recent paper suggests that actually a rather large fraction of all the stars in the halo may in fact be in found in those streams, and thus have been accreted from outside. It is not so much "contrails in the halo" as the contrails are the (stellar) halo. Incidentally, it is not just the MW; there is recent evidence that the story is very much the same for our partner the Andromeda galaxy. Early in this book, I emphasized that the commonness of galaxy collisions, often pre-ordained by their group and cluster environment, upset the view from Hubble's time that galaxies were isolated island universes. A more accurate description would be that galaxies are continually evolving dynamic entities. That notion has come home to roost!

7.4. . . . A n d those yet t o come

We have considered a great deal of evidence for collisions involving the Milky Way in the last couple of sections. On the other side of the coin, there is evidence that the MW has not been involved in a major merger since the time when its disk became well formed. Thus, despite the evidence for episodes of bombard­ ment, it has been a relatively quiet place to live in the world of galaxies. In the future that will change. It may not have been so quiet for our sister galaxy, M31 in Andromeda. The evidence suggests that it has had a significantly more violent upbringing. However, before bringing some of that evidence to the table, I should point out that some of the interpretation rests on our point of view. We have a good one of Andromeda, well outside of it so we can see all parts of it, yet close enough that we can still discern ghostly streams. On the other hand, we should be cautious in citing differences with the MW; some of those differences may be exaggerated by the different modes of observation required for the two systems.

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Figure 7.12 Distribution of some of the recently discoverered (and more well-known) dwarf satellites of the Andromeda galaxy, M31 (from R. Ibata, etal.,Astrophys. /., Vol. 671).

The ghostly streams might be a good place to start a discussion of the halo of M31. Firstly, it has a number of them, just like the MW. The history of their discovery has been parallel to that of the MW, if a few years later. It has also proceeded somewhat more quickly. (It has also been carried out by many of the same astronomers who discovered the MW streams.) The M31 system is not spread across the whole sky, so surveys on the scale required to study the MW streams are not needed. Interestingly, many of the M31 streams, like those of the MW, are in a nearly polar plane. Also in a parallel history, many new dwarf companions have been discovered around Andromeda, some in possible association with streams (Figure 7.12). The total is now close to a couple of dozen. A couple of them, Andromeda XII (And XII, for short) and And XIV appear to be moving at close to the escape velocity of

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the galaxy. The latter is located at a projected distance of more than 160 kpc, way out in the halo. The implication is that these may be relative newcomers to the Andromeda halo, not yet slowed by dynamical friction. Alternatively, they may be just passing through, moving too fast to be captured. This also echoes our discussion of the high speed Magellanic Clouds in the MW halo. The M31 streams include the so-called Giant Southern Stream discovered by Rodrigo Ibata and his team in 2001. It is a large extension of stars to the south­ west of the galaxy disk, and contains a substantial globular cluster given the notso-imaginative name Gl. According to some sources, some of these stars may have been pulled out of the M31 disk by a collision. It is probably more likely that they are the scattered remnants of a disrupted collision partner. Models show that in a more nearly head-on, radial collision, as opposed to a fly-around collision, stripped debris is deposited in partial shells at the outermost points of the orbit. It has been suggested that the southern stream, a corresponding socalled "shelf" in the NE, and perhaps other structures are such shells. Detailed models have been produced (Figure 7.13). If this theory is correct, then these features are essentially the same as those produced by minor mergers in the class of shell galaxies that was rather well studied in the 1980s and early 1990s. When astronomers use radio telescopes to study the hydrogen gas around M31, the view is very different. The M31 halo is no longer a collection of shells and streams. What is observed is a faint gas bridge to M31's large companion M33, as well as a number of clouds scattered through the halo. So now we have a bridge over the stream with shells by the bank? Well, not exactly. The streams and shells tell us about earlier mergers, while the bridge to M33 may tell us about one likely to occur in the relatively near future, in galaxy time. Another clue to the importance of accretion events in M31's halo is a population of intermediate age globular clusters, roughly like those of the Magellanic Clouds. The MW has no such cluster population. One more indication of collisional violence in Andromeda is the discovery in the last few years of a vast extended disk ("sprawling" by one description) in the same plane, but reaching well beyond M31's easily visible disk. This structure extends from a radius of about 15 kpc out as far as 70 kpc. It was also discovered by the Ibata team. As mentioned in earlier chapters, many galaxies have gas disks that extend well beyond their stellar disks. This extended disk is different, since it consists of stars. Its average density profile is like an extension of the inner disk. Its stars have chemical abundances and velocities much like those of the inner disk. Yet it has clumpy and irregular structures that could not persist in a disk, so its discoverers argue that they represent tidal debris. They further speculate that the extended disk was formed primarily by accretion. It is my personal opinion that this speculation may be going a bit too far. I find it easier to believe a scenario more along the following lines. Ancient interactions with dwarf companions orbiting near the disk plane, resulted in some accretion, but also stimulated star formation in an extended gas disk. The latter has since been exhausted or removed, perhaps by later encounters. However, much more study is needed to get beyond speculation.

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Figure 7.13 Numerical model illustrating the formation and development of the streams and shells around M 3 1 . Features produced by individual infalling satellites are shown at the bottom (from A.S. Font, et al., Astrophys. /., Vol. 673).

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This all adds up to a lot of collisional violence in the halo of M31, but the inner parts have not been immune to the disturbances. In 2006, an international group of astronomers (led by David L. Block of the University of the Witwatersrand in South Africa) discovered a small (1.5 kpc by 1.0 kpc) ring of infrared emission in the center of M31 using observations from the Spitzer Space Telescope. Spitzer images also clearly show a (well-known) large ring of star formation in the M31 disk. The discoverers of the small ring produced a model showing how both rings could be produced by a direct collision through the center of M31. (The spiral arms between the two rings were modeled as pre­ existing structures.) That is, in addition to being a shell galaxy, a bridge galaxy and an accretion disk galaxy, M31 may also be a colliding ring galaxy! The putative companion is M32, which is a rather anomalous, and perhaps disturbed, dwarf elliptical (like NGC 205 in Figure 7.2). Thus, M31, M32, and M33 may also be involved more or less actively in collisions at the present. Given evidence of past violence, the Andromeda galaxy does indeed seem to have had a rougher time of it than the MW. However, the final twist in this story is still a few Gyr or so in the future. The MW and M31 are gravitationally bound. A collision is implied. They are far enough apart that they have no history; in fact, in the early ages of the universe the cosmological expansion carried them farther apart. However, their future is written in their stars, and their gas clouds, but most especially in their masses of dark matter. It is indeed collision and merger. The immediate question is when will they collide? The short answer is about 2-5 Gyr, and presently, we cannot nail it down much better. With words that echo those I used above for the Magellanic Clouds and some of the dwarf galaxies, we must wait for accurate proper motion measurements. The radial velocity of M31 is about 120 km/s towards us. As was also true in the earlier cases, we are limited by our ignorance of the mass contained in the outer halos of both galaxies. These uncertainties prevent us from knowing what the closest approach distance will be on the first pass, and the merger time as well. Given these uncertainties, it is easy to understand why the astronomical literature does not contain too many papers on this topic, though it is a perennial favorite in popular astronomy publications. John Dubinski of Toronto has maintained a beautiful animation of a representative evolution for a number of years. Recently, Thomas J. Cox and Abraham Loeb also carried out new models. From these models and the general rules of thumb we have learned from earlier chapters, we can derive a fair understanding of how this interaction will play out. First of all, the fact that both galaxies are relatively large, and of comparable size, will make it a major merger. Given the modest gas resources of both galaxies, the merger will not be too wet. However, this too is a bit uncertain. The Magellanic Clouds have gas to contribute to the MW. If M33 merges with M31 first, will the result be a net addition of gas or a net diminution due to induced star formation? Again, we need better data to accurately model that merger. Thus, it is hard to estimate the magnitude of the star formation fireworks and nuclear activity that will result from the MW-M31 merger.

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As pointed out by Cox and Loeb, the final outcome is easier to predict; it will be an elliptical remnant. They emphasize that it may be a model of a late forming elliptical galaxy. It is likely that the Sun will still be alive at this point, i.e., the Sun will not yet be a dying red giant star, though it will have long been too luminous for life to persist on Earth. The real estate on Mars might be better. The planets orbit so closely to the Sun that it is very unlikely that their orbits will be directly disturbed by the merger process. There is a fair chance that the Sun will be slung out in a tidal structure in either the first or a subsequent encounter. The view from the halo of the merger remnant may be even better than the present view from the disk of the Milky Way. One may ask why we even bother to speculate on Gyr futures when no Earthly species would even be likely to recognize its descendents, if there were any. One could get a bit worked up about the validity of scientific results that have no reasonable chance of being tested, no matter how accurate we might think they are. But who knows, maybe humans or their descendants will last longer than seems reasonable at present, and Gyr predictions will be verified! Although the models of M31 and the MW are really the same as those of any other colliding system, the fact that it is the home system adds a little more interest. Be that as it may, in the Local Group it seems to be possible to do increasingly detailed collision archaeology, and what we have found so far is sobering. Even in an isolated and relatively quiet group, there is much evidence of collisional disruption and disturbance. The results described above, and their concordance with simulations of galaxy formation and growth, suggest that we may be seeing less than half the story of the still continuing build-up of galaxies when we study more distant systems. In the next chapter we will consider what the simulations, and observations on larger scales, tell us about the still bigger picture.

Further Reading

There are many fine sources on the Milky Way and the Local Group, though most were written before the discovery of the halo streams. The authoritative technical textbook is: Binney, J., and Merrifeld, M., Galactic Astronomy, University Press), 1998.

(Princeton: Princeton

Popular level sources include: Bok, J.B., and Bok, P.F., The Milky Way, (Cambridge: Harvard University Press) 1981. Henbest, N., The Guide to the Galaxy, (New York: Cambridge University Press) 1994. Hodge, P.W., Skelton, B.P., and Ashizawa, J., An Atlas of Local Group Galaxies, (Boston: Kluwer Academic Publishers) 2002.

8

A Wider View

In the previous chapter, we took a rather long stroll through our own neighborhood in the world of galaxies, and looked in some detail at the properties there. Although new research has changed our view of the Local Group in the last few years, there is still much more of the familiar than in the many foreign parts we journeyed through in the preceding chapters. Before ending our journey altogether, we will now climb up to the top of a nearby mountain. Our imagination will let us accept the contradictory propositions that the metaphorical mountain is high enough to give us a very wide view of the galaxy world, and that we can attain that height quickly. From there we want to look in particular at some corners of the world of galaxies and their interactions that we have not been able to visit in the preceding chapters. We will glance quickly from one to another of these views. As in the real world of astronomical observation, long light travel times allow us to see the past of the galaxy world as well. We immediately note again the huge range of galaxies, from the small dwarfs with masses only a million times that of the Sun, to giant central cluster galaxies with masses of nearly a trillion solar masses. Again, we wonder about why that huge range exists?

8.1 In the beginning... At the end of Chapter 4 we discussed a theory, the hierarchical model, which helps us understand this huge range of galaxy masses, and many other features of our wide view. Recall that in this theory, objects the size of small dwarf galaxies were the first things to pull themselves together out of the universal smoothness of the young universe. In the most idealized hierarchical model, bigger galaxies are grown out of successive two-by-two mergers. This is of course very unrealistic. As we discussed in Chapter 4, the random statistical aspects of the build-up of galaxies are better described by merger trees, derived from large-scale simula­ tions. However, several other important aspects of this process were not highlighted in the discussion there, and we will consider them a bit more here. The first is that the build-up is not uniform in time. Because the universe was too hot, and gravity was effectively too weak, the formation of the first building blocks could not start before about a million years after the big bang. (This is a very crude description, but the complexities are explained in many excellent cosmology texts.) The initial collapse of these first building blocks took much C. Struck, Galaxy Collisions: Forging New Worlds from Cosmic Crashes, Springer Praxis Books, DOI 10.1007/978-0-387-85371-08, © Springer Science+Business Media, LLC 2011

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longer than this, of order a hundred million years. Only then could the hierarchical merging begin, and it would take a somewhat longer time to really get underway. On the other hand, we have direct evidence from high redshift observations, that some galaxies, probably young ellipticals, had accomplished a large fraction of their build-up by about a couple of billion years after the Big Bang. Thus, in some situations most of the hierarchical growth was accomplished when the age of the universe went from about a third of a Gyr to a few Gyr. Even the latter figure is much less than the present age of the universe of about 13.6 Gyr., so most of the galaxy growth occurred very early. At those early times the universe was smaller, and much denser on average than now. Those building block galaxies were much closer to each other than if they all were spread across the present-day universe in a comparable fashion. This is a part of the reason for so many early collisions, but it is not the main one. The original building blocks are small because they formed in the smallest lumps of slightly higher than average density in the generally very smooth early universe. At the epoch of "recombination," when the universe got cool enough for protons and electrons to come together to form hydrogen atoms, and just before the building blocks were able to start collapsing, density variations in the universe were less than a few parts in a hundred thousand. The universe was very smooth indeed. Objects in our everyday Earthly world are about 27 powers often more dense than the average density of the universe, so a great deal of separation and many orders of magnitude of compression occurred over the age of the universe. Most of it was accomplished by gravitational collapse, and the formation of galaxies, stars, and planets. According to current cosmological theory, regions of over-density any smaller than dwarf galaxies, were smoothed away by various dispersion processes before recombination. This is why planets and stars did not form before galaxies. On the other hand, lumps on any mass scale larger than dwarf galaxies were not erased in the early universe. These could range from dwarf galaxy size to sizes much larger than superclusters of galaxies. The big lumps were not separate from the small lumps. Most small lumps and several of their neighbors were probably contained within a larger lump, which along with several peers was probably contained within a still larger lump, etc. We know this not only from numerical simulations of particular theories, but from observations of the microwave background radiation. This radiation is leftover heat from the Big Bang, and bears the imprint of the density structure of the universe at the (first) time the universe became transparent, which is also about the time of recombination. Several good popular books are available which describe the exciting research on the properties of this radiation over the last couple of decades. There are two ways that a group of building blocks could be contained in a larger over-dense region. One is simply that they could be quite close together, and away from under-dense regions, so that if their matter were spread out over a volume containing all of them, then the density would still exceed the average. The second is that they could truly be lumps on a bigger background lump. That

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is, even if the smaller lumps were taken away, the big lump would still be a region of over-density. The second case is closer to reality on average. A pure twoby-two hierarchical model would have the smallest lumps arranged in close pairs, and those pairs arranged in pairs, and so on. Let us come back to the main point, which is that gravitational collapse forms the first building blocks, and later collapse of the larger lumps containing them orchestrates their collisions and mergers. In the present-day universe, galaxy groups and clusters are incubators of collisions, both because the galaxies bound within them are likely to collide sooner rather than later, and because galaxies tend to keep falling into such large centers of attraction from outside. This process of fated mergers began early in the age of galaxy formation. In the cosmological case, we actually have two effective forces to consider; the inertia of expansion as well as gravity. Before dark energy was discovered, it was believed on average across the universe that these two exactly equaled each other, in such a way that gravity would ultimately stop the expansion, albeit at an infinite time in the future. (This is in accord with the generally accepted inflationary cosmology.) Dark energy provides an additional outward accelera­ tion to space-time, so in the distant future everything will accelerate away from everything else. Although matter and energy cannot travel faster than the speed of light according to Einstein's theory of Special Relativity, his theory of General Relativity allows space-time to stretch faster than the speed of light. This means that not only will the galaxies continue to accelerate away from each other, but eventually they will do so at faster than the speed of light, unless the acceleration by dark energy ceases. When their light cannot travel fast enough to reach us, the galaxies will disappear from sight. Far-future cosmology is fascinating, but very speculative! It has sidetracked us, so let us return to galaxy formation. The self-gravity of a collapsing lump overcomes the cosmic expansion, and in a sense it becomes its own little bound universe. In a universe with dark energy, an early over-dense region must be dense enough to overcome the acceleration, as well as the initial expansion that is not due to dark energy. At early times the acceleration is small, so it has little effect. However, the principle implicit here can be extended - the more dense our lump, the stronger its self-gravity, and its collapse is correspondingly more rapid. The lumpiest lumps clump fastest. Small lumps within bigger lumps collapse first, forming dwarf building blocks. Then the bigger lumps collapse, leading to the merger of the building blocks within. If these are contained within still bigger lumps, then they too will be driven to merge. Soon we have built a proper galaxy. This is a nice picture, being essentially the cosmological version of the ideal hierarchical model, which means that it is too simple. Reality is more complex; remember the merger trees! For starters, the bigger lump at some stage may contain a lot of smaller ones, rather than the few envisioned by the ideal hierarchical model. I am being deliberately vague by using a term like "a lot," since there is probably considerable range across the universe. An interesting example is the case where "a lot" means a great number, all brought together by the collapse of a much larger scale. This is essentially a lumpy monolithic collapse, rather than a

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Stars log n 3 (cm"3)

Gas log n g (cm"3)

Metals [O/H]

0.1 Gyr

0.3 Gyr

0.5 Gyr

1.0 Gyr

Figure 8.1 Snapshots of a supercomputer model of elliptical galaxy formation. Time advances from left to right, with the top row showing the agglomeration of star-forming clumps. The middle and bottom rows show the redistribution of gas and its heavy elements constituents due to winds and other feedback processes (from M. Mori & M. Umemura, Nature, Vol. 440).

hierarchical build-up with many simultaneous interactions on a small scale, on the way to an overall merger. In Section 4.9 we already discussed some of these issues, e.g., how the rapid formation of at least large fractions of elliptical galaxies may have resulted from nearly simultaneous multiple mergers. We also discussed how large-scale simulations like the Millennium Simulation provide helpful insights on how the processes develop, although many questions can only be answered with even more detailed cosmological simulations. In the meantime, some models of multiple mergers have been produced. These use plausible initial conditions on roughly galaxy scales, rather than evolving many realizations of those conditions from early times on larger scales, as in the large-scale cosmological simulations. This allows more spatial resolution, and the inclusion of more detailed gas heating and cooling processes. It amounts to choosing to follow the biography of one specific galaxy, rather than read a more general history of the times. One example is shown in the snapshots of Figure 8.1 produced by Japanese astronomers Masao Mori and Masayuki Umemura. Note that although the Mori and Umemura model is on a much smaller scale

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than the Millennium Simulation, they still required some massive computer hardware, including the so-called Earth Simulator supercomputing complex. Since they begin with only about a dozen lumps, which already contain stars, they assume that a good deal of evolution must have already occurred. Similarly, at the end of the model the proto-elliptical is still surrounded by a large amount of (metal-enriched) gas, so there is a good deal of evolution left to complete. Nonetheless, according to comparisons they made to hydrogen emission observations, the model looks a good deal like some high redshift galaxies. The resolution of such observations is generally limited, but they frequently convey the impression of a lumpy mess. Another essential point can be discerned in the images of Mori and Umemura's models, and even more so in the Millennium and other large-scale simulations. This is that the collapses of lumps and bigger lumps, etc., is not generally a spherical process. This is a general truism of gravitational collapse that has been discovered by several people at different times in different applications. The Russian physicist and astronomer Yakov Zeldovich was the first to emphasize its importance in galaxy formation in the 1970s. He and his group studied a model in which collapse occurred first on large scales, the scales of superclusters. Galaxies later formed via fragmentation within the large scales. Even more than monolithic collapse, this model is the opposite of hierarchical growth. Unfortunately, the galaxies in this model formed too late to be consistent with modern observations of high redshift galaxies, so the model is not tenable. However, it had a dominant characteristic of continuing interest extremely anisotropic collapse. Specifically, the initial collapse on the large scale generally proceeded fastest in one direction. That is, collapse along one line was much faster than collapse in the perpendicular directions. (Of course, the specific direction of first collapse varied more or less randomly from place to place; there was no universal direction.) As a result of the initial collapse, a large initial region of over-density collapsed to a sheet, about as long and wide as at the initial size, but very thin in the collapse direction. To Zeldovich this suggested a Russian blini and the theory came to be called the "pancake" theory. After this initial collapse stage, the structure would continue to collapse in the other directions, while fragmenting into galaxies on the smaller scale. Again, the collapse of one direction would usually outrun the other, and a more filamentary structure would result. (It was not, however, called a sausage!) The next logical step might seem to be collapse in the final direction down to a small dense lump. However, that logic neglects an important point, which is that most regions of over-density are not isolated, but connected to others, e.g., by both being contained in a still larger structure. The consequences of this fact apparently are not too important in the initial collapse of large scales, but by the time filaments develop they are. Filaments are born connected to filaments, and in consequence, filaments do not usually collapse to their centers. A better qualitative generalization would be to say that they drain out their ends to the points of connection with other filaments, like water flowing out of a hose lifted

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in the middle. These connecting points were the densest regions on large scales from the beginning. They apparently do collapse in a more spherical fashion than most over-densities, and they do it so quickly that it is essentially completed while filaments are still forming in their surroundings. These connection points are frequently destined to become large galaxy clusters. The sheet-like pancakes were also connected, and that too has a consequence. Their connections often formed a more or less closed, three-dimensional figure around some point. In the region of that point, the matter all collapsed toward the nearest sheet, leaving the point in the middle of a huge matter void. These voids were first discovered in a large survey of galaxy redshifts (distances) undertaken by a group of Harvard University astronomers, led by Margaret Geller and John Huchra in the 1980s. Many more have been discovered in the much larger surveys carried out since that time. The reader may have noticed that I have gone from speaking of the Zeldovich pancake model in the past tense to (rather roughly) describing large-scale collapse in the present tense. The reason is that many aspects of the pancake model still apply to modern simulations of collapse on the largest scales (Figure 4.10). Collapse on those scales still happens rather as Zeldovich envisioned it. However, collapse and aggregation on the smaller scales does not wait for the large scales; it begins first as in the hierarchical model. According to the simulations, our universe is more complicated than either the ideal hierarchical model or the Zeldovich picture. There is collapse on many scales simultaneously, though at slower rates on the largest scales. All these complexities give rise to spatial variations that, in turn, produce an interesting range of galaxy forming environments, which we will consider further in the next section. The complex interplay of these processes are also responsible for some of the phenomena mentioned in Chapter 4, including the prompt formation of the largest galaxies (usually ellipticals), and the cosmic star formation rate. Recall that the latter is the star formation rate averaged throughout the universe at a given epoch, and that it peaks at a redshift of about 2-3, when the universe was less than about one-third of its present age. We now understand that this was some time after the epoch when the smallest lumps collapsed and started star formation. It was probably closer to the time when the largest galaxies were completing their most vigorous phase of star formation, and somewhat smaller, but more numerous galaxies were turning on theirs. This, in turn, could be the beginning of a process wherein the mass of the galaxies with the most intense star formation decreases with time. This is called "downsizing" in galaxy growth. At the present time, these are galaxies of mass an order of magnitude or more smaller than the Milky Way. Many of the proto­ typical colliding galaxies we studied in earlier chapters are in this mass range. These galaxies are not generally found in large groups or clusters, but in rather less dense environments, which cosmological theory leads us to expect pull themselves together at later times. These are "the last dregs of galaxy formation" to quote Alar Toomre's Yale Galaxy Conference talk. The picture may not be exactly what Toomre envisioned at that time, but in terms of "the last dregs" it

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may not be so very different either. We have learned a great deal in the time since Zeldovich's and Toomre's pioneering works. However, we are probably just at the beginning of observing and understanding the history of galaxy formation in its cosmological context.

8.2 Many different evolutionary pathways in groups

In the last section, we took a very broad scan across the space-time of the galaxies, but now we narrow our focus a little. In earlier chapters, we considered specific kinds of galaxy collisions in some detail. We also considered the effects of specific classes of mergers. For another point of view, in this section we will briefly look at a few specific environments. Actually, we have already looked at one in great detail - the environment of our Local Group of galaxies. As we saw in Chapter 7, at first glance our Local Group does not seem very exciting, with no major mergers (yet). Interactions with dwarfs yes, but even the interaction with the Magellanic Clouds does not quite rise to the level of a minor merger. Yet our group is quite typical of regions of moderate cosmological density. We are surrounded by a number of other groups, which are not exactly identical to our group, but are generally quite similar. We are on the outskirts of the Virgo cluster, a massive cluster within the Virgo supercluster of galaxies. Roughly half of all respectably sized galaxies are contained within large clusters, so being on the outskirts of one could be called a kind of median environment in the world of galaxies. The Virgo cluster (and supercluster) could also be called rather average members of their class. Very large, dense, and most dynamically relaxed galaxy clusters are quite rare. Examples of this type of cluster were catalogued in the 1950s by astronomer George Abell. They are often called "Abell" clusters. The Coma cluster is the nearest example (Figure 8.2). More amorphously structured clusters, which are young in the sense of having many gas-rich galaxies with substantial rates of star formation, are not as rare. They are called "irregular," and the Hercules cluster (Figure 8.3) is a nearby example. (Note that in the world of galaxy clusters "nearby" is measured in units of hundreds of millions of parsecs.) Virgo is somewhere between these extremes, by human standards a sort of thirty-something kind of galaxy cluster. We will look more at the goings-on in clusters in a little bit. Before that, let us consider some other examples of the wide range of different types of galaxy groups. We have noted that the Local Group is rather quiet in terms of galaxy interactions. Let us go beyond this to the extreme of groups that are deceased, or nearly so, in terms of interactions and their star formation rates. In the graveyards of the galaxy world we find the "fossil groups." Trevor D. Ponman and D. Bertram proposed this designation in a brief paper in 1993. They did not discover these groups, but recognized them as being something different than previously thought. Actually, the Ponman and Bertram paper only discussed one group, named Hickson 62, from a well-known group catalog.

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Figure 8 . 2 A montage of Hubble Space Telescope images of the Coma cluster of galaxies. (NASA, ESA, and the Hubble Heritage Team (STScl/AURA), D. Carter, Coma HST ACS Treasury Team).

However, their suggestion that this might be the proto-type of a class stuck, and more examples were soon discussed in the literature. Both fossil and Hickson groups are compact groups, meaning that the members appear much closer together than galaxies in most groups. The first distinguishing characteristic of fossil groups is the presence of a dominant elliptical galaxy, located more or less at the center of the group. The second defining characteristic, discovered by Ponman and Bertram in Hickson 62, is that they are generally surrounded by an X-ray emitting hot gas halo whose size is several times larger than the dimensions of the area containing the visible galaxies. The characteristics of the X-ray gas imply a similarly large and very massive halo of dark matter, much more substantial than would be expected for the elliptical galaxy alone. Ponman and Bertram deduced that the group had originally contained more galaxies, spread over a much larger volume of space. They proposed that the orbits of most of the galaxies had decayed via dynamical friction, and that they had merged into a remnant that became the central elliptical. The original system, Hickson 62, contains several ellipticals, so the merger process is not over yet. However, in general, they estimated that the timescale for orbit decay and merger would typically be about a few Gyr in fossil groups. In this process, most

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Figure 8.3 An image of the Hercules galaxy cluster (V. Andersen and W.C. Keel, KPNO/ AURA/NSF, Univ. of Alabama).

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of the interstellar gas would either be used in vigorous episodes of star formation, or heated and mixed into the hot intergalactic medium. Thus, since the active life of fossil groups was essentially lived on this cosmologically short timescale, what we see today is a remnant. These conjectures are still tentative, and in some cases controversial. For example, it has been pointed out that the properties of the central galaxies in fossil groups are not really (that is, statistically) much different from those of the field ellipticals, i.e., ellipticals that live in no visible group at all. The implication of this finding seemed to be that the central galaxies of fossil groups are no more unique as fossils than many other ellipticals, which do not have small companions and equally large X-ray halos. One point of skepticism about this argument is that we know from the discussion in previous chapters that the violent relaxation and other dynamical processes can produce fairly universal products regardless of the starting conditions. The surroundings of the ellipticals can tell us about their formation conditions, while in the ellipticals themselves that information has been violently erased. Another thing the two types have in common is that both fossil groups and field ellipticals are relatively rare objects. Fossil groups have been estimated to make up about 5-20 percent of all comparable groups, with much of the range of the estimate likely depending on how the comparable sample is defined. At the other end of the spectrum of arguments about fossil groups, it has been suggested that their central galaxies have much in common with the class of brightest cluster galaxies. A few fossil groups have been found to lie at the edge of, or even be contained within, larger groups of galaxies, e.g., they lie near a larger cluster or within a group of groups. Under such circumstances, it is plausible that the fossil central elliptical might find itself at the center of a regular galaxy cluster, and become a brightest cluster galaxy, when the larger structure collapses. Similarly, it has been remarked that the X-ray halos of fossil groups are quite comparable to those of galaxy clusters. Perhaps we can regard fossil groups as galaxy clusters without the usual complement of cluster galaxies, and preparing to form the core of a regular galaxy cluster once they sink to the center of a merged larger structure. If so, then we might also regard field ellipticals as galaxy groups without the usual group galaxies, but isolated, with little prospect for near term growth via merging. The lives of the ellipticals seem to have great variety! Maybe the ellipticals only provide an introduction to the variety of environments for galaxy evolution. We do not have to look very far from the Local Group to find a very different setting for galaxy growth. What I have in mind is the M81 group, mentioned in a previous chapter. In some superficial ways, the M81 group is like the Local Group. It is dominated by two galaxies: M81 and M82. There is a third, NGC 3077, which is not much smaller than M82, but is much farther from M81. These three galaxies are roughly similar to the Milky Way, the Andromeda Galaxy and M33 in the Local Group, though the brightness ratios are not exactly the same. Also like the Local Group, the M81

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group is relatively loose (in terms of galaxy separations), and contains about a couple of dozen dwarf galaxies. However, a closer look reveals many differences. Actually, one does not even have to look that closely at a comparison of hydrogen gas distribution derived from 21 cm radio observations to see drastic differences. In the Local Group, the hydrogen gas is almost entirely found in the disks of the biggest galaxies. Figure 8.4 reveals a very different picture in the M81 group, where great quantities of gas are strewn between the galaxies. From the earlier chapters we are very familiar with this landmark of galaxy collisions. Given all the galaxies that are connected by bridges and loops of gas, it appears that there have been several significant collisions in this group. There are numerous other signs of interaction as well. Recall that M82 has a famous nuclear starburst, which was mentioned in Chapter 4. This starburst has driven a beautiful and well-studied wind (Figure 4.5). Like the starburst, the galaxy is also a prototype. Deep images of it, beginning with one from the 200inch Mount Palomar telescope in the 1960s, gave this galaxy its nickname, the Cigar galaxy. Of course, neither starbursts, nor winds are unique to colliding galaxies, though more common among them. Young stars and star clusters outside of the disk provide more unique signatures, and the M81 system is full of them. The M81 group is close enough, at around 4 Mpc, that individual bright stars can be resolved. Thus, observations of young stars include: individuals, small associations, moderate-to-large star clusters, and even several objects large enough to be called tidal dwarf galaxies. One group of clusters strung along a bridge filament is poetically named "the Garland." The ages of these stellar populations extend from very short, a few million years, up to intermediate ages of a couple of hundred million years. That is, they extend up to a timescale that is very reasonably associated with the onset of strong interactions in the system. It is difficult to see the intermediate age populations in more distant colliding galaxies, because of the fading of star clusters with age, which was discussed above. It is reassuring to see those populations quite well represented in this nearby system. The larger clusters and tidal dwarfs have received some study in the last decade, but I believe a good deal more study would be justified. A good understanding of their dynamical and star-forming history could provide some much-needed models for understanding other members of the class in more distant systems. Speaking of models, it is rather ironic, that this, one of the closest systems of colliding galaxies, has never been modeled as a whole. About a dozen or so dynamical simulations of parts of the system have been published. In some of these, only two of the galaxies are modeled, for example, to account for the structure of M81 induced by its most recent collision. More usually, interactions between the three largest galaxies were modeled to account for the tidal bridges and other material scattered between them. However, to the best of my knowledge, there has never been any serious attempt to model the gas on the right side of Figure 8.4, and extending to the galaxy NGC 2976. (The right side is the west side with the usual east-west reversal in astronomical images.) This

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Figure 8 . 4 Contours superimposed on an optical image of the M81 group show gas distributed throughout the group (from K.M. Chynoweth, et al., Astron. /., Vol. 1 35).

material has long been known; some of it was discovered by Phil Appleton and collaborators at the Westerbork radio telescope in the Netherlands about three decades ago. The lack of models is understandable because modeling interactions between three galaxies is difficult. Trying to fit in NGC 2976 and the object labeled as Cloud 3 in the system, not to mention the gas extending to the lower left, is likely to be much more difficult. In 2004, Juan Carlos Gomez and collaborators produced a good model of the gas distributed between the three major galaxies using a computer algorithm that computed a dynamical model, automatically compared it to the observations, adapted the collision parameters and repeated the process until it found a satisfactory model. Several groups have attempted such automatic modeling

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procedures for individual systems in recent years, including my collaborator Beverly Smith. The procedure has definite limitations, but shows promise for the future. They may provide the best route to a more complete model of the M81 group. On the other hand, it may take more than a model of the interactions of half a dozen objects to explain the group entirely. The Russian astronomer Igor D. Karachentsev has long studied nearby galaxies and groups and has noted that M81 has extended connections, including to the small group around the galaxy NGC 2403, though it is nearly a Mpc distant. M81's far-flung connections in various directions make one wonder about the group's cosmological origins. Beyond the center defined by the three main galaxies, is it at the node of several extended filaments of matter? If so, is its evolution qualitatively akin to that of large galaxy clusters, in that a core of galaxies formed first and then continued to grow by drawing in more and more material? Of course, the mass gravitationally bound to the M81 group is a couple of orders of magnitude less than that of a great cluster. Moreover, the core of the group is still forming in the sense that the observed collisions have been very recent by cosmological standards, and the mergers that will form the dominant core galaxy still lie in the future. Evidently, it took a long time to pull the group together. Its initial cosmological over-density must have been slight in order for its gravity to take so long to overcome the cosmological expansion. Within a few Gyr the three main galaxies of the M81 group will merge and others will be drawn into the core region. What we have learned in previous chapters suggests that most of the gas will be converted into stars or heated and scattered. The stars will be dynamically heated, and the merger remnant will look like a much earlier Hubble type. At some point it may well appear to any human astronomers still observing from the Milky Way as an elliptical in a fossil elliptical group. That is, what we may be observing now in this region of the sky up near the Big Dipper is what a fossil group looks like at birth. The M81 group is a bit deceptive. In the early twentieth century and earlier, with limited optical resolutions and no radio telescope arrays, its primary galaxies looked relatively normal and isolated. It required a closer look, and in multiple wavebands, to reveal the group as a strongly interacting system. That has never been a problem with Stephan's Quintet; see the beautiful image produced in 2009 by the new camera on the Hubble Space Telescope in Figure 8.5. Even from a first glance, its galaxies seem to be vigorously wrestling. However, appearances are a bit deceptive in this system too. The big blue galaxy is actually well in front of, and not associated with the group - a fact not known by E. Stephan, when he discovered the group in the nineteenth century. However, in addition to the four yellowish galaxies visible in the image, another group member lies well off to the left (east) of the image. Thus, it is still a quintet. Figure 8.6 shows a very different view of Stephan's Quintet (or SQ). It is a falsecolor encoding of observations ranging from the mid-infrared to the optical. In the Spitzer Space Telescope image some of the dark dust lanes visible in the HST

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Figure 8.5 Hubble Space Telescope image of four members of Stephan's Quintet (NGC 731 7, 7318a, 731 8b and 7319), plus the blue foreground galaxy (NGC 7320), acquired with the recently upgraded (WFC3) camera. In addition to the four yellowish galaxies shown here, a fifth group member lies well off to the east (left) of the image (NASA, ESA, and the Hubble Heritage Team (STScl/AURA)).

image glow (red) due to their thermal heat emission. Yellowish stars in the optical image are turned into a fainter blue. To the right of the center, a yellowred dust semi-circle and two pink galaxy cores make a bizarre galaxian smiley face with a very funny conical hat. At the tip of the cone, the cluster of yellow clumps is known to be an intense region of star formation. Perhaps a tidal dwarf galaxy is being formed there. We will have more to say about the long, green, vertical stripe of hydrogen emission shortly.

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The galaxies of Stephan's Quintet are named NGC 7317, NGC 7318a, NGC 7318b, etc. It is a mouthful of catalog numbers, and hard for anyone except experts to keep straight. My graduate student Jeong-Sun Hwang has been working on gas dynamical models of this group, and when she discussed her results with me, I frequently found myself confused about which galaxy we were talking about. Even simpler designations like Gl (galaxy 1), G2, G3, etc., do not help much. The galaxy NGC 7318b, the left eye (as we look at it) of the smiley face, is known to have a quite high speed along the line-of-sight relative to the group average. It has been called the Intruder. One quirky day that fact made me think of another famous intruder, Goldilocks. I started calling the biggest galaxy, the one on the left in the figures, Papa (Bear), and the one whose core is the right eye of the smiley face, Mama. The galaxy in the lower right corner of the figures (NGC 7317) show no signs of having recently interacted with the others, so we tend to overlook it. The galaxy mentioned above, off the pictures to the left, evidently has been involved recently, so it gets to be the Baby. Actually, we still tend to call it the eastern galaxy because it is located so far in that direction, and the Intruder is still called the Intruder. However, "Papa" and "Mama" beat NGC 7319 and NGC 7318a. SQ is both obviously and technically a compact group. Canadian astronomer Paul Hickson set forth a quantitative definition for compact groups, and assembled a catalog of 100 of the brightest examples in the northern hemisphere. SQis also known as Hickson Compact Group 92. Many of the galaxies in Hickson groups are clearly interacting, though SQ is a standout even in that catalogue. Even a larger fraction of the interstellar gas has been pulled out of the SQ galaxies and into intergalactic space than in the M81 group. Papa and Mama, for example, appear to contain relatively little gas within their visible extents. Many starforming clumps are also found outside of the galaxy disks, as in M81. Since SQis a couple of dozen times more distant than M81, we can only resolve large clusters in the group, certainly not individual stars as in M81. The fact that there are so many of them is very impressive. Of course, Papa and the Intruder are very disturbed galaxies, much more so than the M81 galaxies. There is no starburst nucleus in SQ to match M82, but Papa's core hosts an active galactic nucleus. Thus, many pieces of evidence indicate interactions as strong or stronger in SQ than in the M81 group. This, together with the fact that several of the galaxies appear to be quite close to each other, suggests that they are closer to merger than the galaxies in M81. Recall that dynamical friction generally increases as the galaxies draw together in major mergers. Given that Mama and the galaxy in the lower right corner of the figures are already ellipticals or elliptical-like, we can expect that the merger remnant would be a larger elliptical. Thus, after the next one or two mergers SQ will be a fossil group. Recently Phil Appleton and collaborators made another fascinating discovery about SQ. Using mid-infrared observations from the Spitzer Space Telescope they found that near the long (green) filament of hydrogen alpha emission in Figure 8.6 there is also strong molecular (rather than atomic) hydrogen emission. Generally, extragalactic astronomers only expect to find abundant molecular

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Figure 8.6 False color rendition of multi-waveband observations of Stephen's Quintet. Visible (continuum) light is shown in blue, hydrogen alpha line emission in green (highlighting a large scale shock wave), and mid-infrared emission observed by the Spitzer Space Telescope is shown in red (NASA, JPL-Caltech, Spitzer Science Center and P.N. Appleton).

material in the centers of late-type galaxies, where the gas densities can be high enough to protect it from ultraviolet radiation, which can break the molecules apart. Having masses of molecules strewn between the galaxies is a novelty to say the least. It is known that molecules can be produced in gas compressed to high densities behind strong shock waves. Shock waves are produced when two gas flows impact each other at supersonic velocities, and evidently that is what has occurred among the galaxies of SQ. The Intruder is an immediate suspect, since it is moving at close to 800 km/s relative to Mama and Papa. Its disk may have crashed into parts of their disks,

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giving rise to the shock wave. One more interesting observational point is that the sites of strong recent star formation are not coincident with the molecular hydrogen emission, as is generally the case within galaxies. Perhaps the shock wave was strong enough to destroy large gas clouds, but still create the conditions for molecule formation. Both X-ray and radio emissions have been detected in this same area. These detections provide further evidence of a great deal of energy dissipation in a shock wave. All in all, SQ is a very unusual environment in the galaxy world. For reasons that I cannot claim to understand fully, SQ has been at least as attractive to modelers as the M81 group. In both cases, and most interacting systems for that matter, the intergalactic gas distribution and other tidal features provide powerful clues to the nature of past encounters. One of the most obvious tidal features in SQ is the broken tidal tail extending from the top around to the bottom (south) of Papa and then away. It turns out that a second (gas-rich) tail extends out on a similar, but much wider, path. It winds out near the strong star formation region called SQA (at the tip of the conical hat), presumably past the shock region, though it cannot be tracked well there, and far to the south (behind the foreground galaxy), and out to the left. These tails have been the inspiration of a number of models. Theories or models of the late 1990s attempted to account for the double tail structure via two encounters between the far eastern galaxy (Baby) and Papa Papa tossing baby into the air repeatedly! Despite the potential for fun metaphors, this scenario is unlikely to be correct. The general rule for multiple collisions between galaxies (derived from many models) is that either the first set of tidal structures has disappeared by the time the second is created, or the first set is scattered and destroyed in the second encounter. The former poses a timing problem in SQ and while the latter is not a mathematical proof, it suggests that the odds against this kind of scenario are long. Even if these issues disappeared, the range of collision parameters needed to produce two parallel tails would seem to be small. Incidentally, this is a very nice example of how computer models limit the scope of theoretical conjectures in this field. Alternative theories have explored the possibility that the tails and related phenomena could be the result of an earlier collision between Papa and Mama, followed by a collision between Papa and Baby. (Mama tells Papa to go take care of the baby?) Baby is implicated in all scenarios because both tails point in that direction. The larger outer tail nearly reaches there, though we do not know about their proximity along the line of sight. A thin dust filament apparently connecting Mama and Papa (extending from the smile of the smiley face), argues for Mama's involvement. Our group has explored some models in this scenario that seem to reproduce the tails reasonably well, though they are preliminary, and the range of viable collision parameters does seem fairly narrow. The latter point is a quantitative one - fairly narrow is not necessarily bad; infinitesimally narrow would be very bad. The features on the western side of the system including the shocked region, the star formation region SQA, and the ring-like (albeit triangular-shaped)

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feature associated with them, likely involve a collision with the Intruder. In producing the shock the Intruder may, in fact, have crashed through part of the outer tail, rather than Mama's outer disk. Actually, since Mama does not have an inner gas disk (she's an elliptical), she may well not have an outer gas disk. However, there is another problem in a scenario that seemed to be pulling the observations together into a coherent picture. X-ray emitting gas generally does not stay hot for very long, unless it is of very low density. We do not expect the gas heated by a collision between the Intruder disk and a tidal tail to be of extremely low density. However, producing long tidal features like some of the stellar filaments in that region generally takes a longer time (e.g., hundreds of millions of years). We have been exploring models wherein Mama is actually some distance away from the plane of Papa's disk and the tidal tails. In this case, the Intruder could have collided with Mama's disk some time before hitting Papa's tail. (Goldilocks tries frantically to get past Mama and Papa!) These models seem to show some promise to account for the wealth of features in this region. Does this all sound very Rube Goldberg, or is it just too much of a fairy tale? Maybe so, but it does highlight the fact that life is likely to be complicated in compact groups, and the formation of ellipticals, including fossil ellipticals, equally so. Most of the models suggest one simplifying factor in the evolution of SQ the significant collisions tend to involve two galaxies at time. In our recent models it is Papa and Baby, then Papa and Mama, and finally the Intruder with Mama first and then Papa's tail. The south-western galaxy does not show any signs of interaction and is not included in any of the models. If this general twoat-a-time result is verified in models of a compact group, then it may prove to be a very useful approximation in detailed studies of specific groups. Another interesting facet of the SQ models is that many collisions occur at high relative velocities. Certainly this is true of those involving the Intruder. It also seems to be true of the interaction between Papa and Baby, since Baby had the kinetic energy needed to move so far from Papa. It is interesting that these collisions have happened quite recently compared to the age of the universe. This is curious because there do not seem be any connections to filaments or other groups nearby. SQhas not been accreting galaxies over that long timescale from any such structure. The bears are alone in the forest. Because of this, the group definitely seems poised to become a fossil in a few Gyr. This contrasts with M81 again, which may have a fossil phase, but has lots of galaxies yet to absorb. Why has SQ chosen this particular time for an active build-up phase? That question may only be answerable in terms of odds derived from the statistics of cosmological structure formation models. Whatever the odds, its active, newfound members probably spent long ages sitting out on the periphery, just barely attracted gravitationally more than they are pulled apart by the cosmological expansion. To get a different point of view on this question of group build-up, let us shift our gaze from M81 and Stephan's Quintet to some rather larger groups or small clusters. For several years, astronomers Kenneth Rines and Antonaldo Diaferio have been using the Sloan (SDSS) survey to study the mass profiles of groups and

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clusters to unusually large radii. Their groups and clusters are also detected by satellite X-ray observations, which among other things, provide confirmation that they are indeed self-gravitating entities, not merely (highly unlikely) chance superpositions of galaxies, or a view down the length of a large-scale filament. The latter type of structure would still generally be a gravitationally bound unit, but without the same symmetry and structural properties of a more spherical unit. These symmetry properties are necessary for the technically sophisticated mathematical model they use to estimate masses to much larger radial distances from the cluster center than previous works. The first finding of this research program is that the cluster mass profiles do indeed fit those of cosmological structure growth simulations quite well. The profiles in both cases are well fit by a formula suggested by Julio F. Navarro, Carlos S. Frenk and Simon D.M. White, and commonly called the NFW profile. Of more relevance to the present discussion are Rines and Diaferio's recent results on 16 galaxy groups. With their method, they find evidence for continuing infall onto the group of other galaxies located near the periphery of the groups. Three of their groups are on the edge of larger clusters, and one is undergoing a group-group merger. In summary, this work indicates that like M81 most groups are growing and will continue to grow, presumably with interactions resulting from this growth. Some are growing little or not at all in terms of infall from extended regions. SQ may be another example of these. Other astronomers have also found evidence that compact groups, for example, are the cores of more extended and diffuse structures. Indeed, given that merger times are short compared with the age of the universe, this phenomenon has been suggested as the reason for the existence of (unmerged) compact groups - that they are continually fed more galaxies from the extended structures. From this point of view compact groups, as well as clusters, are incubators for galaxy interactions. The case studies of this section tell a very different story from that of the early chapters, where relatively simple and symmetric collisions were considered. Clearly, current pictures of dynamical build-up of large-scale structure involve much more complexity than those of the 1970s. On the other hand, the yin to that yang is that because the timescales for collisions are usually short, collisions often happen two at a time, and frequently with a fair amount of symmetry. As advertised in the section title, the collision histories of galaxies follow many diverse pathways.

8.3 The b r i g h t lights of t h e galaxy metropolis

Most of the diverse collision pathways we have considered so far are country lanes in the world of galaxies. We will end this chapter with some brisk walks down some hard city streets. The large, dense Abell clusters are the cities of the galaxy world. Compared to their surroundings they are bright, hot and crowded with galaxies. As in the human world, the long-time denizens of galaxy cities

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tend to move faster than their country cousins. They also dress differently; less casually, with fewer flocculent late-type spirals, and no coveralls in the form of very extended gas disks. Their interactions are faster, even brusque, too. The stress of the dense population has its effects. To the informed (and multiwavelength) eye, newcomers stand out. Let us consider some of these points a little more quantitatively. To begin with, mean velocities of galaxies within large clusters range in the thousands of kilometers per second, while those in pairs and groups are typically in the hundreds of kilometers per second. In the latter case, a companion galaxy can cover a distance about equal to the diameter of the primary disk in about one rotation time in the outer part of that disk. This fact means that internal disk motions can resonate with the companion orbit and strong disturbances can result, as we have seen in many cases earlier. In clusters, a flyby companion can cover a distance about equal to the size of a group in that time. It is gone before the target disk can respond, and resonant effects are unlikely. Thus, collisional disturbances in cluster central regions are generally mild, and mergers in particular, are much less likely. The former point was first remarked on by Lyman Spitzer and Walter Baade in the early 1950s. The point about mergers is a major difference from small groups, where mergers are generally the inevitable conclusion of collisions. It is shotgun weddings in the country, versus short-term relationships in the city. Ironically, this means that individual galaxies can persist longer as unmerged "island universes" in the cores of large clusters than in the cores of compact groups, for example. Not that life is easy in the big city. Clusters are enveloped in the hot gas of their intra-cluster medium. In the previous chapter, I discussed the possible existence of hot gas in the halo of the Milky Way, and how it may be responsible for stripping gas out of the Magellanic Clouds and into the Magellanic Stream. This process has been shown to be even more important in large clusters. Although it is very tenuous in clusters, the intra-cluster gas is widely distributed, and contains a lot of mass. It has been estimated that it holds about as much ordinary baryonic matter as is contained in all the stars and gas clouds of the cluster galaxies. (Clusters almost certainly hold much more dark matter than the sum of the individual halos of their constituent galaxies.) Numerical simulations and observational evidence show that at least the outer gas disk, and in some cases the whole gas disk, of a galaxy passing through the central core of a large cluster, where the intra-cluster medium is densest, is removed by ram pressure stripping. The gas that remains in the central regions, experiences a kind of tidal stress as well. This is because the ram pressure pushes it away from the stellar disk and the center of mass of the dark halo. They pull back in proportion to the displacement. Numerical models show that the resulting vertical compression can generate spiral waves, which transfer angular momentum and compress the disk in the radial direction. I have nicknamed this process "annealing" of the disk. It may drive more star formation, accelerating the evolution of the

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remaining disk by consuming some gas, and dispersing other parts, though this is still conjectural and not confirmed by observation. Hot gas in the galaxy halo is also removed. Since this gas has the potential to cool and feed the galaxy disk, this process is called "starvation." Stripping and starvation will end star formation in cluster disks, either promptly or with some delay. If the high galaxy velocities did not make the environment so dynamically hot, we might speak of galaxies freezing out, and ending their evolution, as a result of these processes. There are yet more difficulties to face in galaxy cities. Modeler Ben Moore and collaborators produced some very influential simulations in the late 1990s showing the effects of a process that had previously only been the subject of speculation - galaxy harassment. The idea is that even though interactions in galaxy clusters may be weak, clusters are crowded places, and interactions are likely to be much more frequent. The effects of each may be small, but the collective effects may not be. Moore and company initially sought to investigate the effects of harassment on dwarf galaxies where they might be greatest, but they are now thought to be relevant for most cluster galaxies. Specifically, these and more recent studies show that the effects of harassment include: 1) the slow erosion of the dark matter halos; 2) the dynamical heating and thickening of stellar disks; and 3) the destruction of small disks. The first effect in the above list works, along with tidal forces from the cluster core and other substructures, to reduce the extent of the dark halos of cluster galaxies. There is observational evidence that they are indeed smaller than those of field galaxies. Effects 2) and 3) work with stripping to diminish and heat cluster galaxy disks. Clusters contain a high fraction of early-type galaxies, and it is clear that one reason why is that survival of gas-rich late-type disks is a tough proposition in the environment of large clusters. Evidently, one gets hardened in the big city. A particularly interesting aspect of harassment is the fact that it is a "secular" or slow and continuous evolutionary process, resulting from galaxy collisions. A number of the processes of galaxy evolution have been mentioned in previous chapters, but up to now we have not seen collisions with very high relative velocities, and which have only mild effects. On the contrary, I have often painted them as violent and capable of completely changing the course of evolution in the galaxies involved. Things are different in large clusters. If I have painted galaxy city as a rather hostile place, then I should point out that it is capable of doing something that a human city cannot. Because of the long-range nature of gravitational forces, galaxy city can give individuals and small groups a big hug! However, it is debatable whether this is actually a good thing. Let me explain. The dark halos of clusters are very massive indeed, and despite their large size, the dark matter density within them is as great as that within galaxy groups. This means that when a galaxy group falls into a cluster and through the cluster core, it experiences a considerable increase in halo gravity with the local cluster dark matter added to its own. It has been estimated that this extra dark matter is enough to draw the group galaxies in to a

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configuration that can be roughly half its original size. This is the hug (or squeeze) referred to above. This compression could increase the galaxy density in the group by nearly an order of magnitude, which in turn means that the rate of random collisions would go up by an even larger factor. Of course, the dark matter over-density is a temporary phenomenon. Once the group moves through the cluster core to its outskirts, the dark matter density will decrease to something close to its original value. However, the over-density will persist for a time of about a few hundred million years, which is long enough for the galaxies to be drawn into the denser configuration. It is probably also long enough for some galaxies to be drawn into collisions with others. These collisions will occur at relative velocities that are more like the typical relative velocities in groups rather than the high random velocities of cluster galaxies. Thus, their effects are likely to be stronger than those of "harassment" collisions. When the infalling group does move out of the cluster core, and the dark matter gravity is diminished, it will rebound towards its original size. If this sounds rather familiar, that's because it is very similar to the colliding ring galaxy phenomenon discussed in Chapter 2, though playing out in three dimensions rather than in a two-dimensional disk. As we discussed there, the rebounding galaxies will likely overshoot their original radial positions, and the group will be even larger than before it fell into the cluster. This may render it vulnerable to disruption due to the action of cluster tidal forces, but we will not pursue that topic. Rather, let us return our attention to the colliding galaxies. The reexpansion of the group may mean that following a single flyby some of the collision partners may be separated indefinitely. In some cases, dynamical friction may have had sufficient time to hold the partners together, and eventually draw them into a merger. To date, these dynamical processes have not been modeled in any detail, nor has evidence for their occurrence been teased out of cosmological structure formation simulations, like the Millennium Simulation. However, this and some of the other processes discussed above provide an attractive explanation for a very interesting observational phenomenon, the Butcher-Oemler Effect. Astron­ omers Harvey Butcher and Augustus (Gus) Oemler discovered this effect in the 1970s, in observations of the colors of galaxies in relatively distant clusters. The effect is that those clusters seemed to have a significantly larger fraction of blue (i.e., strongly star-forming) galaxies, than typical nearby galaxy clusters, which as we have noted, are usually dominated by red, early-type galaxies. As usual in extragalactic astronomy, their discovery was initially rather tenuous. They were only able to observe a few clusters, and the cluster redshifts were not very large by modern standards. Yet subsequent studies have confirmed the effect and provided a great deal more information. For example, the blue galaxies are found preferentially in the outer parts rather than the core of clusters. Also there is evidence that they have experienced interactions, and have recently fallen into the cluster. All of these points are consistent with the induced interaction picture outlined above, though not unique to it. If this picture is correct it is a beautiful example of the interactions between several

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scales - the evolution of galaxies is affected by a change in their group environment as a result of traveling through a large cluster core. A wide view of the galaxy world is very helpful for seeing such processes! We can conclude the panoramic sightseeing of this chapter with one more large-scale example - collisions between galaxy clusters. Recall from the first chapter, that for years astronomers were skeptical that galaxy collisions were frequent enough to be important. Galaxies were viewed as island universe with vast volumes of empty space around them, which made collisions seem exceedingly unlikely. However, Zwicky, Arp, and later Toomre championed the cause, and as it came to be understood that the clustering of galaxies predestined them for collisions (and hierarchical build-up, etc.) the cause was won. The same story could be told about galaxy clusters. In their own way Abell's great galaxy clusters were viewed as "island universes." In fairness, we do not often find a comparably sized partner within a few optical diameters of these large clusters. Thus, we do not clearly see large clusters in the throes of a cluster version of a major merger. (Although a beautiful exception will be discussed below.) As noted above, clusters are often connected by filaments or sheets on super-cluster scales. However, these proved hard to detect until the deep redshift surveys of thousands of galaxies began in the 1990s. Cluster interactions were revealed by more subtle tracers. It began with worries about the possibility that cluster mass estimates based on the velocity measurements of member galaxies, which in equilibrium indicate the strength of the gravitational field, were inaccurate because of contamination. Foreground and background galaxies, superimposed on the cluster, would have quite different line-of-sight velocities, with no gravitational connection to the cluster. It turned out that such interlopers are quite common. In 1980 the British and Australian team of John R. Lucey, Robert J. Dickens, and John A. Dawe found that the relatively nearby Centaurus I cluster was "an extreme case of contamination" to quote from their paper title. About two-thirds of the cluster members had velocities within a few hundred km/s around one velocity, while the other one-third had velocities close to a value that was 1700 km/s greater than the mean of the first group. The authors described this circumstance of apparently having two clusters in one, as "substructure." The idea is that two subclusters with velocities centered around two very different mean values, but with the difference between those mean values being less than the escape velocity of the whole, must be merged, or merging. The substructure was also visible as multiple peaks in detailed surface density maps of clusters. Through the decade of the 1980s, Harvard astronomer Margaret Geller, with student Tim Beers, and other collaborators pursued density and velocity observations of cluster galaxies and found many more examples of subclusters. A few other groups carried out parallel work. By the end of that decade, substructure was viewed as a common, if far from fully understood, phenom­ enon in clusters. Better understanding came from a different waveband, the X-rays. X-ray

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satellites have long detected the emission of the hot intra-cluster gas. However, early X-ray satellites had very little spatial resolution. The Chandra satellite, now in operation for more than a decade, was a revolutionary advance for X-ray astronomy generally, and the study of galaxy clusters in particular. It is capable of resolving many details. In fact, it led to the discovery of weather in the intracluster medium. Well, not exactly. What was discovered was a phenomenon that came to be called "cold fronts" in clusters. Cold fronts not only provide evidence for collisions between clusters, they also provide a good deal of detail about the nature of such collisions. Before discussing cold fronts, I would like to discuss a few of the generalities of cluster collisions to provide a little more context. One of the first things we learned about galaxy collisions was that even when galaxies collide directly, there are virtually no star-star collisions. This is true despite the fact that galaxies are relatively dense collections of stars - relatively dense yes, but there are still great distances between the stars in galaxies. The statements also apply quite well to the visible parts of galaxies in clusters, and direct collisions between the bulges and disks of the members of two colliding clusters are unlikely. Galaxies are relatively dense in clusters, but as can be seen in Figures 8.2 and 8.3, there are still considerable distances between the cluster galaxies. (However, the separation-to-size ratio of stars in galaxies is many orders of magnitude greater than that of galaxies in clusters, so though rare, the latter are not nearly as unlikely as the former.) When two galaxy clusters collide their relative velocities will be similar to the velocities of the galaxies within them, i.e., thousands of kilometers per second. This is similar to the situation for galaxy collisions between members of small to modest sized galaxy groups, though there the typical velocities are in the hundreds of kilometers per second. As we have seen above, the cluster velocities are so much higher than the internal velocities of galaxies, that flyby collisions at these velocities have minor effects, and do not generally lead to mergers. Indirect effects, such as induced collisions within groups falling into clusters can occur. What about collisions between the larger dark halos of galaxies? These seem more likely, but two factors work against them. The first is again the high relative velocities. The second is that we believe that halos of cluster galaxies are smaller than those of field galaxies. Processes like tidal stripping and harassment work to reduce them, even if they were born equally large. Of course, by definition the dark halos of the whole clusters do collide. Since the typical collision velocities are comparable to their internal velocities, we expect dynamical friction to be very effective, and the merger of the cluster halos should be rapid. Here, rapid means on a timescale comparable to the crossing time of one cluster through the other. This result is similar to that in major mergers between galaxies, but even stronger. After the first encounter between clusters, there is not as much separation as we often see in galaxy collisions. The hot intra-cluster gases of the two clusters will also collide directly. The relative kinetic energy of gas particles in the two clusters is also comparable to their random, thermal energies. Technically, this means that for the gas, the collision is transonic, or mildly supersonic. Recall that this is also true for

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collisions of hot galaxy halos, while collisions between cool galaxy disk components are usually highly supersonic. Roughly, this means that the two gas halos should merge relatively quickly, like the dark halos. However, it is not quite that simple, as exemplified by cold fronts. The mean temperature of the hot gas halos of clusters depends on their overall mass. More massive clusters are hotter. When two unequal clusters collide (the usual case), the ratio of the relative velocity to the mean sound speed in the halo gas will be larger for the smaller cluster. This means that the shock wave will be stronger around it. As the smaller cluster falls through the larger, this shock wave forms, and to a large extent isolates the cooler gas from the hotter surrounding gas of the larger cluster. What the Chandra observations show is a relatively cool lump of gas and a sharp boundary with the hot gas of the larger cluster, the cold front. As the clusters merge, the gases will be mixed, and settle to a uniform (hotter) temperature. Thus, the presence of a cold front gives evidence for cluster collisions in progress, and in fact, the properties of the cold front are some of the most sensitive indicators of how advanced a cluster-cluster merger is. One of the most beautiful examples of cluster-cluster mergers is the so-called Bullet cluster, shown in Figure 8.7. The brightest "blobs" in the picture are the galaxies. The deep image captures many background (and foreground) galaxies, but the cluster galaxies can be seen as concentrations in the blue halos. The two blue halos in the image are estimates of the distribution of cluster dark matter. These are derived from measurements of the gravitational lensing of the light of the background galaxies. Lensing is an effect of General Relativity, which depends on the mass distribution of the cluster. The two clusters have already collided and moved through each other. Their motion is nearly perpendicular to the line-of-sight. We know this from X-ray observations of the hot gas halos, which are represented in pink in the figure. Note first that the two halos are still partially overlapping. Recall the fact, discussed in previous chapters, that dissipative gas clouds do not generally pass through or past each other like stars or dark matter particles in galaxy collisions, or the galaxies themselves in cluster collisions. Notice that the pink concentration on the right has the definite conical shape of a bow shock. The concentration on the left is more amorphous. It appears that the halo gas of the smaller cluster on the right formed a stronger bow shock and was able to plow through much of the gas of the larger. However, dissipation in the interaction slowed both gaseous halos, and prevented them from separating as much as the cluster galaxies. A great deal of effort went into producing this multi-waveband image from Chandra, HST, and ground-based data, but it illustrates the dynamics wonderfully. In the galaxy world, Nature is especially good at repeating herself on different scales. We have seen a number of ways in which cluster-cluster collisions are qualitatively similar to galaxy collisions. Physicists would remind us that this is not completely surprising given the fact that the force of gravity has no intrinsic scale. If the object masses and other basic properties are scaled up with the length dimension, then we can expect the same physical processes to play out in a

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Figure 8.7 Optical image of the Bullet cluster of galaxies, with hot, X-ray emitting gas in pink, and an estimate of the distribution of dark matter, derived from the gravitational lensing or bending of the light of background galaxies, shown in blue (NASA/CXC/CfA, M. Markevitch, et al.; NASA/STScl/AURA, ESO WFI, Magellan/Univ. Arizona, D. Clowe, et al.).

scaled up time. There is more to the story than that, but it is an important point. It is also true that there are many differences in detail between interactions at the galaxy and cluster scales. Most of the processes discussed in the last few pages, as well as others that we did not consider, either do not occur on both scales, or occur in quite different ways. Many of these differences can be traced to the fact that the galaxies which make up clusters are larger, but much fewer in number than the stars that make up galaxies. In any case, a brief study of interactions at this larger scale has given us more insight into different types of galaxy collisions, and other galaxy evolutionary processes that are generated at this scale. With those words, it is time to end our wide viewing of galaxy world, and prepare to leave that world.

Further Reading

There are many general-interest books on different aspects of cosmology. There are fewer that focus on galaxy formation and evolution, and among those the following include some discussion of galaxy groups and clusters.

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Keel, W.C., The Road to Galaxy Formation, (Chichester: Springer Praxis) 2007. Harrison, E.R., Masks of the Universe: Changing Ideas on the Nature of the Cosmos, (New York: Cambridge University Press) 2003. Silk, J., The Big Bang, Third Edition," (New York: W. H. Freeman) 2001. A recent, but technical source on galaxy clusters is: Mulchaey, J.S., Dressier, A.M., and Oemler, A., Clusters of Galaxies: Probes of Cosmological Structure and Galaxy Evolution, (Cambridge: Cambridge University Press) 2004. A sourcebook on compact galaxy groups is: Hickson, P., Atlas of Compact Groups of Galaxies, (Basil: Gordon and Breach Science Publishers) 1994.

9

Epilogue: the galactobiology of collisions

As we return from the galaxy world to our own, many questions remain. If the recent rate of discovery continues, a number of those questions should be answered in the next couple of decades. Many powerful tools to address them are being developed. A few examples merit at least a mention. For radio astronomy, the recently upgraded VLA and ALMA arrays were briefly described in Chapter 6. Many instances of the value of radio telescopes for studying gas dynamics in galaxies have been given in previous chapters. To reiterate, gas dynamics is key to understanding starbursts and nuclear activity in the galaxy cores, while the structure of large gaseous tidal features provides important constraints on the nature of galaxy collisions. There are also many exciting developments in optical astronomy. Perhaps the most exciting is the "old" Hubble Space Telescope, which was refurbished with new instruments in May 2009. With an unchanged mirror, HST cannot collect light more efficiently than before, but with the advances in electronic detectors it can use that light significantly more effectively. The biggest advance is in the resolution of its newest cameras. This allows observations of star formation regions in nearby interacting galxies to see much sharper detail, and specifically, to resolve much smaller star clusters, or groups of clusters (see Chapter 6). HST's capability for observing in the ultraviolet will be very helpful for determining the ages of star cluster populations. At higher redshifts and greater distances, the increased resolution will allow observers to discern tidal tails and other structures, and get a much better idea of how galaxy collisions at earlier times differ from those of the present day. We already know that galaxy structure was very different at redshifts of greater than 1. This is evident in the clumpy, irregular structures seen in the high redshift galaxies shown in Figure 9.1. These are isolated and not colliding galaxies. Note, that because of the high redshift, the optical images are comparable to ultraviolet images of nearby galaxies, while the infrared images are like optical images of nearby galaxies. Yet the point is still valid; even the infrared images do not look like normal, low redshift galaxies, even granted their low resolution. Besides the morphological differences, observational evidence confirms some other differences as well. High redshift galaxies are frequently gas-rich. This is not true in all cases, for we learned earlier that large parts of elliptical galaxies were formed at very early times. Thus, they can be seen as red, mature-looking galaxies at quite high redshifts. However, it is still true that most of their peers are gas-rich, and when they are involved in major or minor interactions we C. Struck, Galaxy Collisions: Forging New Worlds from Cosmic Crashes, Springer Praxis Books, DOI 10.1007/978-0-387-85371-09, © Springer Science+Business Media, LLC 2011

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Figure 9.1 Eight high redshift galaxies derived from the Hubble Ultra Deep Field in two views each. The optical Advanced Camera for Surveys (ACS) view is on the left, and a near-infrared view from the NICMOS camera is on the right in each case. Note how clumpy and disturbed these galaxies look, though they are not colliding (NASA/STScI/ AURA, and from B.G. Elmegreen, et al., Astrophys. /., Vol. 692).

expect considerable induced star formation and other activity. This is confirmed by recent observations that have revealed the so-called submillimeter galaxies as gas-rich major mergers with tremendous rates of star formation and strong wind outflows. Another difference between high and low redshift mergers, is that larger galaxies are involved in mergers at high redshift. This is one facet of the "downsizing" effect discussed in earlier chapters. In brief, galaxies comparable to the Milky Way are more likely to have experienced consequential mergers at redshifts of 2 or 3 than near the present time. This was the time of the greatest change brought about by

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collisions in such galaxies. Many of them continue to be modified by evolution in groups and as a result of infall into clusters, but generally, we expect these changes to be less dramatic. The coming generation of instruments will give us a much better view of all this, and that view should be spectacular. A downside to these instrument improvements is that they will amplify what was already intense competition among astronomers to obtain HST observations. Many of the most beautiful images produced by HST over the years have been of relatively nearby colliding galaxies, as seen in previous chapters. Astronomers outside of the field may be inclined to think that HST has done all that it can in this area. I hope it is clear from the previous chapters that such an impression would be very wrong indeed. Another exciting prospect in optical (and near infrared) astronomy is the development of extremely large ground-based telescopes. Currently the largest telescopes, like the Keck, Gemini and Very Large Telescope (VLT), are in the 810m class. The next generation is planned to be in the 20-50m range. Since the light gathering power of a telescope scales with the area of its primary mirror, these telescopes would be able to detect extraordinarily faint sources. With adaptive optics to filter out atmospheric turbulence, these telescopes plan to exceed HST's resolution from the ground. Several such projects are in the planning stage around the world, with hopes of construction within this decade. Such huge mirrors cannot be cast as a single piece, but are assembled from many smaller mirror segments, like the current giants. This also facilitates the adaptive optics. One of the primary reasons for building these immense telescopes is to observe galaxies at the highest possible redshift, and thus, given the light travel time, at very short times after their birth. That is, they plan to considerably advance the direct archaeological study of galaxy evolution. I doubt they will have much time to spend studying more nearby colliding galaxies, at least in the early years, but their awesome capabilities could help to answer many questions in that area as well. The next great space telescope, the James Webb Space Telescope (JWST), will observe in the near to mid-infrared wavebands. It will carry out some of the same research as the extremely large ground-based telescopes, e.g., to study the first galaxies, and their early evolution. The mirror of this telescope is designed to have an overall diameter of 6.5m, and a sunshield that NASA describes as the size of a tennis court. Both of these parts are too big to fit on a rocket, so they will be launched in a folded form, and unwrapped via remote control in space. Launch is anticipated in 2014. The light of very distant galaxies will have been emitted in the optical band at the source and redshifted to the infrared along the way. However, JWST will also be a very powerful telescope for seeing through dust and observing infrared sources in the nearby universe. Some of these instruments will be used for decades, and in some cases we won't see any advances over them in that time period. With their development, we do expect the current rapid rate of discovery to continue in this field, and many of the questions and uncertainties in the previous chapters to receive powerful new inputs.

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However, as we leave the world of galaxies, our final questions and concerns are not only about that world, but also about the connections to our own day-to­ day world. At the beginning of the book, I cautioned that the former was very different, and suggested that because of the vastly different length, time, and mass scales, that such connections might be few and tenuous. That is a good first estimate. On the other hand, I also argued that our evolution was intertwined with galaxy evolution. We live in a relatively average galaxy. A large part of galaxy evolution is the evolution of its stellar populations. The Sun and the structure of our Solar System are crucial to the development and survival of our species, and their characteristics are a function of the formation of the Milky Way, and the history of the previous stellar populations. Especially important is the buildup of the elements heavier than hydrogen and helium, which astronomers call "metals." Obviously, carbon, oxygen, nitrogen, silicon, iron, etc. are essential for life on an Earth-like planet, but such elements were not formed in the Big Bang; they were formed in the fusion furnaces at the cores of stars, and strewn across the interstellar medium at the death of those stars. Large amounts of these elements are produced in rare massive stars, and are dispersed when they end their lives in supernova explosions. Significant amounts of some of them are produced in more common stars of lower mass, which push off their outer layers as the misnamed planetary nebulae at the end of their lives. It appears that several generations of stars were needed to enrich the interstellar gas sufficiently to produce the metallicity observed in the Sun. Recent observations of the parent stars of extrasolar planets suggest that most have a comparable metal abundance to the Sun, so a value close to that level may be needed to obtain a high probability that the accretion disk around a young star will produce a planetary system. These conclusions are still rather tentative, but likely. It appears that we are here because of galaxy evolution, and the buildup of heavy elements. Galaxy collisions are an important driver of evolution, via induced star formation. Certainly they can spur the formation of successive generations of stars, and thus increase the average metal abundance in the star-forming disk of a galaxy. Some merger partners could mix metal-poor gas into their metal-rich companion, possibly reducing the average metallicity of the remnant to a value below a critical value for planet formation. However, most major mergers will involve galaxies of comparable metallicity as well as mass. A minor merger companion might bring in low metallicity gas, but the amount will usually be small relative to the gas mass of the primary. Thus, galaxy collisions may be beneficial for planet formation and the development of life in the universe, at least in the long term. Yet, the ferocious impact that a major merger can have on a galaxy makes us wonder if they could be dangerous in the short term (i.e., on timescales of less than a few hundred million years)? Inevitably, we also wonder what it might look like to be in a galaxy involved in a major merger? Let us consider the last question first. In a major merger, we would essentially

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be in or very close to two galaxies at the same time. Thus, our sky would feature something like two Milky Ways. A number of artists have depicted such a scene, but I usually find such depictions exaggerated. If you think about it, it is hard to depict both the grandeur and the subtlety of the Milky Way simultaneously. If two Milky Ways is the first approximation, one can think of a wide range of refinements, based on what we have learned in the previous chapters. Interactions between disk galaxies induce star formation, especially in waves. Imagine that our Solar System was near a spiral wave with young star clusters formed like the classic beads-on-a-string along the wave. These might appear like a string of Pleiades star clusters on the night sky, perhaps each with a number of blue-white O or B type stars shining much brighter than the rest. Alternatively, since interactions can also induce abnormal gas motions, we might find ourselves passing within a large, dense cloud, with virtually no stars visible for millions of years. Fortunately, even in collisions, such clouds only cover a small fraction of the disk(s), so the odds are against that. As the merger goes to completion, it is rather more likely that we would experience a much greater stellar density. Imagine twice as many stars in the sky, or a brighter core or bulge region in the Milky Way. There are many possibilities, which I will leave you to ponder. What about the dangers of galaxy collisions; are there any? The great forces of Nature may take a long time to play their hand, but in the end the results can be catastrophic. However, despite the vast rearrangements of mass involved, it appears that tiny creatures on a little rocky planet, orbiting a fairly ordinary star, do not have much to fear. Early in the book (Chapter 2) we covered one of the most basic points. That is, because of the vast space between stars, collisions between them are extremely unlikely, even in direct collisions between the visible parts of galaxies. Yet this statement might be a little too facile. Another star does not have to actually hit the Sun to cause a great deal of trouble for a planet like ours. It would be enough to perturb our orbit by a few percent to cause some rather extreme seasons and climate disruptions. However, to do that another star would have to pass as close to the Sun as the outer planets, which is still extremely unlikely. However, there may be a more indirect way to disturb the Earth's orbit (and here I use the Earth as an example of a typical habitable planet) - that is, perturb the orbit of an outer giant planet like Jupiter. A rogue star could do that from an even greater distance, roughly comparable to that of the Kuiper Belt objects in our Solar System (or further if the relative velocity is not too high). This is still an improbable event, but much less so than the previous examples. It is possible that a good number of such close encounters occur in the direct collision between two disks. We now believe that planetary systems are common among the stars, though as yet we have no idea of how common are terrestrial, habitable planets, in association with outer giant planets. Yet, it seems fairly likely that in a direct galaxy collision, a few Earth-like planets in a few solar systems are set up for a very rocky orbital future. This would be a secondary result of the disturbance of the outer planet's orbit, which over time results in a disturbance of the terrestrial planet's orbit.

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A massive passing gas cloud could have a similar effect. Such clouds are larger than the typical distance between stars in the galaxy, so they do not fit within the Solar System. Thus, they cannot get close enough to perturb the orbits of any planet directly. Even if the Solar System was engulfed by one as envisioned above, its mass would not generally be concentrated enough to have a large effect. However, for the clouds too there is a subtle means of attack. We believe that our Solar System, and by extension other solar systems, is surrounded by a hugely extended distribution of comets, the Oort comet cloud. The radius of the Oort cloud may extend as far as 10 s times the Earth's orbital radius, a significant fraction of the distance to the nearest star. At such large distances the Oort comets are only loosely bound to the Sun and their orbits are relatively easily disturbed. If the orbits of many thousands of comets are disturbed, many will be lost to the Solar System, but many others will lose orbital angular momentum. In the latter case, their orbits will become more elliptical, with generally much closer approaches to the Sun. A very small fraction of those will have lost all but a tiny fraction of their angular momentum, and their new orbits will take them within the gravitational reach of the giant planets. A fraction of these will be sent into the inner solar system by interactions with the giant planets, and some will collide with the terrestrial planets. Despite the sifting through these many filters, it has been speculated that the end result might be a "comet shower" onto the Earth. That term makes one visualize a spectacular meteor shower. However, with the comets it would be a shower on geological timescales. Nonetheless, such a shower could be the cause of a mass extinction of life. A great deal has been written about the possibility of this kind of event, though not usually in the context of galaxy collisions. The usual context is that of random events in the galaxy. However, the disequilibrium caused by galaxy collisions could well be a more powerful trigger for such a cascade of events. This topic has been approached from many directions. On one hand, researchers have tried to identify astronomical causes for specific mass extinctions, with great success in the case of the K-T event of 65 Myr ago, which evidently resulted in the extinction of the dinosaurs. On the other hand, astronomers have tried to determine better the probability for such events based on astronomical observations. They have also tried to correlate the frequencies or periodicities at both ends. For example, some have considered the possibility that such events are associated with passages of the Sun through the spiral arms of our galaxy, where the star and gas cloud density are increased, as in a galaxy collision. A recent statistical study by my departmental colleague Martin Pohl and Kansas astronomers found that the timing of extinctions is not consistent with our best understanding of the Sun's orbit in the galaxy and the structure of its spiral arms. At least this specific hypothesis can be tested in that manner. The possibility that galaxy collisions play a role in such events in mostly conjecture at present. This is also true of other potential hazards. The next item on the hazards list is the effect of massive stars. Massive stars can be formed in abundance in merging galaxies, and massive stars die in supernovae. The very term "supernova" sounds

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like something one would not want to be very near. It cannot be good to be close to a massive exploding star. Yet it turns out that supernovae are rather vaporous tigers, whose claws and fangs are not so hard. The first good news about them is that most of their energy comes out in the form of neutrinos, light elementary particles that hardly interact with any other particles. They would stream through the Solar System, the Sun and the Earth with no effect. Similarly, the light emission from the explosion is not hazardous, but spectacular, since supernova can be seen across the galaxy, unless obscured by dust. It turns out that ionizing radiation (ultraviolet, X-rays, and gamma-rays), which are produced by supernovae, are also not a great worry. At least they are not for a planet like Earth, with a substantial atmosphere to shield against them. It is possible that the X-ray flux from a very nearby supernova, e.g., within a few parsecs, could temporarily damage the ozone layer, which could cause a great problem for the well-evolved life on a planet like Earth. However, the odds of being so close to a supernova are small, even in a major merger. In the galaxy core, the odds might be considerably increased, but for many reasons that is not a favorable location for planets with life anyway. For stars less massive than the Sun, stellar flares resulting from surface magnetic activity present greater dangers, but which have nothing to do with the galactic environment. A number of researchers, including my former postdoctoral mentors John Scalo and Craig Wheeler have investigated these types of radiation phenomena in detail. The strong, roughly spherical shock wave produced by a supernova could be another concern. However, as they spread out in three dimensions, the shock strength decreases rapidly. Moreover, the Solar System is protected by the heliosphere, a gas bubble produced by the solar wind, extending out about a hundred astronomical units. If supernova shocks have little effect on mature solar systems, they may yet have an effect on forming solar systems. Rare elements, which can be produced in supernovae, have been found in meteorites, suggesting that shocks did impact the early solar nebula and accretion disk. This is plausible if, like most stars, the Sun formed in a star cluster along with more massive stars. Whether shock compressions may have played some role on the development of the nascent Solar System is largely unknown. Major mergers can also feed and energize active galactic nuclei, and one might wonder what effect waking the monster in the middle might have on solar systems. The rough, short answer is no immediate affect on a solar system located at a galactic radius like ours. For example, the central black hole of the Milky Way is likely to have had periods of significantly higher levels of activity over the course of biological evolution than at present, with no evident effect. However, in the central core of the galaxy the large flux of ionizing radiation could be very damaging. The quantitative details of how much damage and how far out it extends would depend on the specific situation. Thus, there are several processes that could make life very difficult near galactic nuclei. These difficulties are generally exacerbated by mergers. The dual notions that radiation and other effects preclude life in the inner galaxy, while

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very low metallicities in the outermost gas disk prevent habitable planets from forming there, inspire the idea of an annular "galactic habitable zone" in the disks. This concept has been studied independently by astronomers Guillermo Gonzalez and Charles Lineweaver. I think some skepticism about how extensive the uninhabitable zones might be is justified. However, galaxy collisions could certainly alter the habitable zone, in any case. This is even more so in the early lives of galaxies when collisions were more frequent, and could involve the mixing of low metallicity gas and the stimulation of much star formation. Galaxies were likely less "habitable" in the early days for a variety of reasons. The other side of the coin to the hazards of galaxy collisions is the benefits. If the hazards are generally unlikely, the benefits are likewise few in number. We have already noted one - a better view. It could be even better than two milky bands and strings of star clusters. There is the possibility that the Sun could get pulled out of the disk. (That is, out of both the original disk and that of the merger remnant.) Then one could end up with an ideal view of the galaxy, close enough to see bright detail, but with an external view of the whole system, unblocked by dust clouds in the disk. Perhaps that seems like a rather superficial "benefit." However, given the profound impact our view of the stars has had on human culture, from the earliest legends to modern works of art, it might not be so superficial. There could be many cultural, philosophical and religious implications for a civilization that develops with such a "picture window" view of its home galaxy. For most galaxies now, and in the far future of the Milky Way, the view may be that of a member of a large cluster of galaxies, the bright lights of galaxy city. Depending on the specific location, a dweller within such a galaxy could get all of the viewing benefits above, plus many more nearby galaxies, that would be bigger and easier to observe with a small telescope. Moreover, within a great cluster there are many stars between the galaxies. The sky would glow very faintly with their light. Astronomers would be able to detect that in their sensitive observations. The glowing sky sounds very romantic, but to the astronomers it would be a nuisance. We can conclude with a mixed benefit/danger. Cosmic rays are fast moving elementary particles or atomic nuclei originating either in the Sun or from the galaxy as a whole. They impact the Earth's atmosphere at high velocity. Some reach the ground, but most collide with particles in the upper atmosphere and induce secondary cascades of particles through the atmosphere. There is accumulating evidence that these particles are accelerated in the magnetic fields swept up and compressed by supernova shock waves. Most carry an electric charge, and are strongly affected by magnetic fields in the galaxy. This interaction turns their trajectories into diffusive, random walks in the interstellar medium. A greatly increased supernova rate, resulting from interaction induced starbursts, could greatly increase the galactic cosmic ray flux, though it might take some time for them to diffuse through the galaxy. Cosmic rays are a form of radiation that can damage the DNA in cell nuclei. Generally, their flux is very low, so individual organisms do not suffer much damage over the course of their lifetime. This is not necessarily true of electronic

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devices. Cosmic rays can cause faults in electronic memories and computer processing units. They may have a long-term effect in evolution as a source of random mutations in DNA. Most such mutations are likely to be harmful, but the rare one might be very helpful, and promulgated by natural selection. Thus, it is possible that a prolonged increase in the cosmic ray flux might affect the course and rate of biological evolution. Recall that the duration of a galaxy interaction is comparable to the time from the rise of the dinosaurs to the present. A lot of evolution can happen in that time, and perhaps more so with external stimulation. It has been suggested that cosmic rays and the particle showers they generate can seed lightning discharges. It has also been suggested that they can seed high altitude cloud formation. If these theories are confirmed, then cosmic rays could affect weather and climate. The corollary is that increases in their flux could have a stronger effect on climate. The nature of that effect is unknown, like many of the other processes noted in this chapter. Recently, an intriguing hypothesis by the Kansas group led by Adrian Melott, promises to stimulate more research on the effects of cosmic rays, which may ultimately clarify some of the issues about their effects on Earth-like ecology. Melott's idea begins with evidence the group has derived for a 62 Myr periodicity in mass extinctions of species on Earth. It continues with a near coincidence between that period and the period of vertical oscillations of the Sun above and below the Milky Way disk. Many others have found possible periodicities in the history of mass extinctions, as well as correlations with other physical quantities. Given the limited statistics we should be skeptical about such correlations. However, Melott and his collaborators go further than some, and make their conjecture more testable, by focusing on a particular physical mechanism. They point out that the northern hemisphere of the Milky Way is moving towards the Virgo galaxy cluster, and that ram pressure against the intergalactic medium could push the hot gas in the Milky Way's halo back near the disk in that direction. The shock front mediating this pressure could compress magnetic fields, which in turn, could accelerate more cosmic ray particles. Melott's idea is that when the Solar System moves closest to this front, Earth experiences an enhanced flux of cosmic rays. It is hard to approximate this flux enhancement, let alone its effects. However, Melott estimates that it is possible, with maximal fluxes, for the effects on ionization in the atmosphere to be considerable. Atmospheric ionization could affect cloud formation, as discussed above, and a variety of other meteorological factors in ways that modify global climate and lead to species extinction. The proposed shock front and directed cosmic ray enhancement are difficult to impossible to observe at present. Only about a dozen extinction events are used to derive the period. Thus, this is very speculative science, and in science most speculations do not pan out. Nevertheless, this one is worth our attention because it highlights a subtle means by which global events in galaxy evolution could have a profound effect on planetary biology. There are others that we will not consider.

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Figure 9 . 2 Portion of the famous Hubble Space Telescope Ultra Deep Field image showing many layers of galaxies extending out to a significant fraction of the size of the universe (NASA, ESA, and the HUDF09 Team).

In conclusion, we have discovered that, despite their fast sizes and long time scales, there are ways that galaxy collisions could change planetary ecologies, and the destinies of beings like ourselves. These processes suggest a great interconnectedness in the universe, but also the limitations of the connections between scales. There is much left to learn, and here at the end we have wandered some ways out on limbs of speculation. So let us leave these speculations about the connections between the worlds, and come back to our own. We can take a last look at the world of galaxies in Figure 9.2. Hopefully, we have covered the basics of the galaxy world well enough in this book for the reader to make some sense of news stories about new research on it, and continue to explore it through such bulletins. I also hope that I have conveyed some of the beauty and mystery of that world, of which we are a very small part. Ultimately, the most important benefit in learning about galaxy collisions is the wonder and inspiration that comes from letting our inquisitive

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minds wander around the universe in a way that, because of the vast scales, our physical bodies cannot. Further reading

Lineweaver, C. H., Fenner, Y., and Gibson, B. K., "The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way," Science, Vol. 303, p. 59, 2004.

Glossary

AGN - active galactic nucleus. A nearly point-like (in most observations) source of exceptionally strong multi-wavelength emissions found in the nuclei of some galaxies. The consensus model for the source of this emission is accretion from a disk onto a supermassive black hole, e.g., of mass 10 6 -10 10 times the mass of the Sun. arcsecond - one sixtieth (1/60) of an arcminute, which is one sixtieth of a degree of angle on the sky. The Sun and the Moon subtend about half a degree on the sky. An arcsecond is about 1800 times smaller than this angle. CFSR - cosmic star formation rate, defined as the average star formation rate in galaxies at a specific time in the evolution of the universe. Doppler shift, redshift - all spectral lines have wavelengths characteristic of the differences between specific energy levels in a given type of ion or atom. If an emission source is moving away from the observer, then those lines are shifted to the red and a longer wavelength. The Doppler formula relates the spectral redshift to the source recession velocity. Because of the cosmological expansion most galaxies are moving away from us, and their spectral lines are redshifted. dynamical friction - originally understood as a collective effect that results in the transfer of orbital energy from satellite galaxies to the stars or dark matter particles of a larger primary galaxy. Subsequently generalized to include collective effects between comparable galaxies, which might include a temporary orbital energy increase, like an "anti-friction." feedback effects - energy and momentum inputs to the interstellar gas deriving from young stars or AGN. In the former case, the inputs derive from young star winds or jets, supernova explosions, or from ultraviolet emissions from the surfaces of hot stars. In the latter case, the input is usually from strong jets originating in the AGN, or high energy (UV, X-ray or gamma ray) photoemissions. Gyr - (gigayear) is a billion (109) years. Ha or hydrogen alpha - is a red emission line of wavelength 656 nm, produced by transitions from the first excited state of atomic hydrogen. Hubble type - refers to type in the simple tuning fork galaxy classification scheme derived by Hubble. In this scheme, elliptical galaxies are classified by their apparent elliptical shape on the sky, which can be quantified using the ratio C. Struck, Galaxy Collisions: Forging New Worlds from Cosmic Crashes, Springer Praxis Books, DOI 10.1007/978-0-387-85371-0, © Springer Science+Business Media, LLC 2011

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of the longest to shortest length. The spiral galaxies are arranged into two families, depending on whether or not they have a bar component. Unbarred spirals of type Sa have large bulges and relatively tightly wound spiral arms. Sc galaxies have small bulges and open arms, while Sb types are intermediate. Hubble Law - because of the expansion of the universe, spectral lines from distant galaxies are shifted to the red. The Doppler formula relates this redshift to the recession velocity of the galaxy. Hubble discovered that the cosmological recession speed is approximately proportional to the distance of the galaxy. V = Hd, where d is the galaxy distance, V is its velocity, and H is now called the Hubble constant. Relativistic cosmology theories can explain this law, and possible deviations from it. km/s - kilometers per second. One kilometer per second is 3600 kilometers per hour. kpc - is a thousand parsecs (see pc). Mpc - is a million parsecs (see pc). Myr - is a million years. n m - nanometer, one thousand-millionth (10 9 ) of a meter. pc - parsec, about 3.26 light years or about 31 trillion (3.1 x 1013) kilometers. proper motion - motion of an astronomical object in the plane of the sky, as opposed to motion toward or away from the observer. radial velocity - motion of an astronomical object velocity along the line of sight toward or away from the observer. redshift, Doppler shift - all spectral lines have wavelengths characteristic of the differences between specific energy levels in a given type of ion or atom. If an emission source is moving away from the observer, then those lines are shifted to the red and a longer wavelength. The Doppler formula relates the spectral redshift to the source recession velocity. Because of the cosmological expansion most galaxies are moving away from us, and their spectral lines are redshifted. rotation curve - a graph of the azimuthal velocity of stars or gas clouds in a galaxy disk as a function of distance from the center. Disk orbits are usually assumed to be approximately circular, so the azimuthal velocity is assumed to approximately equal to the circular velocity, and thus, is representative of the gravitating mass interior to that point. SDSS - Sloan Digital Sky Survey, an ongoing digital (CCD detector) survey of the a large fraction of the sky, made with a dedicated 2.5 m telescope at Apache Point Observatory in New Mexico, USA. For details see http://www.sdss.org/. solar mass - mass is usually measured in units of the Sun's mass, which is 2.0 x 10 30 kilograms.

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starburst - though not rigorously defined, the term generally refers to star formation in a galaxy at a rate of at least several times greater than usual for the given galaxy type. The term also implies that the high rate of star formation is a temporary or transient phenomenon. However, this latter property is not independent, since high rates of star formation are self-limiting either as a result of gas consumption, or by generating gas expulsion via a galactic wind. TDG - tidal dwarf galaxy. By definition TDGs are dwarf galaxies formed in the tidal debris, e.g., bridges or tails, of galaxy collisions. Perhaps the only galaxies known to be forming ab initio, at the present time. ULIRG - ultra-luminous infrared galaxy. The most luminous galaxies in the universe, generally defined as having 10 12 times the luminosity of the Sun or more, emitted in all wavebands, but especially in the infrared. The radiation source is generally a super-starburst, and possibly also an AGN.

Resources

Websites

Wikipedia: the ubiquitous online encyclopedia has general and historical articles, as well as articles on specific galaxy systems. The NASA Extragalactic Database: largely a professional online resource, but with a wonderful image collection, and sometimes interesting comments on the objects. NED, as its known, also contains "Level 5," a compendium of review or summary articles on all things extragalactic. The NASA/Smithsonian Astrophysical Data System: ADS is another professional resource, in this case providing links to the professional literature, but also very useful for research on recent historical figures. Books

The present author has published a couple of pedagogical, if somewhat technical, review articles on the subject. The original sources of these are: Struck, C , Physics Reports, Vol. 321, p. 1, 1999. Struck, C , "Galaxy Collisions - Dawn of A New Era," in Astrophysics Update 2: Topical and Timely Reviews on Astrophysics, ed. John W. Mason, (Springer-Praxis, Chichester), pp 115-158, 2006. These articles are also available online at the N.E.D. Level 5 website. A source of three comparable review articles (by R. C. Kennicutt, Jr., F. Schweizer, and J. E. Barnes) is the book: Galaxies: Interactions and Induced Star Formation: Saas-Fee Advanced Course 26, eds. D. Friedli, L. Martinet, and D. Pfenniger (Springer, New York) 1998. There are, of course, a number of textbooks on extragalactic astronomy, with some discussion of the physical processes involved in galaxy collisions. Most of these tend to be at a rather advanced level, though often with descriptive introductory discussions of various topics. The most technical and authoritative of these is the magisterial sourcebook of Binney and Tremaine: Binney, J., and Tremaine, S., Galactic Dynamics, 2nd Ed. (Princeton University Press, Princeton) 2008.

Index

Abell galaxy clusters, 239, 251-2, 255 AGN (active galactic nuclei), 105, 115-122, 133, 152, 267 Alice in Wonderland, 2 Allen, David, 140 ALMA (Atacama Large Millimeter Array), 161, 261 alternate gravity theories, 17 Ambartsumian's knot, 83-4 Andromeda galaxy (M31), 7-8, 171, 201-3, 226-231 extended disk, 228 Giant Southern Stream, 228 globular clusters, 228 rings, 230 satellite system, 217-230 Antennae galaxies (NGC 4038/4039), 79-81, 104, 136-7, 167, 173, 194 star clusters, 174-6, 178-9, 184 Appleton, Phillip, 40, 51, 56, 244, 247 Arad, Itai, 102 Arcturus, 219-220 Arecibo Observatory, 207 Arp, Halton, 25-8, 29, 94, 156-7 Astrophysical Data System (ADS), NASA, 116, 205, 222 Atlas of Peculiar Galaxies (Arp), 26-27, 29, 86, 156-7, 197 Arp 24, 86-7 Arp 25, 88 Arp 26 (M101), 88-91 Arp 72, 177 Arp 82, 163, 177 Arp 84, 163 Arp 87, 182 Arp 105, 82-4 Arp 148 (Mayall's object), 183-4 Arp 220, 104 Arp 226, see NGC 7252 Arp 240, 163

Arp Arp Arp Arp Arp Arp

242, 163 261, 184-5 269, 177 284, 46 179-181 285, 84 305, 84

Baade, Walter, 8, 11, 252 Bagnuolo, William, 109 Barnes, Joshua, 81, 111-112, 123-4, 137, 170, 194 bars, see galaxies, bars bars-within-bars, 108 Barton, Elizabeth, 168 Bastian, Nate, 178 Bay of Fundy, 57 beads-on-a-string, 50 Beers, Timothy, 255 Bekenstein, Jakob, 119 Bekki, Kenji, 214 Berentzen, Ingo, 75 Bertram, D., 239-240 Besla, Gurtina, 211-214 binary stars, 59-62 Binney, James, 231 black hole, supermassive nuclear, 115, 119121 bulge correlations, 121 Milky Way nucleus, 115 Block, David L., 230 Bok,J. Bart, 231 Bournaud, Frederic, 81, 85, 176 bridge, tidal, formation of, 66-8, 142 Bullet cluster (of galaxies) 257-8 Burbidge, Geoffrey and Margaret, 15 Burstein, David, 19 Bushouse, Howard, 165-7 Butcher, Harvey, 254 Butcher-Oemler effect, 254 Byrd, Gene, 91, 170

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Galaxy Collisions

Canis Major dwarf galaxy, 222-3 cannibalism, galactic, 31, 97 Carter, David, 140 Cartwheel galaxy, 32-4, 39, 51-3, 163 caustic edges, 36, 68, 72 CCD detector, 164-6 Centaurus I galaxy cluster, 255 centrifugal force, 21, 34, 40, 107, 193 Cepheid variable stars, 8 period-luminosity relation, 8 Chandar, Rupali, 176 Chandra X-ray Observatory, 52, 256-8 Chandrasekhar, Subrahmanyan, 96-8, 119-120, 145-9 Cleary, M. N., 205 cold accretion, 153 cold fronts (in galaxy clusters), 256-8 collective effects, 145-6 color, (broadband) astronomical, 156-7 Coma galaxy cluster, 239-240 Combes, Francoise, 86-7, 91 Comerford, Julia M., 120 compact galaxy groups, 32, 239-240, 247-250 compression, wave, 36-37 Coriolis force, 60 cosmic rays, 268-9 cosmic star formation rate, 196-7, 238 cosmological structure formation models, 129-135, 152 counter-rotating disks, 144, 152 Cox, Thomas J., 230-1 dark energy, 18, 235 dark matter, cold, see Lambda cold dark matter cosmology elementary particles, 17 halos, general, 3, 15-17, 25, 64 density profiles, 22 dynamically hot, 39 Davies, Roger, L., 19 Dawe, John A. 255 De Vaucouleurs, G., 99 depletion, gas, 110, 127 Diafero, Antonaldo, 250-1 Dickens, Robert J., 255 Dickey, John, 85 Doppler shift, 9,15, 205

Dottori, Horacio, 79 downsizing, 135, 197-8, 238, 262 Draco dwarf galaxy, 217-8 dry mergers, 133-5, 198 Dubinski, John, 78, 230 Due, Pierre-Alain, 81, 167, 176 Dupraz, Christophe, 142, 146 dust grains, infrared emission, 105 dynamical friction, 95-7, 100-2, 119-120, 139, 145-9, 226-8, 247 dynamical heating, 150 Eagle nebula, 182-3 Earth-Moon system, 35, 57-9 effective potential, 61-2 Einstein, Albert, 9, 17, 93, 235 elliptical galaxies, classification, 11-14 Fundamental Plane, 20, 216 merger formation, 93-9, 102, 130-7, 150, 236-8 properties, global, 19 elliptical orbits, 35, 86, 211 Elmegreen, Bruce, 71, 81, 194 Elmegreen, Debra, 71, 91 epicycles, 49, 67, 69, 147 exploding galaxies, 29 external infall (matter), 74 Faber, Sandra, 19-21 Faber-Jackson relation, 19-21 Fall, Michael, 176 feedback effects (winds, supernovae, AGN), 114-115, 123-130, 150, 179, 188-194 Fisher, J. Richard, 20-1 Fornax dwarf galaxy, 219 fossil galaxy groups, 239-242, 247 Frenk, Carlos S., 251 galactic habitable zone, 268 galaxies, bars, 13-14, 55, 186 formation, induced, 72-5, 108, 113, 127, 146, 152, 169 bulge, 12-14 density profile, 22 dynamically hot, 39 characteristic scales, 1

Index disks, see spiral galaxies distances, 8 formation, 233-259 halos, see dark matter naked-eye, 3 galaxy clusters, 238-241, 251-9, 268 collisions, 255-7 GALEX ultraviolet satellite observatory, 177, 180, 185, 188 Galilei, Galileo, 3 Gallagher, John S. Jay), 165 Gardiner, Lance, 211, 214 Geller, Margaret, 238, 255 Gemini Observatory, 121, 263 General Relativity theory, 17, 235 Gieles, Mark, 178 Gilmore, Gerard, 218 globular star clusters, 171, 174-5, 192 distance, 8 Milky Way, 219 Gomez, Juan Carlos, 244 Gonzalez, Guillermo, 268 Goodall, Jane, 1 gravitational runnel, 59-62 gravitational instability, local, 51, 69, 191-4 global, 113, 186, 189-191 threshold density, 125, 190-2, 195 gravitational lensing, 257-8 gravitational shock, 182 gravitational wake, 96-98, 145-6 gravitational waves (General Relativity), 120 Great Pancake, 223-6 Gunn, James, 164 Hammer, F., 198 Hancock, Mark, 180 harassment, galaxy, 253-4 Harrison, E. R., 259 Hawking, Stephen, 119 Helin, Eleanor, 33 Henbest, Nigel, 231 Hercules galaxy cluster, 239-241 Hernquist, Lars, 75, 78, 81, 111-112, 123-4, 127, 150-1 Herschel, Caroline, 5 Herschel, William, 5, 217 distance determination, 5

281

Milky Way model, 5 M51 observations, 23, 62-3 Hertzsprung, Ejnar, 8, 18 Hibbard, John E., 80, 170 Hickox, Ryan, C , 122-3 Hickson galaxy groups, see compact galaxy groups Hickson, Paul, 247, 259 hierarchical build-up (galaxy formation), 121, 130-5, 224, 233-7 Higdon, James, 51 HII region (ionized hydrogen), 158-9 Hoag's (Art) objects, 47-8 Hodge, Paul W., 231 Holmberg, Erik, 24-5, 64, 169 Hopkins, Philip F., 152 Howard, Sethanne, 170 Hubble Atlas of Galaxies (A. Sandage), 28-9, 204 Hubble, Edwin, 11, 32 expansion law, 9-10, 215 galaxy distances, 8, 64 Realm of the Nebulae, 2, 11, 204 tuning fork classification, 11-15, 18, 23, 32, 156-7 Hubble Space Telescope, 31-3, 41, 53, 71-2, 79-80, 110, 118, 165-7, 176-9, 245-6, 261-3, 270 discovery of super star clusters, 136, 173-176 galaxy distances, 9 Huchra, John, 157, 238 Hudson Bay, 57 Hunter, C , 204, 211, 214 Hwang, Jeong-Sun, 247 hydrogen emission, Balmer lines (including hydrogen alpha), 158-9, 165 21 cm radio line, 107, 162 Ibata, Rodrigo A., 218, 228 IC 342 galaxy group, 202 IC 2163/NGC 2207, see NGC 2207/IC 2163 impulse, velocity, 68 infall, cosmological, 189, 195-6 infant mortality, see star clusters, destruction Inoue, Shigeki, 147-9 interference pattern, wave, 49

282

Galaxy Collisions

International Astronomical Union, 175 interstellar gas clouds, atomic hydrogen, 36 molecular, 36, 108, 124, 163, 171, 179, 187 thermal phases, 36, 124 Io, 57 IRAS (Infrared Astronomical Satellite), 103-5, 157, 162 Irwin, Michael J., 218 island universe theory, 2, 28, 226, 252, 255 ISO satellite, 174 Jackson, Robert E., 19-21 James Webb Space Telescope (JWST), 263 Jeans, James, 64, 186 Jog, Chanda, 86-7, 91, 185 Jogee, Sharda, 198 Johanson, Peter H., 102 Johnston, Kathryn V., 219 Kallivayalil, Nitya, 207, 211 Kalnajs, Agris, 120, 145-6 Kandrup, Henry, 101 Kant, Immanuel, 4 Kapteyn, Jacobus, 6 Karachentsev, Igor D., 245 Karmen, Theodore von, 34 Kaufman, Michele, 81 Keck Observatory, 121, 263 Keel, William, 105, 158, 259 Kennicutt, Robert C , Jr., 158, 188-192, 199 Kepler, Johannes, 4 theology, 4 Third law, 15 Knierman, Karen, 176, 192 Koribalski, Barbel, 85 Landau damping, 102 Large Magellanic Cloud, 171-2, 179, 201-3, 210, 214 Larson, Richard, 131, 155-7, 165, 168, 199 LCDM (lambda cold dark matter) cosmology, 130, 133, 224 Leavitt, Henrietta, 8 Libeskind, Noam I., 224 Lindblad, Bertil, 24-5, 28 Lindblad resonances 69, 107 Lineweaver, Charles, 268, 271

Liu, X., 120 Local Group (of galaxies), 171, 178, 185, 201-231, 239, 242-3 Loeb, Abraham, 230-1 lop-sided galaxies, 86-91 low-surface-brightness galaxies, 189, 199 Luban, Pnina, 88 Lucey, John, R. 255 Lutz, Dieter, 79 Lynden-Bell, Donald, 100-2, 137, 146, 223 Lynds, Roger, 34-6, 40-2, 56 Madore, Barry F., 27-9, 56 Magellanic Clouds, 201-215, 239 distance, 8 Leading Arm 209-210, 213 proper motion, 205-7, 230 star clusters in, 174 visible appearance, 3, 201 Magellanic Stream, 205-213, 220, 252 Majewski, Steven R. 219 Malin, David, 98, 140-1 Mars, 57 Mathews, William G., 114 Mathewson, Don H., 204 Melott, Adrian, 269 merger trees, 131-5, 233-5 Merrifeld, Michael, 231 Messier, Charles, 5, 23, 62-3, 107, 217 microwave background radiation, 234 Mihos, J. Christopher, 75, 78, 124, 127, 150-1 Milky Way, 201-231 dark halo, 210-211, 224 dwarf companions, 79, 85, 142, 217-227 early models of, 5-6, 62 halo streams, 215-227 rotation curve, 210-211 star-forming regions, 179 visible appearance, 2-4 Millenium Simulation, 133-5, 236-7 minor mergers, 122, 135, 139-153, 197-8, 228, 264 Mirabel, I. Felix, 79, 81, 137, 167, 204 Moon, see Earth-Moon System Moore, Ben, 253 Mori, Masao, 236-7 Moulton, F. R., 6 Mount Wilson Observatory, 32, 217

Index Mount Palomar 200-inch telescope, 26, 32 Mulchaey, John S., 259 multiple galaxy collisions, 25-6, 32, 236 Murray, J. D., 205 M31, see Andromeda galaxy M33 (Pinwheel galaxy in Triangulum), 8, 23, 201-5, 228-230 M51 (Whirlpool galaxy), 6, 23-4, 62-4, 67, 70-2, 163, 178-9 M64 (Black-Eye galaxy), 144 M81, galaxy group, 107-8, 202, 242-251 spiral galaxy, 107 M82 (Cigar galaxy), 105-110, 114, 242-3 M83 galaxy group, 202 M101, see Arp 26 Navarro, Julio, 251 nebulous stars (nebulae), 4 neutrino, 17 Newberg, Heidi Jo, 220 Newton, Isaac first law of motion, 59-60 law of gravitation, 3, 15, 17, 25 theology, 4 NFW halo profile, 251 NGC 205, 201-3 NGC 628, 159 NGC 1097, 108-9 NGC 2207/IC 2163, 71-2, 176, 185 NGC 2403, 245 NGC 2976 (in M81 group), 243-4 NGC 3077 (in M81 group), 242 NGC 3921 (merger remnant), 175 NGC 3923 (shell galaxy) 141 NGC 4038, see Antennae galaxies NGC 4650a, 54 NGC 5128 (Centaurus A), 23-4, 116-117 galaxy group, 202 NGC 5291, 85 NGC 5474, 90-1 NGC 5907 (loops), 224-5 NGC 6221/6215, 85 NGC 7714, see Arp 284 NGC 7252 (Atoms for Peace galaxy), 78, 98-9, 104, 175 Noguchi, Masafumi, 73-5, 211, 214 nuclear activity (galaxies), 75, 155

283

Occam's razor, 29 ocean tides, 57 ocular galaxy 71-2, 113 Oemler, Augustus, 254 Oort comet cloud, 266 orbit crossing zone, 36-7, 50, 68 orbit inclination, 42 orbit mixing, 49 Ostriker, J., 96, 98 Palomar (Observatory), 166, 217, 243 Sky Survey, 25, 219 pancake theory (galaxy formation), 237-8 Parker, Barry, 137 Parsons, William, Third Earl of Rosse, 6 Leviathan telescope, 6, 63 M51 observations, 6-7, 63-4 Peterson, Bradley W., 179-181 phase mixing, 102 Pohl, Martin, 266 polar ring galaxies, 54-5, 116 Ponman, Trevor D., 239-240 post-starburst, 118, 179 Press, William, 130-132 proximity effect (separation-star formation relation), 168-170, 175 quasars, 115 merger formation, 115-123 Quinn, Peter, 140 ram pressure stripping, 209, 213, 252, 269 random walk, 100 recombination, cosmological, 234 redshift, 9-11 Renaud, Florent, 194 resonance rings, 55 resonant orbits, 145-8 retrograde interactions, 86-91, 108, 168-9 Rich, R. Michael, 80 Rines, Kenneth, 250-1 ring galaxies, 31-56, 71-2, 107-8, 176, 182-4, 254 Roche lobe overflow, 59, 142-3, 152 Rosse, Third Earl of, see Parsons, William rotation curves, see, Spiral galaxy Rubin, Vera, 15 Russell, Henry Norris, 18

284

Galaxy Collisions

Sacred Mushroom galaxy, 38 Sagittarius dwarf galaxy, 142, 217-223 globular clusters (M54, Pal 12), 219 Sandage, Alan, 29 Hubble Atlas of Galaxies, 29, 156-7 Sanders, David B., 115-118, 121-3 Sargent, Wallace, 109 Saturn's rings, 69 Saviane, Ivo, 80 Scalo, John, 267 Schechter, Paul, 130-2 Scheepmaker, Remco, 178 Schmidt (Schmidt-Kennicutt) Law, 187-194 Schmidt, Maartin, 187 Schmidt telescope, 25, 33 Schombert, James, 166-7 Schweizer, Francois, 79, 91, 98-99, 137, 173-4 Sculptor galaxy group, 202 Searle, Leonard, 109-110 secular changes, 3 Seguin, P., 142, 146 self-regulation, in galaxy disks, 188-191 Seyfert galaxies, 115 Shapley, Harlow, 11, 23-4, 64, 217 shell galaxies, 140-2, 146, 228 Shoemaker, Eugene, 33 Silk, Joseph, 259 sinking satellites, 145, 151 Slipher, Vesto, 7, 9 Sloan Digital Sky Survey (SDSS), 121, 128, 175, 192, 215-216, 219-222, 250 Small Magellanic Cloud, 178-9, 201, 210 Smith, Beverly J., 46, 169-170, 191, 245 Solar System, 57, 264-9 Solomon, Phillip, 185 Solvay conference, 93 spiral galaxy, classification, 12-14 disks density waves, 40 exponential (density) profile, 22 rotation, 16, 18, 20 thickening, 150-1 Tully-Fisher relation, 20 spiral nebulae, 6, 8, 16, 18 spiral waves, induced, 65-91, 108, 113, 127, 152, 169, 186 one-armed, 86-7

Spitzer, Lyman, 140, 252 Spitzer Space Telescope, 14-15, 31, 46, 64, 162, 169, 230, 245-7 SINGS project, 190-2 standard candles, 8-9 star clusters, formation, 170-8, 184 destruction, 178-182 R136a, 171-3 super, 173-6, 186, 192 Westerlund 1, 172-3 starbursts, 103-110, 114-116, 127-9, 175, 179, 189-191, 198 nearby, dwarf, 173 stars, evolution of, 18, 181 main sequence, 18 starvation, galaxy, 253 Stephan's Quintet, 245-251 Stuart, B. V., 169-170 submillimeter galaxies, 262 Sundelius, Bjorn, 71 Sundin, Maria, 71 super-Antennae galaxy, 104 superclusters, of galaxies, 234, 255 supersonic collisions, 50 swing amplification, 70 synchronous rotation, 59 Taffy galaxies, 44-6, 143, 184 Tarantula Nebula, 171-2 Terlevich, Roberto, 19 thermal radiation spectrum, 18 Thomasson, Magnus, 81, 86 tidal bridge, color, 162-3 formation, 66, 68 star formation in, 166, 176, 179, 191-2, 198, 243 tidal dwarf galaxies (TDGs), 75, 79-85, 167, 176, 192, 226, 243, 246 tidal forces, 57-62, 75, 113, 193 tidal shocking, 102, 140-3 tidal tails, color, 162-3 dispersal, 96 formation, 67-8, 75-85 hydrogen alpha emission, 165 merger remnants, 98

Index star formation, 166, 176, 191-2, 198 Tinsley, Beatrice, 93, 109, 155-7, 165, 168, 199 Tolkien, J. R. R., 71 Toomre, Alar, 64-5, 70, 79, 91, 111-113, 116, 137, 214 disk stability criterion, 188, 193-195 Magellanic Clouds-Milky Way collision, 204, 211, 214 mergers-to-ellipticals, 27, 94-9, 102, 130-2, 173 ring galaxy theory, 34-6, 40-3, 56 Toomre, Juri, 27, 64-5, 79, 91, 111-113, 116 Toomre merger sequence, 95, 173 Tremaine, Scott, 146-7 Tully, R. Brent, 20-1 tuning fork (galaxy classification), see Hubble, Edwin Turner, Kenneth C , 204 Two Micron All Sky Survey (2MASS), 219-223 ULIRG (ultraluminous infrared galaxy), 103-5, 110, 114-115, 123, 133, 155, 165, 197 Umemura, Masayuki, 236-7 Valluri, Monica, 102 Veilleux, Sylvain, 137 velocity-star formation relation, 168-9 Villumsen, Jens, 97 violent relaxation, 100-2, 223, 242 Virgo galaxy cluster, 28, 239, 269

285

VLA (Very Large Array radio telescope), 160-1, 261 VLT (Very Large Telescope) Observatory, 121, 263 Von Neumann, John, 96-8 Vorontsov-Velyaminov, Boris A., 25-7 Walker, Ian R., 150-1 Wallin,John, 166-7, 169-170 Wannier, Peter, 204 Weedman, Daniel, 110 Weinberg, Martin, 146-7 Westphal, James, 164 Wheeler, J. Craig, 267 Whirlpool galaxy, Messier 51 see M51 white holes, 29 White, Simon D. M., 97, 251 Whitmore, Bradley C , 137, 174-6 wind, galactic, 105-7, 114-115, 128, 150, 194, 198 Wright, Thomas, 4-5 An Original Theory, 4 Wrixson, Girard, 204 Yale Galaxies Conference, 93-4, 100, 238 young star clusters (YSCs), 84, 106, 110, 126, 129, 184, 243 merger-induced formation, 167, 173 wave-induced formation, 49, 70, 77 Zang, Thomas, 70 Zeldovich, Yakov, 237-9 Zwicky, Fritz, 25-6, 29, 32-4, 40