A review article for non-specialists, Physics Reports, 321, 1, 1999

Abstract. Theories of how galaxies, the fundamental constituents of large-scale structure, form and evolve have undergone a dramatic paradigm shift in the last few decades. Earlier views were of rapid, early collapse and formation of basic structures, followed by slow evolution of the stellar populations and steady buildup of the chemical elements. Current theories emphasize hierarchical buildup via recurrent collisions and mergers, separated by long periods of relaxation and secular restructuring. Thus, collisions between galaxies are now seen as a primary process in their evolution.

This article begins with a brief history; we then tour parts of the vast array of collisional forms that have been discovered to date. Many examples are provided to illustrate how detailed numerical models and multiwaveband observations have allowed the general chronological sequence of collisional morphologies to be deciphered, and how these forms are produced by the processes of tidal kinematics, hypersonic gas dynamics, collective dynamical friction and violent relaxation.

Galaxy collisions may trigger the formation of a large fraction of all the stars ever formed, and play a key role in fueling active galactic nuclei. Current understanding of the processes involved is reviewed. The last decade has seen exciting new discoveries about how collisions are orchestrated by their environment, how collisional processes depend on environment, and how these environments depend on redshift or cosmological time.

Table of Contents

1. INTRODUCTION TO COLLIDING GALAXIES

1.1 Overview

1.2 Orders of Magnitude

1.3 Background and Early History

1.4 The Importance of Collisions

1.5 Nature's Galaxy Experiments

2. SOME PHENOMENOLOGY: WHAT'S OUT THERE?

2.1 Morphological Classification of Collisional Forms

2.2 Physical Classification?

2.3 The Naming of Things

3. TRANSIENT EVENTS I: SOME WAVE MORPHOLOGIES AND THEIR CAUSES (Yxx).

3.1 Ring galaxies (YDe0)

3.2 Symmetric Caustic Waves

3.3 Ring relatives: Bananas, Swallows and Others

3.4 From Rings to Spirals

3.5 Tidal Spirals and Oculars (YDx+)

3.6 Fan Galaxies and One Arms (YDx-)

3.7 Gas vs. Stars in Waves

4. TRANSIENT EVENTS II: DEATH AND TRANSFIGURATION.

4.1 Transient Mass Transfer and Bridges

4.1.1 Splashes: Bridges and Infall in Direct Collisions (YDd0)

4.1.2 Models and Splash Physics

4.1.3 Slings: Tidally Torqued Bridges (YDx+)

4.1.4 Observations of Tidal Bridges and Star Formation

4.2 Complete Collisional Disruption

4.3 Transient Summary

5. COMING BACK (Ixx, Oxx)

5.1 Dynamical Friction - Bringing it Back - Background and history - Recent Developments

5.2 Simulational Examples of Dynamical Friction

5.2.1 Sinking satellites

5.2.2 Bobbing satellites

5.3 Halo Braking

5.4 Tidal Stretching: Tails and "Antennae" (IXd+)

5.5 Shells and Ripples

5.6 Induced Bars

5.6.1 Collisional Bar Formation

5.6.2 Effects of Induced Bars

5.6.3 Longevity, Frequency and Other Matters

5.7 Intermediate Summary

6. MERGERS - ALL THE WAY BACK (Oxx)

6.1 Overview and Historical Highlights

6.2 Major Merger Dynamics

6.2.1 Violent Relaxation

6.2.2 Gas Funneling

6.3 Minor mergers: disk heating and aging

6.4 New disks

6.4.1 Ellipticals with Disks

6.4.2 Counter-rotating Disks in S0 and Sa Galaxies

6.4.3 Polar Ring Galaxies

6.5 Multiple Mergers

7. INDUCED STAR FORMATION

7.1 Color, H&#alpha; and Other Indicators of Global Enhancements

7.2 Spectral Line Diagnostics

7.2.1 SF Histories

7.2.2 IMF Variations and the Example of M82

7.3 SF Region Morphologies

7.4 Mechanisms and Modes

7.4.1 Star Formation Enhancements

7.4.2 Star Formation Suppression

8. ACTIVE GALACTIC NUCLEI IN COLLISIONAL GALAXIES

8.1 Phenomenology

8.2 Fueling Mechanisms

9. ENVIRONMENTS AND REDSHIFT DEPENDENCE

9.1 Groups and Compact Groups

9.2 Dense Clusters

9.2.1 cD Galaxies

9.2.2 Collisions and Harassment in Clusters

9.2.3 The Cluster Environment: Stripping and Cluster Tidal Effects

9.2.4 Cluster-Cluster Collisions

9.3 High-Redshift Collisions

10. CONCLUSIONS

ACKNOWLEDGEMENTS

REFERENCES