Galaxy Collisions

A review article for non-specialists, Physics Reports, 1999 (321, 1).
Note this version is identical to that at A nice, screen-readable version can be found at, and there is the publisher's version (including convenient download versions) at

Curtis Struck

Dept of Physics and Astronomy, Iowa State University, Ames, IA 50010 USA



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, Ha 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




Figures and Captions