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Article Contents

ABSTRACT

1.INTRODUCTION

1.1.Why Study Rings?

2.RINGS AS COLLISIONAL OBJECTS

2.1.The Collision Idea

2.2.Morphology of Ring Galaxies and R ing Galaxy Samples

2.3.Some Early Ideas about Ring Forma tion

3.OBSERVATIONS OF RING GALAXIES

3.1.The Photometry and Colors of Ring Galaxies

3.2.Observation of the Diffuse and Mo lecular Gas Content of Ring Galaxies

3.3.Observations of Ring Kinematics

3.3.1. The Cartwheel Galaxy and its Companions

3.3.2 Model Velocity Fields

3.4.Star Formation Properties of Ring Galaxies

3.5.Radio Emission and AGN Activity in Ring Galaxies

3.6.Rings in Bulge-dominated Systems and Hoag-like Galaxies

4.SIMPLE KINEMATIC MODELS OF RINGS AND RING-LIKE GALAXIES

4.1.Caustic Waves

4.1.1. Singularity Theory in Galaxy Formation and Ring Galaxies

4.1.2. Symmetric Ring Caustics in the Kinematic Orbit Approximation

4.2.Varieties of Symmetric Rings

4.3.Multiple Encounters?

4.4.Asymmetric Encounters

5.SELF-CONSISTENT STELLAR DYNAMICAL MODELS OF RING GALAXIES

6.THE GAS DYNAMICS OF RING GALAXIES

6.1.Lessons from Simple Models

6.2.Numerical Simulations of Ring Gal axy Hydrodynamics

6.2.1.The First Simulations: 1970s

6.2.2.The Next Generation: 1980s

6.2.3.The Current Generation of Models: 1990s

6.3.Models of the Cartwheel

6.4.What Are the Spokes?

6.5.Explorations of Heating and Cooling Effects

7.FUTURE DIRECTIONS

7.1.Observational Directions

7.2.The Next Steps in Modeling

REFERENCES

APPENDIX 1: Multiple Encounter Kinematics

APPENDIX 2: Simple Models for Asymmetric Stellar Ring-Like Waves