Published in Fundamentals of Cosmic Physics, Vol. 16, pp. 111-220, 1996.

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COLLISIONAL RING GALAXIES

P.N. Appleton and Curtis Struck-Marcell


Erwin W. Fick Observatory and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011


Abstract. We review the current state of knowledge of both the observational and theoretical nature of collisional ring galaxies. Ring galaxies represent a class of colliding galaxy in which nearly symmetrical density waves are driven into a disk as a result of an almost bulls-eye collision with another galaxy. Since the basic dynamics of the collision is now quite well understood, the ring galaxies can be used as a form of cosmic perturbation "experiment" to explore various properties of galactic disks. For example, as the density wave expands into the disk, it triggers the birth of large numbers of massive stars. This provides us with an opportunity to study the evolution of stars and star clusters in the wake of the ring. We review the now extensive observations of ring galaxies from the early photographic measurements to recent infrared, radio and optical studies. We also present a simple analytical treatment of the ring-making collisions and compare them to recent N-body and gas-dynamical models. The importance of ring galaxies lies in their relative simplicity compared with other colliding systems and the possibility that low-angular momentum collisions might have been more common in the past.


Keywords: Interacting galaxies, Colliding galaxies, Star formation in galaxies, Ring galaxies, Dynamics of interacting systems, Starburst activity in galaxies


Table of Contents

INTRODUCTION
Why Study Rings?

RINGS AS COLLISIONAL OBJECTS
The Collision Idea
Morphology of Ring Galaxies and R ing Galaxy Samples
Some Early Ideas about Ring Forma tion

OBSERVATIONS OF RING GALAXIES
The Photometry and Colors of Ring Galaxies
Observation of the Diffuse and Mo lecular Gas Content of Ring Galaxies
Observations of Ring Kinematics
The Cartwheel Galaxy and its Companions
Model Velocity Fields
Star Formation Properties of Ring Galaxies
Radio Emission and AGN Activity in Ring Galaxies
Rings in Bulge-dominated Systems and Hoag-like Galaxies

SIMPLE KINEMATIC MODELS OF RINGS AND RING-LIKE GALAXIES
Caustic Waves
Singularity Theory in Galaxy Formation and Ring Galaxies
Symmetric Ring Caustics in the Kinematic Orbit Approximation
Varieties of Symmetric Rings
Multiple Encounters?
Asymmetric Encounters

SELF-CONSISTENT STELLAR DYNAMICAL MODELS OF RING GALAXIES

THE GAS DYNAMICS OF RING GALAXIES
Lessons from Simple Models
Numerical Simulations of Ring Galaxy Hydrodynamics
The First Simulations: 1970s
The Next Generation: 1980s
The Current Generation of Models: 1990s
Models of the Cartwheel
What Are the Spokes?
Explorations of Heating and Cooling Effects

FUTURE DIRECTIONS
Observational Directions
The Next Steps in Modeling

REFERENCES

APPENDIX 1: Multiple Encounter Kinematics

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

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