1.1. Why Study Rings?

Ring galaxies are rare amongst samples of bright galaxies. Despite this, we believe that ring galaxies are important for the following reasons.

• Ring galaxies represent a class of colliding galaxies in which the disk of at least one of the participants of the collision remains sufficiently intact after the collision to allow its structure and star formation properties to be studied in detail. This is not often the case in colliding galaxy systems. In a sense, the ring galaxy serves as a galaxy-scale perturbation experiment that allows us to study the hydrodynamic and stellar evolutionary processes within the host disk. In many cases, the gravitational perturbation of the small companion galaxy is expected to be modest, driving low amplitude waves through its disk. In other cases the companion is more substantial, and large three-dimensional effects and strong non-linear behavior is predicted. Models of rings have now been developed by a number of independent groups, and such tools can be used to make predictions which can be compared with observation.

• Ring galaxies are the sites of vigorous non-nuclear star formation. Typical knots in ring galaxies have optical luminosities similar to, or in excess of, the Large Magellanic Cloud. The colors and spectra and extreme luminosity of the HII regions in the rings suggest large numbers of OB stars are being born at high rates. Ring galaxies also have generally high global far-infrared (IR) luminosities and large far-IR color temperatures compared with "normal" galaxies. Recent observations also show that they contain substantial quantities of molecular gas. These properties are shared with nuclear starburst galaxies.

• Ring galaxies can provide a means of studying the time evolution of star formation across and inside the ring. The simplest models predict that stars form in the expanding ring. A star formation burst formed at the leading edge of the expanding density wave is expected to produce a monotonic color gradient in the wake of the ring, as evolving stars drift inwards from their birth sites. There is now mounting evidence for such color gradients and other indicators of stellar evolution behind the ring.

• Models indicate that off-center, moderate impact-parameter collisions often produce a strongly asymmetric ring initially, and many known ring galaxies appear to fall into this category. Interesting azimuthal and vertical variations are predicted from the models in these cases and these can provide interesting tests of the mechanisms of star formation which can be compared with observations.

• Because of its geometrical simplicity, as compared with more dynamically messy galaxy mergers, the collisional ring galaxy can be used to explore differences between the behavior of gas and stars in collisions. Dissipation can aid the development of local gravitational instabilities in the gas, causing the collapse and shearing of gas complexes, star formation and perhaps the formation of `spokes' similar to those found in the Cartwheel.