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The strong star formation observed in many rings and discussed above is good evidence in itself that many ring galaxy disks are gas rich. In fact, there is probably a strong selection effect biasing us toward the discovery of bright, blue star-forming rings with late-type precursors. Red stellar rings in early-type disks will not announce themselves as loudly, and even when discovered are often accounted for as the result of non-collisional dynamical processes. Nonetheless, in many of the well-known systems, most notably the Cartwheel, the rings are essentially a gas dynamical phenomenon.

This being the case, an important question is how the gas response to the dynamical disturbance differs from that of the stars? It is likely that the radial epicyclic motions are likely to be very supersonic in most systems, so collisional shock dynamics probably play an important role. However, even this conclusion is a bit simplistic, and must be qualified. The structure of any shocks in the ring waves will depend on the nature and structure of the interstellar medium in the precursor galaxy. Although there is beginning to be some observational input in this area, it is still largely unknown territory. Indeed, the question might be turned around in that studies of rings may in some cases be used to determine conditions in the precursor disk.

However, the largest question in this area remains - how does the ring wave stimulate star formation? We will see below that simulations are beginning to provide some possible answers. We have long hoped that the relative simplicity of ring waves would allow us to understand them in enough detail to learn some generally applicable lessons about wave-induced star formation. Recent results from simulations appear to support this conjecture.

Finally, there are questions concerning nonlinear feedbacks, e.g., what effects do the wave and the induced star formation have on the interstellar gas downstream of the ring? Also, how do these effects interact with the strong rarefaction behind the compression wave? Answers to these questions require a realistic treatment of the effects of young star activity.

In the rest of this section we will review how the questions above have been addressed in models produced to date. We will focus on models specifically designed to simulate the ring galaxy phenomenon. However, there are clearly strong connections to gas dynamical modeling of general tidal interactions and collisions, especially as regards star formation activity and the resulting feedbacks. There are also connections to a very large literature of models of gas dynamics and wave-induced star formation in isolated galaxies. This includes the literature on the maintenance and the effects of spiral density waves, and of waves driven by stellar bars. An examination of these connections is beyond the scope of this review.

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