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abstract
Interacting ring galaxies are often observed to have a clumpy or knotty structure in their rings. These knots are thought to be sites of intense massive star formation. In at least one ring galaxy, the Cartwheel, clumps are also seen in spokes that extend from an inner ring and terminate at the large outer ring. The star formation rate in the spokes is believed to be significantly lower than that in the outer ring. Little, if any, star formation is thought to be taking place in the inner ring. There is evidence that little star formation had occured in the Cartwheel before the present collision-induced episode. This would imply that the Cartwheel's disk was in a sense ''stable'' before the collision, or at least not violently unstable.
A series of numerical experiments using the method of Smoothed Particle Hydrodynamics is presented which seeks to reproduce the clumpy appearance of the Cartwheel starting from a ''stable'' precollision disk. High resolution numerical runs are used to elucidate which physical processes and conditions are important in feeding the instabilities, inhibiting gravitational collapse to very small length scales, and producing sheared spoke-like features that are clumpy, but nevertheless exhibit reduced rates of star formation with respect to the outer ring.
