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abstract
A subclass of interacting galaxies, ring galaxies, provides a unique laboratory for studying unusually large bursts of non-nuclear star formation. The rings in these systems are often large (10s of kiloparsecs) and contain what appear to be associations of giant H{\small II} regions.
As a basis for future modeling of star forming regions in observed ring galaxies we present a series of combined n-body/gas numerical experiments on ring formation and evolution. Three different mass ratios between the ``host'' galaxy and ``intruder'' galaxy are considered. It is shown that a collision between equal-mass galaxies produces a qualitatively different response in the ``host'' galaxy when compared to a collision in which the ``intruder'' is one-fourth as massive or less.
Large bulk flows of material occur in a collision between equal-mass galaxies and three-dimensional effects are pronounced. A single, massive ring is present, which is composed of material that originates from a large range of radii in the disk. Strong shocks occur in the gaseous component of the disk and these dissipational processes cause the gaseous ring to expand more slowly than does the collisionless stellar ring.
When the ``intruder'' is one-fourth or less as massive as the disk galaxy, the ring behaves much more like a true density wave which propagates through the disk. A second ring forms in the inner region of the disk during the lifetime of primary outer ring. Gas in the disk accumulates on the outer edge of the primary ring so that the center of the thin gaseous ring is located ahead of the center of broader stellar ring.
