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The dominant stellar rings and partial rings that are observed in the disks of some galaxies are believed to have been caused by collision with another galaxy of comparable or somewhat smaller mass. Here we present the results of a numerical exploration of the morphologies and dynamics of collisions between two similar mass galaxies and compare our results to optical observations of this type of ring galaxy. The experiments are comprised of combined N-body/Smooth Particle Hydrodynamic, 3-D numerical simulations of collisions between a rotating disk galaxy, composed of gas, stars, and dark matter, with a spherical galaxy composed of stars and dark matter only. The collisions take place at impact parameters ranging between zero and 0.9 times the radius of the disk, R, in a direction roughly parallel to the spin axis of the disk. The ratio of the mass of the disk galaxy to that of the spherical galaxy is varied between 1 and 10. The results of these numerical experiments are intercompared and it is found that the largest density increases in the gas are obtained for collisions with an impact parameter of approximately 0.5 R, at a time somewhat after the closest approach of the nuclei of the two galaxies. In general, high volume densities are obtained in the gas at those places where kinetic theory predicts orbit crossing and, if we assume that star formation in these galaxies is triggered by shocks in high density regions, we can accurately predict the locations of the intense bursts of star formation observed to occur in these galaxies. We find that the more off-center the collision, the larger is the resulting warp in the disk, and the larger the eventual displacement of the nucleus of the disk galaxy from the center. These characteristics will allow us to constrain the orbits of observed systems and to investigate the star formation histories and dynamics of individual galaxies.
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