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S. A. Lamb, N. C. Hearn (University of Illinois at Urbana-Champaign), R. A. Gerber (Lawrence Berkeley National Lab.)
Disturbed galaxies can have very large velocity gradients across their disks indicating much larger than average azimuthal velocities. Some display extended, narrow dust features delineating the edges of one-armed or two-armed spiral density enhancements in the gas. Phenomena such as these can be attributed to the dynamical effects of galaxy collisions, which often produce unusual velocity distributions in, and large-scale shocking of interstellar gas. Here we present a limited numerical survey of the velocity distributions produced in gaseous galactic disks by impact with another galaxy along an axis roughly parallel to the disk rotation axis. The simulations used in this survey are 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 mass of the disk galaxy varies between one and ten times that of the elliptical, and the collisions take place at impact parameters spanning the radius of the disk. The sequences of 3-D numerical models are produced using a combined N-body/Smooth Particle Hydrodynamic code which allows us to follow the evolution of gas, stars, and dark matter fully self-consistently. The disturbed morphologies and unusual velocity distributions that result from real encounters can then be used in combination with these models to put strong constraints on the dynamical history of the encounter and the timing of star formation in the disk galaxy. We show that the collision dynamics account for the extreme velocity gradients in NGC 1144, one component of the Arp 118 system.
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