2001ApJ...551..651H

Colliding galaxies are excellent laboratories for studying galactic evolution and global star formation. Computer simulations of galaxy collisions, in which at least one galaxy has a significant gaseous component, show the production of density enhancements and shock waves in the interstellar medium. These high-density regions coincide with the locations of recent, large-scale star formation in observations of some real colliding galaxies. Thus, combined n-body/hydrodynamic computer simulations can be used to explore the history and conditions of star-forming regions in colliding galaxies. We compare multiwavelength observations of the Arp 119 system with a combined n-body/SPH simulation of colliding galaxies. Most of the observations used here are gleaned from the literature. Additionally, we obtained new near-infrared (J- and H-band) observations of this system, using the NIRIM camera at the Mount Laguna observatory. These new data add information about the underlying, old stellar population. Arp 119 (CPG 29) is comprised of a southern member, Arp 119S (Mrk 984), which has an extremely disturbed appearance, and a northern member, Arp 119N, a gas-poor elliptical. The morphology of both members can be fitted well by a simulation in which a gas-rich disk galaxy has been impacted by an equal-mass elliptical that had a trajectory approximately perpendicular to the plane of the disk and passed through the disk slightly off-center. From our comparison, we find that the progression of recent large-scale star formation in this galaxy can be accounted for by a single outwardly propagating collision-induced density wave in the gas. We deduce that the star formation rate in this density wave was not a smooth function of time but that, so far, three major episodes of star formation have occurred at roughly 25-30 Myr intervals. An initial burst took place very soon after the peak of the collision between the two galaxies, and the latest burst is still ongoing. The fit of the simulations to the observations indicates that this collision occurred approximately 71 Myr ago. At the current, projected separation of 53 kpc (assuming H₀=75 km.s^–1.Mpc^–1), we obtain a current relative space velocity between the two galaxies of approximately 850 km.s^–1. This is strong evidence that the collision partner was Arp 119N and not some currently more distant galaxy. Furthermore, the high relative velocity of the pair and the paucity of gas currently to be found in the elliptical may explain the very high velocity gas observed in the greatly disturbed disk galaxy, Arp 119S.