Progressive Starbursts and High Velocities in the Infrared-luminous, Colliding Galaxy Arp 118

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Abstract
In this Letter, we demonstrate for the first time the connection between the spatial and temporal progression of star formation and the changing locations of the very dense regions in the gas of a massive disk galaxy (NGC 1144) in the aftermath of its collision with a massive elliptical galaxy (NGC 1143). These two galaxies form the combined object Arp 118, a collisional ring galaxy system. The results of three-dimensional, time-dependent, numerical simulations of the behavior of the gas, stars, and dark matter of a disk galaxy and the stars and dark matter of an elliptical during a collision are compared with multiwavelength observations of Arp 118. The collision that took place approximately 22 Myr ago generated a strong, nonlinear density wave in the stars and gas in the disk of NGC 1144, causing the gas to become clumped on a large scale. This wave produced a series of super-star clusters along the arcs and rings that emanated from the central point of impact in the disk. The locations of these star-forming regions (revealed by multiwavelength observations, including radio continuum, Hα, and CO emission) match those of the regions of increased gas density predicted by the time sequence of models. The models also predict the large velocity gradients observed across the disk of NGC 1144. These are due to the rapid radial outflow of gas coupled with large azimuthal velocities in the expanding ring caused by the impact of the massive intruder.
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