Starburst-driven Starbursts in the Heart of Ultraluminous Infrared Galaxies

There is increasing evidence for the presence of blue super star clusters inthe central regions of ultraluminous infrared galaxies like Arp 220.Ultraluminous galaxies are thought to be triggered by galaxy mergers, and ithas often been argued that these super star clusters may form during violentcollisions between gas clouds in the final phase of the mergers. We nowinvestigate another set of models which differ from previous ones in that theformation of the super star clusters is linked directly to the very intensestarburst occurring at the very center of the galaxy. Firstly we show that ascenario in which the super star clusters form in material compressed by shockwaves originating from the central starburst is implausible because the objectsso produced are much smaller than the observed star clusters in Arp 220. Wethen investigate a scenario (based on the Shlosman-Noguchi model) in which theinfalling dense gas disk is unstable gravitationally and collapses to formmassive gaseous clumps. Since these clumps are exposed to the external highpressure driven by the superwind (a blast wave driven by a collective effect ofa large number of supernovae in the very core of the galaxy), they can collapseand then massive star formation may be induced in them. The objects produced inthis kind of collapse have properties consistent with those of the observedsuper star clusters in the center of Arp 220.Comment: 13 pages, 1 figure, ApJ (Letters) in pres