Starburst-driven starbursts in the heart of ultraluminous infrared galaxies

There is increasing evidence for the presence of blue super-star clusters i n the central regions of ultraluminous infrared galaxies like Arp 220. Ultr aluminous galaxies are thought to be triggered by galaxy mergers, and it ha s often been argued that these super-star clusters may form during violent collisions between gas clouds in the final phase of the mergers. We now inv estigate another set of models that differ from previous ones in that the f ormation of the super-star clusters is linked directly to the very intense starburst occurring at the very center of the galaxy. First, we show that a scenario in which the super-star clusters form in material compressed by s hock waves originating from the central starburst is implausible because th e objects so produced an much smaller than the observed star clusters in Ar p 220. We then investigate a scenario (based on the Shlosman-Noguchi model) in which the infalling dense gas disk is unstable gravitationally and coll apses to form massive gaseous clumps. Since these clumps are exposed to the external high pressure that is driven by the superwind (a blast wave that is driven by the collective effect of a large number of supernovae in the v ery core of the galaxy), they can collapse and then massive star formation may be induced in them. The objects produced in this kind of collapse have properties consistent with those of the observed super-star clusters in the center of Arp 220.