Massive star clusters in ongoing galaxy interactions – clues to cluster formation 1

We present HST WFPC2 observations, supplemented by ground-based Hα data, of the star-cluster populations in two pairs of interacting galaxies, selected for being in very different kinds of encounters seen at different stages. Dynamical information and n-body simulations provide the details of encounter geometry, mass ratio,m and timing. In NGC 5752/4, we are seeing a weak encounter well past closest approach, after about 2.5× 10 years. The large spiral NGC 5754 has a normal population of disk clusters, while the fainter companion NGC 5752 exhibits a rich population of luminous clusters with a flatter luminosity function. The strong, ongoing encounter in NGC 6621/2, seen about 1.0× 10 past closest approach between roughly equal-mass galaxies, has produced an extensive population of luminous clusters, particularly young and luminous in a small region between the two nuclei. This region is dynamically interesting, with such a strong perturbation in the velocity field that the rotation curve reverses sign. From these results, in comparison with other strongly interacting systems discussed in the literature, cluster formation requires a threshold level of perturbation, with stage of the interaction a less important factor. The location of the most active star formation in NGC 6621/2 draws attention to a possible role for the Toomre stability threshold in shaping star formation in interacting galaxies. The rich cluster populations in NGC 5752 and 6621 show that direct contact between gas-rich galaxy disks is not a requirement to form luminous clusters, and that they can be triggered by processes happening within a single galaxy disk (albeit triggered by external perturbations). Subject headings: galaxies: individual (NGC 5752/4) – galaxies: individual (NGC 6621/22) — galaxies: interactions — galaxies: star clusters