HST Observations of Young Star Clusters in Interacting Galaxies

We present early results from the analysis of HST imaging observations for several pairs of interacting galaxies. We include two cases that were specifically chosen to represent a strong early (young) encounter and a weak late (old) encounter. The goals of the project include a determination of the timing, frequency, strength, and characteristics of the young star clusters formed in these two limiting cases of tidal encounters. 1. Results from the Analysis of HST Imaging Observations Multi-band HST imaging data have been obtained for several interacting galaxy systems. These include the Cartwheel Ring Galaxy (Borne et al. 1996; Struck et al. 1996; Appleton et al., in preparation), Arp 81 and Arp 297 (Keel & Borne, in preparation), and a large sample of ULIRGs (Ultra-Luminous Infrared Galaxies; Borne et al. 2000, and references therein). WFPC2 B-band (F450W) and I-band (F814W) images have been obtained for the Cartwheel, Arp 297, and Arp 81. WFPC2 I-band and NICMOS H-band (F160W) images have been obtained for the ULIRGs. Additional HST archival WFPC2 V-band (F606W) images are also available for some of the ULIRGs (Farrah et al. 2001). A wide variety of scientific results from these studies have been published or are in preparation. Here, we report specifically on results pertaining to the young star cluster (YSC) population in these interacting galaxy systems. It appears certain that the formation of YSCs in interacting galaxies is collision-induced (Whitmore et al. 1999). A key question is: At what phase of the interaction do the clusters form? While a rich population of ∼500 very blue YSCs is seen in the Cartwheel, especially in its expanding collision-induced ring,