Two Populations of Young Massive Star Clusters in Arp 220
  We present new optical observations of young massive star clusters in Arp 220, the nearest ultraluminous infrared galaxy, taken in UBVI with the Hubble Space Telescope ACS/HRC camera. We find a total of 206 probable clusters whose spatial distribution is centrally concentrated toward the nucleus of Arp 220. We use model star cluster tracks to determine ages, luminosities, and masses for 14 clusters with complete UBVI indices or previously published near-infrared data. We estimate rough masses for 24 additional clusters with I < 24 mag from BVI indices alone. The clusters with useful ages fall into two distinct groups: a ``young'' population (< 10 Myr) and an intermediate-age population (~300 Myr). There are many clusters with masses clearly above 10^6 Msun and possibly even above 10^7 Msun in the most extreme instances. These masses are high enough that the clusters being formed in the Arp 220 starburst can be considered as genuine young globular clusters. In addition, this study allows us to extend the observed correlation between global star formation rate and maximum cluster luminosity by more than an order of magnitude in star formation rate. 
Christine D. Wilson, William E. Harris, Rebecca Longden, N. Z.   Scoville