Caught in the Act: The Hierarchical Formation of Abell 1185

Abell 1185 is a nearby example of a rich galaxy cluster still undergoing violent hierarchical assembly. Its brightest member galaxy, NGC 3550, is offset by 150 kpc from the cluster's dynamical center and is in the throes of cannibalizing several neighbors. Abell 1185 is also home to a population of intergalactic globular clusters, as well as the interacting system Arp 105, a spectacular collision between two galaxies that has spawned a compact region of intense star formation known as Ambartsumian's Knot as well as a 100-kpc-long tidal tail with embedded star clusters and newborn dwarf galaxies. Abell 1185 offers a unique opportunity to witness the ongoing birth of a dynamically young cluster and its constituents, a present-day analog to cluster formation at earlier epochs. We propose to carry out a comprehensive WFC3+ACS imaging survey of A1185 to learn more about the ongoing creation of galactic and intergalactic stellar populations in this dynamic environment. The resulting dataset will be of comparable legacy value to the ACS Treasury Survey of the Coma Cluster (GO 10861) for a fraction of the investment of time, providing a snapshot of a rich cluster environment that represents an important earlier stage in galaxy cluster evolution.