Star Formation in Cluster-Cluster Mergers

Radio continuum emission at 1.4 GHz was used to identify active galaxies in a sample of 20 nearby Abell clusters. The radio emission indicates either an active galactic nucleus or current star formation, and optical spectroscopy was used to evaluate which of these dominates for each of the radio-selected galaxies. In an analysis which parallels the blue fractions of Butcher-Oemler studies, we calculated radio galaxy fractions for the clusters. One cluster in particular shows a dramatic increase in activity relative to the others: the cluster-cluster merger Abell 2255. We compare the results for Abell 2255 with those for Abell 2256 to assess the role of cluster-cluster mergers on the star formation activity of member galaxies. From these clusters, as well as several identified in other studies, a picture emerges in which substructure and cluster dynamical activity are of great importance in understanding evolutionary phenomenon such as the Butcher-Oemler Effect. Increased fractions of active galaxies (be they the blue galaxies of Butcher-Oemler studies or radio galaxies as discussed here) naturally result as clusters are built through mergers of smaller groups in hierarchical formation scenarios. The observed spread in fractions of active galaxies at any given redshift reflects the spread in cluster dynamical state.