The Dynamics of the Merging Galaxy Cluster System A2256: Evidence for a New Subcluster

We present 236 new radial velocities of galaxies in the cluster A2256 measured with the WIYN Hydra multi-object spectrograph. Combined with the previous work of Fabricant, Kent, and Kurtz (1989), we have velocities for a total of 319 galaxies of which 277 are cluster members. In addition to the new radial velocities, we present a 3x3 image mosaic in the R band of the central 19'x19' region of A2256, from which we obtained photometry for 861 galaxies. These data provide strong evidence for a merger event between two groups. In addition, we present evidence for the presence of a third group, on the outer reaches of the system, that is just now beginning to merge with the system.