Abell 2111: An Optical and Radio Study of the Richest Butcher-Oemler Cluster

Abstract

We present an in-depth analysis of the Butcher-Oemler cluster A2111, including new optical spectroscopy plus a deep Very Large Array (VLA) radio continuum observation. These are combined with optical imaging from the Sloan Digital Sky Survey (SDSS) to assess the activity and properties of member galaxies. Prior X-ray studies have suggested A2111 is a head-on cluster merger, a dynamical state which might be connected to the high level of activity inferred from its blue fraction. We are able to directly assess this claim, using our spectroscopic data to identify 95 cluster members among 196 total galaxy spectra. These galaxy velocities do not themselves provide significant evidence for the merger interpretation, however they are consistent with it provided the system is viewed near the time of core passage and at a viewing angle >~30 degrees different from the merger axis. The SDSS data allow us to confirm the high blue fraction for A2111, f_b = 0.15 +/- 0.03 based on photometry alone and f_b = 0.23 +/- 0.03 using spectroscopic data to remove background galaxies. We are able to detect 175 optical sources from the SDSS in our VLA radio data, of which 35 have redshift information. We use the SDSS photometry to determine photometric redshifts for the remaining 140 radio-optical sources. In total we identify up to 26 cluster radio galaxies, 14 of which have spectroscopic redshifts. The optical spectroscopy and radio data reveal a substantial population of dusty starbursts within the cluster. The high blue fraction and prevalence of star formation is consistent with the hypothesis that dynamically-active clusters are associated with more active member galaxies than relaxed clusters.