A Deep Radio Survey of Abell 2125. II. Accelerated Galaxy Evolution during a Cluster-Cluster Merger

Using our extensive radio, optical, near-IR and X-ray imaging andspectroscopy, we consider the reason for the unusually large number of radiodetected galaxies, mostly found outside the cluster core, in Abell 2125(z=0.2465, richness class 4). With 20-cm VLA data, we detect continuum emissionfrom 90 cluster members. The multiwavelength properties of these galaxiessuggest that most of the radio emission is due to an enhanced star-formationrate. The dynamical study of Miller et al (2004) suggests that Abell 2125 isundergoing a major cluster-cluster merger, with our view within 30 degrees ofthe merger axis and within 0.2 Gyr of core passage. The combination ofprojection effects and the physical processes at work during this special timein the cluster's evolution seem likely to be responsible for the unusual levelof activity we see in the cluster. We argue that tidal effects on individualcluster members, often far from the cluster core, are responsible for theincreased star formation. Our results are consistent with the idea that diskgalaxies during this phase of a cluster's evolution undergo rapid evolution,through a burst of star formation, on their way to becoming S0's.