Butcher-Oemler effect and radio continuum

Abstract

The Abell clusters 2125 and 2645 have been imaged in the radio at 20 cm to the same luminosity limit using the Very Large Array. Both the clusters are very similar in richness and redshift, but Abell 2125 has a higher blue galaxy fraction (0.19) compared to Abell 2645 (0.03). The cluster Abell 2125 appears to be qualitatively different from Abell 2645 in having an order of magnitude more radio sources associated with the cluster. The radio sources belonging to Abell 2125 are distributed over a region (about 15 Mpc) much larger than the central regions of the cluster. The origin of radio emission could be due both to AGN-type of activity and/or due to massive star formation in these galaxies. On the average, the optical counterparts of the radio sources in Abell 2125 farther frorn the cluster center show signs of star formation and blue color. Most of the optical counterparts of the radio sources within the central regions (N 3 Mpc) of Abell 2125 are red in color. No radio ernission was detected to the .5a limit of 3.2 x l022 W HZ-I from most of the blue galaxies in the central regions of the cluster which are responsible for the Butcher-Oernler effect in Abell 2125.