On the different radio source populations in the Butcher-Oemler clusters Abell 2125 and 2645

Abstract

The Abell clusters 2125 and 2645 have different radio source populations, despite being very similar in richness (Abell class 4) and redshift (~0.25). The number density of radio sources in Abell 2125 is almost an order of magnitude more than that in Abell 2645, based on observations to the same optical and radio luminosities of the two clusters. About 30% of the radio sources in Abell 2125 shows signs of star formation, with the largest concentration of them in the southwest clump 2 Mpc from the cluster center. There is a bimodal distribution of Abell 2125 members in radio luminosity, with the majority below a spectral luminosity at 20 cm of 10^23 W Hz^-1. This entire low-luminosity class of galaxies is absent in Abell 2645. Based on earlier observations, the blue galaxy fractions in Abell 2125 and Abell 2645 are known to be 0.19 and 0.03, respectively. Most of the blue galaxies responsible for the Butcher-Oemler effect in Abell 2125 were not detected in radio to a 20 cm luminosity limit of 1.4x10^22 W Hz^-1. The current supernova rate in these blue galaxies is unlikely to be more than a few times the current Galactic supernova rate. The different dynamical states of the two clusters might be responsible for the differences in the radio source populations observed in these two clusters, as well as the Butcher-Oemler effect.