Cluster Mergers as Triggers of Star Formation and Radio Emission: A Comparative Study of the Rich Clusters Abell 2125 and 2645

Abstract

We report a detailed optical study of the clusters Abell 2125 and 2645. These clusters are very similar in redshift (z ≈ 0.25) and richness (Abell class 4), yet contrast strongly in blue fraction and radio-galaxy populations. In 1984 Butcher & Oemler reported that A2125 and A2645 have blue-galaxy fractions of 0.19 and 0.03, respectively, while more recent radio observations with the VLA and subsequent optical identifications on the digital Palomar Sky Survey show an apparent excess of radio galaxies in A2125 relative to A2645 (Dwarakanath & Owen). Our spectroscopic observations confirm this difference. We find 27 radio galaxies to be members of A2125 and only four in A2645, based on (nearly) complete observations to the same limiting magnitude and radio flux density. The radio galaxies in A2125 extend over about 5 Mpc (assuming H0 = 75 km s-1 Mpc-1) along a band running from northeast to southwest of the cluster center. About half the radio galaxies are red and have optical spectra that resemble old stellar populations. The other half are blue with emission lines, most of which indicate an origin in star formation rather than AGN. Many of the blue galaxies are in a distinct clump located about 2 Mpc in projection from the cluster center. The excess population of radio galaxies in A2125 occurs entirely at radio luminosities less than 1023 W Hz-1, where one expects star formation to be primarily responsible for the radio emission. Most of these radio galaxies have optical properties most consistent with systems later than E/S0. However, the optical line luminosities are often weaker than one would expect for the star formation rates implied by the radio emission. Thus we suspect that dust obscuration, larger than is usually found locally, hides most of the star-forming regions optically. The existence of a cluster-cluster merger in progress in A2125 seems likely to play some role in these phenomena, although the details are obscure.