Cluster mergers as triggers of star formation and radio emission: A comparative study of the rich clusters Abell 2125 and 2645

We report a detailed optical study of the clusters Abell 2125 and 2645. The se clusters are very similar in redshift (z approximate to 0.25) and richne ss (Abell class 4), yet contrast strongly in blue fraction and radio-galaxy populations. In 1984 Butcher & Oemler reported that A2125 and A2645 have b lue-galaxy fractions of 0.19 and 0.03, respectively, while more recent radi o observations with the VLA and subsequent optical identifications on the d igital Palomar Sky Survey show an apparent excess of radio galaxies in A212 5 relative to A2645 (Dwarakanath & Owen). Our spectroscopic observations co nfirm 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 li miting magnitude and radio flux density. The radio galaxies in A2125 extend over about 5 Mpc (assuming H-0 = 75 km s(-1) Mpc(-1)) along a band running from northeast to southwest of the cluster center. About half the radio ga laxies are red and have optical spectra that resemble old stellar populatio ns. 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 l uminosities less than 10(23) W Hz(-1), where one expects star formation to be primarily responsible for the radio emission. Most of these radio galaxi es have optical properties most consistent with systems later than E/SO. Ho wever, the optical line luminosities are often weaker than one would expect for the star formation rates implied by the radio emission. Thus we suspec t that dust obscuration, larger than is usually found locally, hides most o f the star-forming regions optically. The existence of a cluster-cluster me rger in progress in A2125 seems likely to play some role in these phenomena , although the details are obscure.