The Butcher-Oemler Effect at Moderate Redshift

We present the results of Butcher-Oemler–style analysis of three moderate-redshift (0.1 < z < 0.2) clusters that have bimodal X-ray surface brightness profiles. We find that at least two of these clusters exhibit unusually high fractions of blue galaxies (fb's) as compared with clusters at comparable redshifts studied by Butcher & Oemler. This implies that star formation is occurring in a high fraction of the galaxies in the two clusters. Our results are consistent with hierarchical clustering models in which subcluster-subcluster mergers create shocks in the intracluster medium. The shocks, in turn, induce simultaneous starbursts in a large fraction of cluster galaxies. Our study therefore lends weight to the hypothesis that the Butcher-Oemler effect is an environmental, as well as evolutionary, phenomenon.