2000AJ....119.1090M

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 (f_b{apos}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.