The Butcher-Oemler effect at moderate redshift

We present the results of Butcher-Oemler-style analysis of three moderate-r edshift (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 hi gh fractions of blue galaxies (f(b)'s) as compared with clusters at compara ble redshifts studied by Butcher & Oemler. This implies that star formation is occurring in a high fraction of the galaxies in the two clusters. Our r esults are consistent with hierarchical clustering models in which subclust er-subcluster mergers create shocks in the intracluster medium. The shocks, in turn, induce simultaneous starbursts in a large fraction of cluster gal axies. Our study therefore lends weight to the hypothesis that the Butcher- Oemler effect is an environmental, as well as evolutionary, phenomenon.