An environmental Butcher-Oemler effect in intermediate-redshift X-ray clusters

We present uniform Canada-France-Hawaii Telescope (CFHT) Megacam g and r photometry for 34 X-ray-selected galaxy clusters drawn from the X-ray Multi-Mirror (XMM) Large Scale Structure (LSS) survey and the Canadian Cluster Comparison Project (CCCP). The clusters possess well-determined X-ray temperatures spanning the range 1 < kT(keV) < 12. In addition, the clusters occupy a relatively narrow redshift interval (0.15 < z < 0.41) in order to minimize any redshift-dependent photometric effects. We investigate the colour bimodality of the cluster galaxy populations and compute blue fractions using criteria derived by Butcher & Oemler. We identify a trend to observe increasing blue fraction versus redshift in common with numerous previous studies of cluster galaxy populations. However, in addition we identify an environmental dependence of cluster blue fraction in that cool (low-mass) clusters display higher blue fractions than hotter (higher mass) clusters. Finally, we tentatively identify a small excess population of extremely blue galaxies in the coolest X-ray clusters (essentially massive groups) and note that these may be the signature of actively starbursting galaxies driven by galaxy-galaxy interactions in the group environment.