The environmental dependence of galaxy colors in intermediate redshift X-ray-selected clusters

We present a wide-field imaging study of the colors of bright galaxies in 12 X-ray selected clusters and groups of galaxies at z ~ 0.3. The systems cover one of the largest ranges in X-ray luminosity (Lx ~ 10^43 - 10^45 erg/s), and hence mass, of any sample studied at this redshift. We find that the `red' galaxies form a tight color-magnitude relation (CMR) and that neither the slope nor zero-point of this relation changes significantly over the factor of 100 in X-ray luminosity of our sample. Using stellar population synthesis models we find our data allow a maximum possible change of 2 Gyrs in the typical age of the galaxies on the CMR over the range of Lx of our sample. We also measure the fraction of blue galaxies (fb) relative to the CMR in our clusters and find a low value of fb ~ 0.1 and find that there is no correlation between fb and Lx over our large Lx range. However, both the CMR and fb do depend on cluster radius, with the zero-point of the CMR shifting blueward in B-R by 0.10 +/- 0.036 magnitudes out to 0.75 times the virial radius, equivalent to a luminosity weighted age gradient of ~ 2.5 Gyrs per log(radius). It thus appears that the global cluster environment, in the form of cluster mass (Lx), has little influence on the properties of bright cluster galaxies, whereas the local environment, in the form of galaxy density (radius), has a strong effect. The range of ~ 100 in Lx corresponds to a factor of ~ 40 in ram-pressure efficiency, suggesting that ram-pressure stripping, or other mechanisms that depend on cluster mass like tidal stripping or harassment, are unlikely to be solely responsible for changing the galaxy population from the `blue' star forming galaxies, that dominate low density environments, to the `red' passive galaxies that dominate cluster cores.(abridged