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 (<M* + 2) in a sample of 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 ~ 1043-1045 ergs s-1), 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 covered by our sample. Using stellar population synthesis models, we find that our data allow a maximum possible change of 2 Gyr in the typical age of the "red" 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 consistent with other X-ray-selected cluster samples. We find that there is no correlation between fb and LX over our large LX range. However, we do find that both the CMR and fb depend significantly on cluster radius, with the zero point of the CMR shifting blueward in B - R by 0.10 ± 0.036 mag out to a radius of 0.75 times the virial radius. This color change is equivalent to a luminosity-weighted age gradient of ~2.5 Gyr per log(radius) and is consistent with previous studies of the radial change in the zero point of the CMR. It thus appears that the global cluster environment, in the form of cluster mass (LX), has little influence on the properties of the 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, thus 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, which dominate low-density environments, to the "red" passive galaxies, which dominate cluster cores.