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 similar to 0.3. The systems cover one of the largest ranges in X-ray luminosity ( L-X similar to 10(43)-10(45) 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 L-X of our sample. We also measure the fraction of blue galaxies (f(b)) relative to the CMR in our clusters and find a low value of f(b)similar to 0.1 consistent with other X-ray-selected cluster samples. We find that there is no correlation between fb and L-X over our large L-X 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 similar to 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 (L-X), 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 similar to 100 in L-X corresponds to a factor of similar to 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.