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 12X-ray selected clusters and groups of galaxies at z ~ 0.3. The systems coverone of the largest ranges in X-ray luminosity (Lx ~ 10^43 - 10^45 erg/s), andhence 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 slopenor zero-point of this relation changes significantly over the factor of 100 inX-ray luminosity of our sample. Using stellar population synthesis models wefind our data allow a maximum possible change of 2 Gyrs in the typical age ofthe galaxies on the CMR over the range of Lx of our sample. We also measure thefraction of blue galaxies (fb) relative to the CMR in our clusters and find alow value of fb ~ 0.1 and find that there is no correlation between fb and Lxover our large Lx range. However, both the CMR and fb do depend on clusterradius, 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 aluminosity weighted age gradient of ~ 2.5 Gyrs per log(radius). It thus appearsthat the global cluster environment, in the form of cluster mass (Lx), haslittle influence on the properties of bright cluster galaxies, whereas thelocal 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-pressureefficiency, suggesting that ram-pressure stripping, or other mechanisms thatdepend on cluster mass like tidal stripping or harassment, are unlikely to besolely responsible for changing the galaxy population from the `blue' starforming galaxies, that dominate low density environments, to the `red' passivegalaxies that dominate cluster cores.(abridged)Comment: 18 pages, 17 figures. Accepted for publication in Ap