Evolution of the Color-Magnitude Relation in High-Redshift Clusters: Blue Early-Type Galaxies and Red Pairs in RDCS J0910+5422

The color-magnitude relation has been determined for the RDCS J0910+5422 cluster of galaxies at redshift z = 1.106. Cluster members were selected from HST Advanced Camera for Surveys (ACS) images, combined with ground–based near–IR imaging and optical spectroscopy. Postman et al. (2005) morphological classifications were used to identify the early-type galaxies. The observed early–type color–magnitude relation (CMR) in (i775 z850) versus z850 shows an intrinsic scatter in color of 0.060±0.009 mag, within 1 ′ from the cluster X–ray emission center. Both the ellipticals and the S0s show small scatter about the CMR of 0.042 ± 0.010 mag and 0.044± 0.020 mag, respectively. From the scatter about the CMR, a mean luminosity–weighted age t > 3.3 Gyr (zf > 3) is derived for the elliptical galaxies, assuming a simple stellar population modeling (single burst solar metallicity). This is consistent with a previous study of the cluster RDCS1252.9-292 at z=1.24 (Blakeslee et al.). Strikingly, the S0 galaxies in RDCS J0910+5422 are systematically bluer in (i775 z850) by 0.07 ± 0.02 mag, with respect to the ellipticals. The blue S0s are predominantly elongated in shape; the distribution of their ellipticities is inconsistent with a population of axisymmetric disk galaxies viewed at random orientations, suggesting either that they are intrinsically prolate or there is some orientation bias in the S0 classification. The ellipticity distribution as a function of color indicates that the face-on S0s in this particular cluster have likely been classified as elliptical. Thus, if anything, the offset in color between the elliptical and S0 populations may be even more significant. The color offset between S0 and E corresponds to an age difference of � 1 Gyr, for a singleburst solar metallicity model. Alternatively, it could be the result of a different star formation history; a solar metallicity model with an exponential decay in star formation will reproduce the offset for an age of 3.5 Gyr, i.e. the S0s have evolved gradually from star forming progenitors. The color offset could also be reproduced by a factor of �2 difference in metallicity, but the two populations would each need to have very small scatter in metallicity to reproduce the small scatter in color. The early–type population in this cluster appears to be still forming. The blue early-type disk galaxies in RDCS J0910+5422 likely represent the direct progenitors of the more evolved S0s that follow the same red sequence as ellipticals in other clusters. Thirteen red galaxy pairs are observed and the galaxies associated in pairs constitute �40% of the CMR galaxies in this cluster. This finding is consistent with the conclusions of van Dokkum and Tran et al. that most of the early–type galaxies grew from passive red mergers.