The XMM Cluster Survey: Galaxy Morphologies and the Color-Magnitude Relation in XMMXCS J2215.9 - 1738 at z = 1.46

We present a study of the morphological fractions and color-magnitude relation (CMR) in the most distant X-ray selected galaxy cluster currently known, XMMXCS J2215.9 - 1738 at z = 1.46, using a combination of optical imaging data obtained with the Hubble Space Telescope Advanced Camera for Surveys, and infrared data from the Multi-Object Infrared Camera and Spectrograph, mounted on the 8.2 m Subaru telescope. We find that the morphological mix of the cluster galaxy population is similar to clusters at z ~ 1. Within the central 0.5 Mpc, approximately ~62% of the galaxies identified as likely cluster members are ellipticals or S0s; and ~38% are spirals or irregulars. Therefore, early-type galaxies were already entrenched as the dominant galaxy population in at least some clusters approximately ~4.5 Gyr after the big bang. We measure the CMRs for the early-type galaxies, finding that the slope in the z ₈₅₀-J relation is consistent with that measured in the Coma cluster, some ~9 Gyr earlier, although the uncertainty is large. In contrast, the measured intrinsic scatter about the CMR is more than three times the value measured in Coma, after conversion to rest-frame U - V. From comparison with stellar population synthesis models, the intrinsic scatter measurements imply mean luminosity-weighted ages for the early-type galaxies in J2215.9 - 1738 of ≈3 Gyr, corresponding to the major epoch of star formation coming to an end at z_f ≈ 3-5. We find that the cluster exhibits evidence of the "downsizing" phenomenon: the fraction of faint cluster members on the red sequence expressed using the Dwarf-to-Giant Ratio (DGR) is 0.32 ± 0.18 within a radius of 0.5R ₂₀₀. This is consistent with extrapolation of the redshift evolution of the DGR seen in cluster samples at z < 1. In contrast to observations of some other z > 1 clusters, we find a lack of very bright galaxies within the cluster.