The XMM Cluster Survey: Galaxy Morphologies and the color-magnitude Relation in XMMXCS J2215:9 - 1738 AT z = 1.46

M. Hilton, S. A. Stanford, J. P. Stott, C. A. Collins, B. Hoyle, M. Davidson, M. Hosmer, S. T. Kay, A. R. Liddle, E. Lloyd-Davies, R. G. Mann, N. Mehrtens, C. J. Miller, R. C. Nichol, A. K. Romer, K. Sabirli, M. Sahlén, P. T. P. Viana, M. West, K. Barbary, K. S. Dawson, J. Meyers, S. Perlmutter, D. Rubin, N. Suzuki

Abstract
We present a study of the morphological fractions and color{magnitude relation 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 8.2m 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 color{magnitude relations for the early type galaxies, finding that the slope in the z₈₅₀ - 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 color{magnitude relation 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 zf ~ 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 0.5R₂₀₀. 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.

Keywords
galaxies: clusters: individual (XMMXCS J2215.9-1738) - galaxies: elliptical and lenticular, cD - galaxies: evolution - X-rays: galaxies: clusters

The Astrophysical Journal
Volume 697, Page 436
May 2009

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