The XMM Cluster Survey: Galaxy morphologies and the color-magnitude relation in J2215.9 – 1738 AT z = 1.46

Matt Hilton, S. Adam Stanford, John P. Stott, Chris A. Collins, Ben Hoyle, Michael Davidson, Mark Hosmer, Scott T. Kay, Andrew R. Liddle, Ed Lloyd-davies, Robert G. Mann, Nicola Mehrtens, Christopher J. Miller, Robert C. Nichol, A. Kathy Romer, Kivanc Sabirli, Martin Sahlen, Pedro T. P. Viana, Michael J. West, Kyle BarbaryKyle S. Dawson, Joshua Meyers, Saul Perlmutter, David Rubin, Nao Suzuki

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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 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 0.5R 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.
Original languageEnglish
Pages (from-to)436-451
JournalThe Astrophysical Journal
Issue number1
Early online date1 May 2009
Publication statusPublished - 20 May 2009


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