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The XMM cluster survey: Galaxy morphologies and the color-magnitude relation in XMMXCS J2215.9 - 1738 at z = 1.46

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  • Matt Hilton
  • S. Adam Stanford
  • 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 Sahlén
  • Pedro T P Viana
  • Michael J. West
  • Kyle Barbary
  • Kyle S. Dawson
  • Joshua Meyers
  • Saul Perlmutter
  • David Rubin
  • Nao Suzuki
<mark>Journal publication date</mark>1/05/2009
<mark>Journal</mark>The Astrophysical Journal
Issue number1
Number of pages16
Pages (from-to)436-451
Publication StatusPublished
<mark>Original language</mark>English


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.