Discovery of an X-Ray-luminous Galaxy Cluster at z = 1.4

We report the discovery of a massive, X-ray-luminous cluster of galaxies at z = 1.393, the most distant X-ray-selected cluster found to date. XMMU J2235.3-2557 was serendipitously detected as an extended X-ray source in an archival XMM-Newton observation of NGC 7314. VLT FORS2 R- and z-band snapshot imaging reveals an overdensity of red galaxies in both angular and color spaces. The galaxy enhancement is coincident in the sky with the X-ray emission; the cluster red sequence at R-z ≃ 2.1 identifies it as a high-redshift candidate. Subsequent FORS2 multiobject spectroscopy unambiguously confirms the presence of a massive cluster based on 12 concordant redshifts in the interval 1.38 < z < 1.40. The preliminary cluster velocity dispersion is 762 ± 265 km s-1. VLT ISAAC Ks- and J-band images underscore the rich distribution of red galaxies associated with the cluster. Based on a 45 ks XMM-Newton observation, we find that the cluster has an aperture-corrected unabsorbed X-ray flux of fX = (3.6 ± 0.3) × 10-14 ergs cm-2 s-1, a rest-frame X-ray luminosity of LX = (3.0 ± 0.2) × 1044 h ergs s-1 (0.5-2.0 keV), and a temperature of kT = 6.0 keV. Though XMMU J2235.3-2557 is likely the first confirmed z > 1 cluster found with XMM-Newton, the relative ease and efficiency of discovery demonstrates that it should be possible to build large samples of z > 1 clusters through the joint use of X-ray and large ground-based telescopes.