The XMM Cluster Survey: A Massive Galaxy Cluster at z = 1.45

We report the discovery of XMMXCS J2215.9-1738, a massive galaxy cluster at z = 1.45, which was found in the XMM Cluster Survey. The cluster candidate was initially identified as an extended X-ray source in archival XMM data. Optical spectroscopy shows that six galaxies within a ~60'' diameter region lie at z = 1.45 ± 0.01. Model fits to the X-ray spectra of the extended emission yield kT = 7.4 keV (90% confidence); if there is an undetected central X-ray point source, then kT = 6.5 keV. The bolometric X-ray luminosity is L X = 4.4 × 10 44 ergs s -1 over a 2 Mpc radial region. The measured T X , which is the highest for any known cluster at z > 1, suggests that this cluster is relatively massive for such a high redshift. The redshift of XMMXCS J2215.9-1738 is the highest currently known for a spectroscopically confirmed cluster of galaxies.