The XMM Cluster Survey : A Massive Galaxy Cluster

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 6 galaxies within a ∼60 arcsec diameter region lie at z = 1.45 ± 0.01. Model fits to the X-ray spectra of the extended emission yield kT = 7.4 −1.8 keV (90% confidence); if there is an undetected central X-ray point source then kT = 6.5 −1.8 keV. The bolometric X-ray luminosity is Lx = 4.4 +0.8 −0.6 × 10 44 ergs s over a 2 Mpc radial region. The measured Tx, which is the highest known for a 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. Subject headings: galaxies: clusters — galaxies: evolution — galaxies: formation