The Xmm Cluster Survey : a Massive Galaxy Cluster At

We report the discovery of XMMXCS J2215.9 1738, a massive galaxy cluster at , which was found z p 1.45 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 p 1.45 0.01 Model fits to the X-ray spectra of the extended emission yield keV (90% confidence); if there is 2.7 kT p 7.4 1.8 an undetected central X-ray point source, then keV. The bolometric X-ray luminosity is 2.6 kT p 6.5 L p 1.8 X ergs s 1 over a 2 Mpc radial region. The measured , which is the highest for any known cluster 0.8 44 4.4 # 10 T 0.6 X at , suggests that this cluster is relatively massive for such a high redshift. The redshift of XMMXCS z 1 1 J2215.9 1738 is the highest currently known for a spectroscopically confirmed cluster of galaxies. Subject headings: galaxies: clusters: general — galaxies: evolution — galaxies: formation