2006ApJ...646L..13S

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^+2.7_–1.8keV (90% confidence); if there is an undetected central X-ray point source, then kT=6.5^+2.6_–1.8keV. The bolometric X-ray luminosity is L_X=4.4^+0.8_–0.6x10⁴⁴ 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.