An X-ray selected galaxy cluster at z=1.11 in the ROSAT Deep Cluster Survey

We report the discovery of an X-ray luminous galaxy cluster at z=1.11. RDCS J0910+5422 was selected as an X-ray cluster candidate in the ROSAT Deep Cluster Survey on the basis of its spatial extent in a ROSAT Position Sensitive Proportional Counter image. Deep optical and near-IR imaging reveal a red galaxy overdensity around the peak of the X-ray emission, with a significant excess of objects with J-K and I-K colors typical of elliptical galaxies at z similar to 1. Spectroscopic observations at the Keck II telescope secured nine galaxy redshifts in the range 1.095 < z < 1.120, yielding a mean cluster redshift of = 1.106. Eight of these galaxies lie within a 30 radius around the peak X-ray emission. A deep Chandra ACIS exposure on this field shows extended X-ray morphology and allows the X-ray spectrum of the intracluster medium to be measured. The cluster has bolometric luminosity Lx = 2.48(-0.26)(+0.33) x 10(44) ergs s(-1), temperature kT = 7.2(-1.4)(+2.2) keV, and mass within r = 1 Mpc of 7.0x10(14) M-circle dot (H-0 = 65 km s(-1) Mpc(-1), Omega(m) = 0.3, and Omega(Lambda)= 0.7). The spatial distribution of the cluster members is elongated, which is not due to an observational selection effect, and followed by the X-ray morphology. The X-ray surface brightness pro le and the spectrophotometric properties of the cluster members suggest that this is an example of a massive cluster in an advanced stage of formation with a hot intracluster medium and an old galaxy population already in place at z > 1.