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~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 <z>=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 L_x=2.48^+0.33_-0.26×10⁴⁴ ergs s^-1, temperature kT=7.2^+2.2_-1.4 keV, and mass within r=1 Mpc of 7.0×10¹⁴ M_solar (H₀=65 km s^-1 Mpc^-1, Ω_m=0.3, and Ω_Λ=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 profile 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. Based in part on observations obtained at the W. M. Keck Observatory.