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 Lx = 2.48 × 1044 ergs s-1, temperature kT = 7.2 keV, and mass within r = 1 Mpc of 7.0 × 1014 M⊙ (H0 = 65 km s-1 Mpc-1, Ωm = 0.3, and ΩΛ = 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.