An X-Ray–Selected Galaxy Cluster at [ITAL][CLC]z[/CLC][/ITAL] = 1.11 in the [ITAL]ROSAT[/ITAL] Deep Cluster Survey

We report the discovery of an X-ray luminous galaxy cluster at z =1.11. RDCSJ0910+5422 was selected as an X-ray cluster candidate in the ROSAT Deep ClusterSurvey on the basis of its spatial extent in a Rosat PSPC image. Deep opticaland near-IR imaging reveal a red galaxy overdensity around the peak of theX-ray emission, with a significant excess of objects with J-K and I-K colorstypical of elliptical galaxies at z ~ 1.0. Spectroscopic observations at theKeck II telescope secured 9 galaxy redshifts in the range 1.095=1.106. Eight of these galaxies liewithin a 30 arcsec radius around the peak X-ray emission. A deep Chandra ACISexposure on this field shows extended X-ray morphology and allows the X-rayspectrum of the intracluster medium to be measured. The cluster has abolometric luminosity L_x = 2.48^{+0.33}_{-0.26} x 10^44 ergs/s, a temperatureof kT = 7.2^{+2.2}_{-1.4} keV, and a mass within r = 1 Mpc of 7.0 x 10^14 M_sun(H_0=65 km/s/Mpc, Omega_m = 0.3, and Lambda = 0.7). The spatial distribution ofthe cluster members is elongated, which is not due to an observationalselection effect, and followed by the X-ray morphology. The X-ray surfacebrightness profile and the spectrophotometric properties of the cluster memberssuggest that this is an example of a massive cluster in an advanced stage offormation with a hot ICM and an old galaxy population already in place at z >1.Comment: 19 pages, 7 figures: Figures 1,4,6 included as separate jpg files. Accepted for publication in The Astronomical Journa