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 PSPC 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 \sim 1 .
Spectroscopic observations at the Keck II telescope secured 9 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 arcsec 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 a bolometric luminosity L _ { x } = 2.48 ^ { +0.33 } _ { -0.26 } \times 10 ^ { 44 } ergs s ^ { -1 } , a temperature of kT = 7.2 ^ { +2.2 } _ { -1.4 } keV , and a mass within r = 1 Mpc of 7.0 \times 10 ^ { 14 } M _ { \sun } ( 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 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 ICM and an old galaxy population already in place at z > 1 .