1999AJ....118...76R

We report the discovery of an X-ray-luminous galaxy cluster at z=1.26. RX J0848.9+4452 was selected as an X-ray cluster candidate in the ROSAT Deep Cluster Survey on the basis of its spatial extent. Deep optical and near-IR imaging have revealed a galaxy overdensity around the peak of the X-ray emission, with a significant excess of red objects with J-K colors typical of elliptical galaxies at z>1. Spectroscopic observations at the Keck II Telescope have secured six galaxy redshifts in the range 1.257<z<1.267 (<z≥1.261) within a 35" radius around the peak X-ray emission. This system lies only 4'.2 away (5.0 h^–1₅₀ comoving Mpc, q₀=0.5) from the galaxy cluster ClG J0848+4453, which was identified at z=1.273 in a near-IR field galaxy survey and is also known to be X-ray luminous. Assuming that the X-ray emission is entirely due to hot intracluster gas, both these systems have similar rest-frame luminosities L_X~1x10⁴⁴ ergs.s^–1 (0.5-2.0 keV band). In combination with our spectrophotometric data for the entire 30 arcmin² field, this suggests the presence of a superstructure consisting of two collapsed, possibly virialized, clusters, the first detected to date at z>1.