An X-Ray-Selected Galaxy Cluster at z = 1.26^{* **}

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 42 away (5.0 h 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 ≈ 1 × 10⁴⁴ 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.