Spectroscopic Confirmation of A Radio - Selected Galaxy

We report the discovery of a galaxy overdensity at z = 1.11 associated with the z = 1.110 high-redshift radio galaxy MG 0442+0202. The group, CL 0442+0202, was found in a near-infrared survey of z > 1 radio galaxies undertaken to identify spatiallycoincident regions with a high density of objects red in I − K ′ color, typical of z > 1 elliptical galaxies. Spectroscopic observations from the Keck I telescope reveal five galaxies within 35 of MG 0442+0202 at 1.10 < z < 1.11. These member galaxies have broad-band colors and optical spectra consistent with passively-evolving elliptical galaxies formed at high redshift. Archival ROSAT observations reveal a 3σ detection of soft X-ray emission coincident with CL 0442+0202 at a level five times greater than expected for the radio galaxy. These data are suggestive of a rich galaxy cluster and inspired a 45 ks Chandra X-Ray Observatory observation. As expected, the radio galaxy is unresolved to Chandra, but is responsible for approximately half of the observed X-ray flux. The remaining ROSAT flux is resolved into four point sources within 15 of the radio galaxy, corresponding to a surface density two orders of magnitude higher than average for X-ray sources at these flux levels (S0.5−2keV > 5×10 −16 ergs cm s). One of these point sources is identified with a radio-quiet, type II quasar at z = 1.863, akin Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation.