Confirmation of a radio-selected galaxy overdensity at z=1.11

We report the discovery of a galaxy overdensity at z=1.11 associated with the z=1.110 high-redshift radio galaxy MG1 J04426+0202 (hereafter MG 0442+0202). The group, Cl 0442+0202, was found in a near-infrared survey of z>1 radio galaxies undertaken to identify spatially coincident 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<1.11. These member galaxies have broadband colors and optical spectra consistent with passively evolving elliptical galaxies formed at high redshift. Archival ROSAT observations reveal a 3 sigma detection of soft X-ray emission coincident with Cl 0442+0202 at a level 5 times greater than expected for the radio galaxy. These data suggest a rich galaxy cluster and inspired a 45 ks Chandra X-Ray Observatory observation. As expected, the radio galaxy is unresolved by Chandra but is responsible for approximately half 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 2 orders of magnitude higher than average for X-ray sources at these flux levels [S(0.5-2 keV)>5x10(-16) ergs cm(-2) s(-1)]. One of these point sources is identified with a radio-quiet type II quasar at z=1.863, akin to sources recently reported in deep Chandra surveys. The limit on an extended hot intracluster medium in the Chandra data is S(1-6 keV)<1.9x10(-15) ergs cm(-2) s(-1) (3 sigma, 30 '' radius aperture). Though the X-ray observations do not confirm the existence of a massive bound cluster at z>1, the success of the optical/near-infrared targeting of early-type systems near the radio galaxy validates searches using radio galaxies as beacons for high-redshift large-scale structure. We interpret Cl 0442+0202 as a massive cluster in the process of formation.