Confirmation of a Radio-selected Galaxy Overdensity atz=1.11

We report the discovery of a galaxy overdensity at z=1.11 associated with thez=1.110 high-redshift radio galaxy MG0442+0202. The group, CL0442+0202, wasfound in a near-infrared survey of z>1 radio galaxies undertaken to identifyspatially-coincident regions with a high density of objects red in I-K' color,typical of z>1 elliptical galaxies. Spectroscopic observations from the Kecktelescope reveal five galaxies within 35" of MG0442+0202 at 1.10 5e-16 erg/cm2/s. One of these point sources is identified with aradio-quiet, typeII quasar at z=1.863, akin to sources recently reported indeep Chandra surveys. The limit on an extended hot intracluster medium in theChandra data is S(1-6keV) < 1.9e-15 erg/cm2/s (3-sigma, 30" radius aperture).Though the X-ray observations do not confirm the existence of a massive, boundcluster at z>1, the success of the optical/near-infrared targeting ofearly-type systems near the radio galaxy validates searches using radiogalaxies as beacons for high-redshift large-scale structure. We interpretCL0442+0202 to be a massive cluster in the process of formation.Comment: 23 pages, 7 figure