Extremely Red Objects in the Field of QSO 1213−0017: A Galaxy Concentration at [ITAL][CLC]z[/CLC][/ITAL] = 1.31

We have discovered a concentration of extremely red objects (EROs; R-K>6) inthe field of the z=2.69 quasar QSO 1213-0017 (UM 485), which is significantlyoverabundant compared to the field ERO surface density. The optical/near-IRcolors of the EROs and numerous other red galaxies in this field are consistentwith elliptical galaxies at z=1-2. HST optical images for a subset of galaxiesshow regular morphologies, most of them being disky or diffuse and without anyobvious evidence for interactions. Ground-based IR images show similarmorphologies, indicating any dust reddening in these objects is spatiallyuniform. Optical spectroscopy with the W. M. Keck Telescope has found that fourof the red galaxies lie at =1.31, and a fifth lies in the foreground atz=1.20. Of the =1.31 galaxies, one is a reddened AGN while the remainingthree have rest-frame UV absorption-line spectra characteristic of old (fewGyr) stellar populations, similar to the old red galaxy LBDS 53W091 at z=1.55.Including the MgII absorber seen in the QSO spectrum, we find five galaxies at=1.31 spread over 1.5 h_50^{-1} Mpc on the sky. These results suggest wehave discovered a coherent structure of old galaxies at high-redshift, possiblyassociated with a massive galaxy cluster.Comment: 37 pages including 11 Postscript figures. To appear in the June 2000 issue of the Astronomical Journa