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

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Abstract
We have discovered a concentration of extremely red objects (EROs; R-K > 6) in the field of the z = 2.69 quasar QSO 1213-0017 (UM 485), which is a significant overabundance compared with the field ERO surface density. The optical/near-IR colors of the EROs and numerous other red galaxies in this field are consistent with elliptical galaxies at z = 1–2. HST optical images for a subset of galaxies show regular morphologies, most of them being disky or diffuse and without any obvious evidence for interactions. Ground-based IR images show similar morphologies, indicating that any dust reddening in these objects is spatially uniform. Optical spectroscopy with the W. M. Keck Telescope has found that four of the red galaxies lie at z ≈ 1.31 and a fifth lies in the foreground at z = 1.20. Of the z ≈ 1.31 galaxies, one is a reddened AGN while the remaining three have rest-frame UV absorption-line spectra characteristic of old (a few gigayears) stellar populations, similar to the old red galaxy LBDS 53W091 at z = 1.55. Including the Mg II absorber seen in the QSO spectrum, we find five galaxies at z ≈ 1.31 spread over 1.5 h Mpc on the sky. These results suggest that we have discovered a coherent structure of old galaxies at high redshift, possibly associated with a massive galaxy cluster.
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