Large scale structures around radio galaxies at z ~ 1.5^*

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany e-mail: agalamet@eso.org
² Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109, USA
³ Observatoire Astronomique de Strasbourg, 11 rue de lUniversité, 67000 Strasbourg, France
⁴ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
⁵ Institut d'Astrophysique de Paris, CNRS, Université Pierre et Marie Curie, Paris, France
⁶ Mullard Space Science Laboratory, UCL, Holmbury St Mary, Dorking, Surrey, RH5 6NT, UK
⁷ Spitzer Science Centre, Caltech, 1200 East California Boulevard, Pasadena, CA 91125, USA

Received: 25 March 2009
Accepted: 27 August 2009

Abstract

We explore the environments of two radio galaxies at z ~ 1.5, 7C 1751+6809 and 7C 1756+6520, using deep optical and near-infrared imaging. Our data cover 1515 arcmin² fields around the radio galaxies. We develop and apply BzK color criteria to select cluster member candidates around the radio galaxies and find no evidence of an overdensity of red galaxies within 2 Mpc of 7C 1751+6809. In contrast, 7C 1756+6520 shows a significant overdensity of red galaxies within 2 Mpc of the radio galaxy, by a factor of 3.1±0.8 relative to the four MUSYC fields. At small separation (r < 6″), this radio galaxy also has one  evolved galaxy candidate, one z > 1.4 star-forming galaxy candidate, and an AGN candidate (at indeterminate redshift). This is suggestive of several close-by companions. Several concentrations of red galaxies are also noticed in the full 7C 1756+6520 field, forming a possible large-scale structure of evolved galaxies with a NW-SE orientation. We construct the color-magnitude diagram of red galaxies found near 7C 1756+6520 (r < 2 Mpc), and find a clear red sequence that is truncated at K_s ~ 21.5 (AB). We also find an overdensity of mid-infrared selected AGN in the surroundings of 7C 1756+6520. These results are suggestive of a proto-cluster at high redshift.