Spectroscopic confirmation of a galaxy cluster associated with 7C 1756+6520 at z = 1.416

¹ European Southern Observatory, Karl-Schwarzschild-Strae 2, 85748 Garching, Germany e-mail: agalamet@eso.org
² Observatoire Astronomique de Strasbourg, 11 rue de l'Université, 67000 Strasbourg, France
³ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109, USA
⁴ Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
⁵ Space Radiation Laboratory, MS 220-47, California Institute of Technology, Pasadena, CA 91125, USA

Received: 4 March 2009
Accepted: 17 April 2010

Abstract

We present spectroscopic follow-up of an overdensity of galaxies photometrically selected to be at 1.4 < z < 2.5 found in the vicinity of the radio galaxy 7C 1756+6520 at z = 1.4156. Using the DEIMOS optical multi-object spectrograph on the Keck 2 telescope, we observed a total of 129 BzK-selected sources, comprising 82 blue, star-forming galaxy candidates (sBzK) and 47 red, passively-evolving galaxy candidates (pBzK*), as well as 11 mid-infrared selected AGN candidates. We obtain robust spectroscopic redshifts for 36 blue galaxies, 7 red galaxies and 9 AGN candidates. Assuming all foreground interlopers were identified, we find that only 16% (9%) of the sBzK (pBzK*) galaxies are at z < 1.4. Therefore, the BzK criteria are shown to be relatively robust at identifying galaxies at moderate redshifts. Twenty-one galaxies, including the radio galaxy, four additional AGN candidates and three red galaxy candidates are found with 1.4156 ± 0.025, forming a large scale structure at the redshift of the radio galaxy. Of these, eight have projected offsets <2 Mpc relative to the radio galaxy position and have velocity offsets <1000 km s^-1 relative to the radio galaxy redshift. This confirms that 7C 1756+6520 is associated with a high-redshift galaxy cluster. A second compact group of four galaxies is found at z ~ 1.437, forming a sub-group offset by Δv ~ 3000 km s^-1 and approximately 1.'5 east of the radio galaxy.