2010A&A...516A.101G

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 <2Mpc relative to the radio galaxy position and have velocity offsets <1000km/s 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∼3000km/s and approximately 1.5' east of the radio galaxy.