Spectroscopic confirmation of a galaxy cluster associated with
  7C1756+6520 at z=1.416
  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 7C1756+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 7C1756+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 Dv~3000km/s and
approximately 1.5' east of the radio galaxy.
