The DRaGONS survey: a search for high-redshift radio galaxies and heavily obscured active Galactic nuclei

We present the first results from the Distant Radio Galaxies Optically Non-detected in the SDSS (DRaGONS) urvey. Using a novel selection technique for identifying high-redshift radio galaxy (HzRG) candidates, a large sample is compiled using bright (S_{1.4 GHz} > 100 mJy) radio sources from the FIRST survey having no optical counterpart in the SDSS. Near-IR K-band imaging for 96 targets allows preliminary identification of HzRG candidates through the well-known K-z relation, for subsequent spectroscopic observation. We measure magnitudes brighter than K ≈ 19.5 for 70 of the 96, and limiting magnitudes for the remainder. Redshifts based on a linear fit to the K-z diagram give a mean 〈z〉 = 2.5 and median z = 2.0, showing that this method should be efficient at identifying a large number of HzRGs. This selection is also sensitive to a previously unseen population of anomalously red radio galaxies (r -K > 6.5-7), which may indicate significant obscuration at moderate redshifts. These account for>10% of our sample and may represent a sizable radio-loud population missing from current optically selected AGN samples. We also identify 479 EROs to K = 17.5 in the fields surrounding our HzRG candidates, in a noncontiguous area of 2.38 deg². There is a small overdensity of EROs surrounding the radio galaxy candidates over random fields and some evidence for a physical association of the EROs with the radio galaxies. The clustering of all K ≤ 19.0 galaxies around the radio targets, however, reveals no evidence of a global galaxy excess, strengthening our conclusion that the EROs trace an overdensity not evident in the overall galaxy population.