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) survey. Using a novel selection technique for identifying high-redshift radio galaxy (HzRG) candidates, a large sample is compiled using bright (S_1.4GHz>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 =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.

Based on observations collected at Kitt Peak National Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under cooperative agreement with the National Science Foundation.