Sensitive radio survey of obscured quasar candidates

Abstract

We study the radio properties of moderately obscured quasars in samples at both low (z similar to 0.5) and high (z similar to 2.5) redshift to understand the role of radio activity in accretion, using the Karl G. Jansky Very Large Array (VLA) at 6.0 GHz and 1.4 GHz. Our z similar to 2.5 sample consists of optically selected obscured quasar candidates, all of which are radio-quiet, with typical radio luminosities of nu L-nu [1.4 GHz] less than or similar to 10(40) erg s(-1). Only a single source is individually detected in our deep (rms similar to 10 mu Jy) exposures. This population would not be identified by radio-based selection methods used for distinguishing dusty star-forming galaxies and obscured active nuclei. In our pilot A-array study of z similar to 0.5 radio-quiet quasars, we spatially resolve four of five objects on scales similar to 5 kpc and find they have steep spectral indices with an average value of alpha = -0.75. Therefore, radio emission in these sources could be due to jet-driven or radiatively driven bubbles interacting with interstellar material on the scale of the host galaxy. Finally, we also study the additional population of similar to 200 faint (similar to 40 mu Jy-40 mJy) field radio sources observed over similar to 120 arcmin(2) of our data. 60 per cent of these detections (excluding our original targets) are matched in the Sloan Digital Sky Survey (SDSS) and/or Wide-Field Infrared Survey Explorer (WISE) and are, in roughly equal shares, active galactic nuclei (AGN) at a broad range of redshifts, passive galaxies with no other signs of nuclear activity and infrared-bright but optically faint sources. Spectroscopically or photometrically confirmed star-forming galaxies constitute only a smallminority of the matches. Such sensitive radio surveys allow us to address important questions of AGN evolution and evaluate the AGN contribution to the radio-quiet sky.