The VLA Survey of the Chandra Deep Field South. IV. Source Population

TL;DR

This study analyzes 256 faint radio sources in the Chandra Deep Field South using multi-wavelength data, revealing a significant population of distant AGN and clarifying the composition of sub-millijansky radio sources.

Contribution

It provides a detailed characterization of faint radio sources, highlighting the importance of X-ray data and uncovering a substantial population of distant AGN missed in previous surveys.

Findings

Faint radio sources include a significant population of distant AGN.

Star-forming galaxies and AGN are roughly equal in the sub-millijansky regime.

Approximately 20% of sources are radio-quiet AGN.

Abstract

We present a detailed analysis of 256 radio sources from our deep (flux density limit of 42 microJy at the field centre at 1.4 GHz) Chandra Deep Field South 1.4 and 5 GHz VLA survey. The radio population is studied by using a wealth of multi-wavelength information in the radio, optical, and X-ray bands. The availability of redshifts for ~ 80% of the sources in our complete sample allows us to derive reliable luminosity estimates for the majority of the objects. X-ray data, including upper limits, for all our sources turn out to be a key factor in establishing the nature of faint radio sources. Due to the faint optical levels probed by this study, we have uncovered a population of distant Active Galactic Nuclei (AGN) systematically missing from many previous studies of sub-millijansky radio source identifications. We find that, while the well-known flattening of the radio number counts below 1 mJy is mostly due to star forming galaxies, these sources and AGN make up an approximately equal fraction of the sub-millijansky sky, contrary to some previous results. The AGN include radio galaxies, mostly of the low-power, Fanaroff-Riley I type, and a significant radio-quiet component, which amounts to approximately one fifth of the total sample. The ratio of radio to optical luminosity depends more on radio luminosity, rather than being due to optical absorption.