The VLA-COSMOS 3 GHz Large Project: Multiwavelength counterparts and the composition of the faint radio population ^⋆

¹ University of ZagrebPhysics Department Bijenička cesta 32 10002 Zagreb Croatia
e-mail: [vs;ivand]@phy.hr
² INAF–Osservatorio Astronomico di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
³ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Istituto di Radioastronomia di Bologna, INAF, via P. Gobetti, 101, 40129 Bologna, Italy
⁵ Spitzer Science Center, California Institute of Technology, 220-6, Pasadena, CA 91125, USA
⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁷ Argelander-Institut für Astronomie, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁸ Aix Marseille Université, Laboratoire d’Astrophysique de Marseille, 38 rue Frédéric Joliot-Curie, 13388 Marseille, France
⁹ Institut d’Astrophysique de Paris, Sorbonne Universités, UPMC, Univ. Paris 06 et CNRS, UMR 7095, 98bis bd Arago, 75014 Paris, France
¹⁰ Department of Physics & Astronomy, Clemson University, Clemson, SC 29634, USA
¹¹ Max Planck Institute for Extraterrestrial Physics, Giessenbachstr. 1, 85748 Garching, Germany

Received: 9 December 2016
Accepted: 12 April 2017

Abstract

We study the composition of the faint radio population selected from the Karl G. Jansky Very Large Array Cosmic Evolution Survey (VLA-COSMOS) 3 GHz Large Project, which is a radio continuum survey performed at 10 cm wavelength. The survey covers a 2.6 square degree area with a mean rms of ~ 2.3 μJy/beam, cataloging 10 830 sources above 5σ, and enclosing the full 2 square degree COSMOS field. By combining these radio data with optical, near-infrared (UltraVISTA), and mid-infrared (Spitzer/IRAC) data, as well as X-ray data (Chandra), we find counterparts to radio sources for ~93% of the total radio sample reaching out to z ≲ 6; these sources are found in the unmasked areas of the COSMOS field, i.e., those not affected by saturated or bright sources in the optical to near-infrared (NIR) bands. We further classify the sources as star-forming galaxies or AGN based on various criteria, such as X-ray luminosity; observed mid-infrared color; UV–far-infrared spectral energy distribution; rest-frame, near-UV optical color that is corrected for dust extinction; and radio excess relative to that expected from the star formation rate of the hosts. We separate the AGN into subsamples dominated by low-to-moderate and moderate-to-high radiative luminosity AGN, i.e., candidates for high-redshift analogs to local low- and high-excitation emission line AGN, respectively. We study the fractional contributions of these subpopulations down to radio flux levels of ~11 μJy at 3 GHz (or ~20 μJy at 1.4 GHz assuming a spectral index of –0.7). We find that the dominant fraction at 1.4 GHz flux densities above ~200 μJy is constituted of low-to-moderate radiative luminosity AGN. Below densities of ~100 μJy the fraction of star-forming galaxies increases to ~ 60%, followed by the moderate-to-high radiative luminosity AGN (~ 20%) and low-to-moderate radiative luminosity AGN (~ 20%). Based on this observational evidence, we extrapolate the fractions down to sensitivities of the Square Kilometer Array (SKA). Our estimates suggest that at the faint flux limits to be reached by the (Wide, Deep, and UltraDeep) SKA1 surveys, a selection based only on radio flux limits can provide a simple tool to efficiently identify samples highly (>75%) dominated by star-forming galaxies.