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

Smolčić, Vernesa and Delvecchio, I. and Zamorani, G. and Baran, Nikola and Novak, M. and Delhaize, J. and Schinnerer, E. and Berta, S. and Bondi, M. and Ciliegi, P. and Capak, P. and Civano, F. and Karim, A. and Le Fevre, O. and Ilbert, O. and Laigle, C. and Marchesi, S. and McCracken, H. J. and Tasca, L. and Salvato, M. and Vardoulaki, E. (2017) The VLA-COSMOS 3 GHz Large Project: Multiwavelength counterparts and the composition of the faint radio population. Astronomy and Astrophysics, 602. Art. No. 2-21. ISSN 0004-6361

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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 highredshift 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.

Item Type: Article
Keywords: radio continuum, galaxies – catalogs
Date: June 2017
Subjects: NATURAL SCIENCES > Physics > Astronomy and Astrophysics
Additional Information: © ESO 2017. Received 9 December 2016. Accepted 12 April 2017. Published online 13 June 2017.Acknowledgements. Zgal group: European Union’s Seventh Framework program under grant agreement 333654 (CIG, “AGN feedback”; N.B., V.S.) and grant agreement 337595 (ERC Starting Grant, “CoSMass”; V.S., I.D., J.D., M.N.). C.L. acknowledges the funding by a Discovery Early Career Researcher Award (DE150100618). Partially based on data obtained with the European Southern Observatory Very Large Telescope, Paranal, Chile, under Large Programs 175.A-0839 (the zCOSMOS Spectroscopic Redshift Survey) and 185.A-0791 (the VIMOS Ultra Deep Survey). A. K. and E. V. acknowledge support from the collaborative research center 956 (sub-project A1) funded by the Deutsche Forschungsgemeinschaft (DFG).
Divisions: Faculty of Science > Department of Physics
Publisher: EDP Sciences
Depositing User: Vernesa Smolčić
Date Deposited: 06 Nov 2017 14:25
Last Modified: 06 Nov 2017 14:25

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