The VLA-COSMOS 3 GHz Large Project: AGN and host-galaxy properties out to z ≲ 6

¹ Department of PhysicsFaculty of Science, University of Zagreb, Bijenička cesta 32, 10000 Zagreb Croatia
e-mail: ivand@phy.hr
² INAF–Osservatorio Astronomico di Bologna, via Piero Gobetti 93/3, 40129 Bologna, Italy
³ Australian Research Council Centre of Excellence for All-sky Astrophysics (CAASTRO), 44 Rosehill Street Redfern, NSW 2016, Australia
⁴ International Centre for Radio Astronomy Research, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
⁵ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road DH1 3LE, UK
⁶ Institute of Cosmology and Gravitation, University of Portsmouth, Dennis Sciama Building, Portsmouth PO1 3FX, UK
⁷ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁸ Institute for Computational Cosmology (ICC), Department of Physics, Durham University, South Road, Durham, DH1 3LE, UK
⁹ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
¹⁰ INAF–Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
¹¹ National Radio Astronomy Observatory, NM 87801 Socorro, NM, USA
¹² Cavendish Laboratory, Cambridge, CB3 0HE UK
¹³ California Institute of Technology, MC 249-17, 1200 East California Boulevard, Pasadena, CA 91125, USA
¹⁴ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁵ Department of Astronomy, California Institute of Technology, MC 249-17, 1200 East California Blvd, Pasadena, CA 91125, USA
¹⁶ Harvard Smithsonian Center for Astrophysics, 60 Garden st, Cambridge, MA 02138, USA
¹⁷ Astronomisches Institut, Ruhr-Universität Bochum, Universitätstrasse 150, 44801 Bochum, Germany
¹⁸ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
¹⁹ Institut d’Astrophysique de Paris, Sorbonne Universités, UPMC Univ Paris 06 et CNRS, UMR 7095, 98bis bd Arago, 75014 Paris, France
²⁰ Department of Physics & Astronomy, Clemson University, Clemson, SC 29634, USA
²¹ Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1312, 85741 Garching, Germany

Received: 21 July 2016
Accepted: 21 October 2016

Abstract

We explore the multiwavelength properties of AGN host galaxies for different classes of radio-selected AGN out to z ≲ 6 via a multiwavelength analysis of about 7700 radio sources in the COSMOS field. The sources were selected with the Very Large Array (VLA) at 3 GHz (10 cm) within the VLA–COSMOS 3 GHz Large Project, and cross-matched with multiwavelength ancillary data. This is the largest sample of high-redshift (z ≲ 6) radio sources with exquisite photometric coverage and redshift measurements available. We constructed a sample of moderate-to-high radiative luminosity AGN (HLAGN) via spectral energy distribution decomposition combined with standard X-ray and mid-infrared diagnostics. Within the remainder of the sample we further identified low-to-moderate radiative luminosity AGN (MLAGN) via excess in radio emission relative to the star formation rates in their host galaxies. We show that at each redshift our HLAGN havesystematically higher radiative luminosities than MLAGN and that their AGN power occurs predominantly in radiative form, while MLAGN display a substantial mechanical AGN luminosity component. We found significant differences in the host properties of the two AGN classes, as a function of redshift. At z< 1.5, MLAGN appear to reside in significantly more massive and less star-forming galaxies compared to HLAGN. At z> 1.5, we observed a reversal in the behaviour of the stellar mass distributions with the HLAGN populating the higher stellar mass tail. We interpret this finding as a possible hint of the downsizing of galaxies hosting HLAGN, with the most massive galaxies triggering AGN activity earlier than less massive galaxies, and then fading to MLAGN at lower redshifts. Our conclusion is that HLAGN and MLAGN samples trace two distinct galaxy and AGN populations in a wide range of redshifts, possibly resembling the radio AGN types often referred to as radiative- and jet-mode (or high- and low-excitation), respectively, whose properties might depend on the different availability of cold gas supplies.