The VLA-COSMOS 3 GHz Large Project: The infrared-radio correlation of star-forming galaxies and AGN to z ≲ 6

¹ Department of PhysicsFaculty of Science, University of Zagreb, Bijenička cesta 32 10000 Zagreb, Croatia
e-mail: jacinta@phy.hr
² Astronomy Centre, Department of Physics & Astronomy, University of Sussex, Brighton, BN1 9QH, UK
³ Argelander Institute for Astronomy, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁴ INAF–Osservatorio Astronomico di Bologna, via Piero Gobetti 93/3, 40129 Bologna, Italy
⁵ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁶ National Radio Astronomy Observatory, 520 Edgemont Rd, Charlottesville, VA 22903, USA
⁷ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
⁸ INAF–Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
⁹ Department of Astronomy, California Institute of Technology, MC 249-17, 1200 East California Blvd, Pasadena, CA 91125, USA
¹⁰ National Radio Astronomy Observatory, Socorro, NM, USA
¹¹ Cavendish Laboratory, Cambridge, UK
¹² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹³ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
¹⁴ Institut d’Astrophysique de Paris, Sorbonne Universités, UPMC Univ. Paris 06 et CNRS, UMR 7095, 98bis bd Arago, 75014 Paris, France
¹⁵ Department of Physics and Astronomy, Clemson University, Kinard Lab. of Physics, Clemson, SC 29634-0978, USA
¹⁶ Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1312, 85741 Garching, Germany
¹⁷ International Centre for Radio Astronomy Research, Curtin University, Perth, WA 6102, Australia

Received: 29 July 2016
Accepted: 17 February 2017

Abstract

We examine the behaviour of the infrared-radio correlation (IRRC) over the range 0 <z ≲ 6 using new, highly sensitive 3 GHz observations with the Karl G. Jansky Very Large Array (VLA) and infrared data from the Herschel Space Observatory in the 2 deg² COSMOS field. We distinguish between objects where emission is believed to arise solely from star-formation, and those where an active galactic nucleus (AGN) is thought to be present. We account for non-detections in the radio or in the infrared using a doubly-censored survival analysis. We find that the IRRC of star-forming galaxies, quantified by the infrared-to-1.4 GHz radio luminosity ratio (q_TIR), decreases with increasing redshift: q_TIR(z) = (2.88 ± 0.03)(1 + z)^{− 0.19 ± 0.01}. This is consistent with several previous results from the literature. Moderate-to-high radiative luminosity AGN do not follow the same q_TIR(z) trend as star-forming galaxies, having a lower normalisation and steeper decrease with redshift. We cannot rule out the possibility that unidentified AGN contributions only to the radio regime may be steepening the observed q_TIR(z) trend of the star-forming galaxy population. We demonstrate that the choice of the average radio spectral index directly affects the normalisation, as well as the derived trend with redshift of the IRRC. An increasing fractional contribution to the observed 3 GHz flux by free-free emission of star-forming galaxies may also affect the derived evolution. However, we find that the standard (M82-based) assumption of the typical radio spectral energy distribution (SED) for star-forming galaxies is inconsistent with our results. This suggests a more complex shape of the typical radio SED for star-forming galaxies, and that imperfect K corrections in the radio may govern the derived trend of decreasing q_TIR with increasing redshift. A more detailed understanding of the radio spectrum is therefore required for robust K corrections in the radio and to fully understand the cosmic evolution of the IRRC. Lastly, we present a redshift-dependent relation between rest-frame 1.4 GHz radio luminosity and star formation rate taking the derived redshift trend into account.