GOODS-Herschel: radio-excess signature of hidden AGN activity in distant star-forming galaxies^⋆

¹ Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
e-mail: agnese.del-moro@durham.ac.uk
² Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/Service d’Astrophysique, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
³ Pontificia Universidad Católica de Chile, Departamento de Astronomía y Astrofísica, Casilla 306, Santiago 22, Chile
⁴ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA
⁵ Department of Astronomy University of Massachusetts, LGRT-B618, 710 North Pleasant Street, Amherst, MA 01003, USA
⁶ National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719, USA
⁷ Kapteyn Astronomical Institute, University of Groningen, Groningen, The Netherlands
⁸ ASTRON, Netherlands Institute for Radio Astronomy, Post box 2, 7990AA Dwingeloo, The Netherlands
⁹ Leiden Observatory, Leiden University, Post box 9513, 2300RA Leiden, The Netherlands
¹⁰ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Australia
¹¹ Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA 16802, USA
¹² Department of Physics & ITCP, University of Crete, 71003 Heraklion, Greece
¹³ US Planck Data Center, MS220-6 Caltech, Pasadena, CA 91125, USA
¹⁴ Observatoire de Paris, LERMA, CNRS, UMR 8112, 61 Av. de l’Observatoire, 75014 Paris, France
¹⁵ INAF – Osservatorio Astronomico di Bologna, via Ranzani, 1, 40127 Bologna, Italy
¹⁶ Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover, NH 03755, USA
¹⁷ Smithsonian Astrophysical Observatory, 60 Garden St., Cambridge, MA 02138, USA
¹⁸ UK Astronomy Technology Centre, Science and Technology Facilities Council, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁹ Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
²⁰ Institute for Astronomy, University of Hawaii, Manoa, HI 96822; Canada-France-Hawaii Telescope Corp., Kamuela, HI 96743, USA
²¹ Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Chinese Academy of Sciences, Hefei, 230026 Anhui, PR China

Received: 25 June 2012
Accepted: 28 September 2012

Abstract

Context. A tight correlation exists between far-infrared and radio emission for star-forming galaxies (SFGs), which seems to hold out to high redshifts (z ≈ 2). Any excess of radio emission over that expected from star formation processes is most likely produced by an active galactic nucleus (AGN), often hidden by large amounts of dust and gas. Identifying these radio-excess sources will allow us to study a population of AGN unbiased by obscuration and thus find some of the most obscured, Compton-thick AGN, which are in large part unidentified even in the deepest X-ray and infrared (IR) surveys.

Aims. We present here a new spectral energy distribution (SED) fitting approach that we adopt to select radio-excess sources amongst distant star-forming galaxies in the GOODS-Herschel (North) field and to reveal the presence of hidden, highly obscured AGN.

Methods. Through extensive SED analysis of 458 galaxies with radio 1.4 GHz and mid-IR 24 μm detections using some of the deepest Chandra X-ray, Spitzer and Herschel infrared, and VLA radio data available to date, we have robustly identified a sample of 51 radio-excess AGN (~1300 deg^-2) out to redshift z ≈ 3. These radio-excess AGN have a significantly lower far-IR/radio ratio (q < 1.68, 3σ) than the typical relation observed for star-forming galaxies (q ≈ 2.2).

Results. We find that ≈45% of these radio-excess sources have a dominant AGN component in the mid-IR band, while for the remainders the excess radio emission is the only indicator of AGN activity. The presence of an AGN is also confirmed by the detection of a compact radio core in deep VLBI 1.4 GHz observations for eight of our radio-excess sources (≈16%; ≈66% of the VLBI detected sources in this field), with the excess radio flux measured from our SED analysis agreeing, to within a factor of two, with the radio core emission measured by VLBI. We find that the fraction of radio-excess AGN increases with X-ray luminosity reaching ~60% at L_X ≈ 10⁴⁴ − 10⁴⁵ erg s^-1, making these sources an important part of the total AGN population. However, almost half (24/51) of these radio-excess AGN are not detected in the deep Chandra X-ray data, suggesting that some of these sources might be heavily obscured. Amongst the radio-excess AGN we can distinguish three groups of objects: i) AGN clearly identified in infrared (and often in X-rays), a fraction of which are likely to be distant Compton-thick AGN; ii) moderate luminosity AGN (L_X ≲ 10⁴³ erg s^-1) hosted in strong star-forming galaxies; and iii) a small fraction of low accretion-rate AGN hosted in passive (i.e. weak or no star-forming) galaxies. We also find that the specific star formation rates (sSFRs) of the radio-excess AGN are on average lower that those observed for X-ray selected AGN hosts, indicating that our sources are forming stars more slowly than typical AGN hosts, and possibly their star formation is progressively quenching.