Letter to the Editor

The far-infrared/radio correlation as probed by Herschel^*

¹ UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK e-mail: rji@roe.ac.uk
² Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
³ Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1312, 85741, Garching, Germany
⁴ ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁵ INAF - Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
⁶ Laboratoire AIM-Paris-Saclay, CEA/DSM/Irfu – CNRS – Université Paris Diderot, CE-Saclay, pt courrier 131, 91191 Gif-sur-Yvette, France
⁷ Herschel Science Centre, European Space Astronomy Centre, Villanueva de la Cañada, 28691 Madrid, Spain
⁸ Department of Physics & Astronomy, University of California, Irvine, CA 92697, USA
⁹ Cardiff School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
¹⁰ Astrophysics Group, Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK
¹¹ California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
¹² Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹³ Instituto de Astrofísica de Canarias (IAC) and Departamento de Astrofísica, Universidad de La Laguna (ULL), La Laguna, Tenerife, Spain
¹⁴ Laboratoire d'Astrophysique de Marseille, OAMP, Université Aix-marseille, CNRS, 38 rue Frédéric Joliot-Curie, 13388 Marseille cedex 13, France
¹⁵ Dipartimento di Astronomia, Università di Bologna, Via Ranzani 1, 40127 Bologna, Italy
¹⁶ Department of Astrophysical and Planetary Sciences, CASA 389-UCB, University of Colorado, Boulder, CO 80309, USA
¹⁷ INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, I-40127 Bologna, Italy
¹⁸ Observational Cosmology Laboratory, Code 665, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁹ Astronomy Centre, Department of Physics & Astronomy, University of Sussex, Brighton BN1 9QH, UK
²⁰ Dipartimento di Astronomia, Università di Padova, vicolo Osservatorio, 3, 35122 Padova, Italy
²¹ INAF - Osservatorio Astronomico di Roma, via di Franscati 33, 00040 Monte Porzio Catone, Italy
²² Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada
²³ Institut d'Astrophysique Spatiale (IAS), bâtiment 121, Université Paris-Sud 11 and CNRS (UMR 8617), 91405 Orsay, France
²⁴ Infrared Processing and Analysis Center, MS 100-22, California Institute of Technology, JPL, Pasadena, CA 91125, USA
²⁵ School of Physics and Astronomy, The University of Manchester, Alan Turing Building, Oxford Road, Manchester M13 9PL, UK
²⁶ Institute for Astronomy, University of Hawaii, Honolulu, HI 96822, USA
²⁷ Canada-France-Hawaii Telescope, Kamuela, HI, 96743, USA
²⁸ Institut d'Astrophysique de Paris, UMR 7095, CNRS, UPMC Univ. Paris 06, 98bis boulevard Arago, 75014 Paris, France
²⁹ National Radio Astronomy Observatory, PO Box O, Socorro NM 87801, USA
³⁰ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
³¹ Space Science and Technology Department, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK
³² Institute for Space Imaging Science, University of Lethbridge, Lethbridge, Alberta T1K 3M4, Canada
³³ Astrophysics, Oxford University, Keble Road, Oxford OX1 3RH, UK
³⁴ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, Hertfordshire AL10 9AB, UK

Received: 30 March 2010
Accepted: 23 April 2010

Abstract

We set out to determine the ratio, q_IR, of rest-frame 8–1000-μm flux, S_IR, to monochromatic radio flux, S_{1.4 GHz}, for galaxies selected at far-infrared (IR) and radio wavelengths, to search for signs that the ratio evolves with redshift, luminosity or dust temperature, T_d, and to identify any far-IR-bright outliers – useful laboratories for exploring why the far-IR/radio correlation (FIRRC) is generally so tight when the prevailing theory suggests variations are almost inevitable. We use flux-limited 250-μm and 1.4-GHz samples, obtained using Herschel and the Very Large Array (VLA) in GOODS-North (-N). We determine bolometric IR output using ten bands spanning λ_obs = 24–1250 μm, exploiting data from PACS and SPIRE (PEP; HerMES), as well as Spitzer, SCUBA, AzTEC and MAMBO. We also explore the properties of an L_IR-matched sample, designed to reveal evolution of q_IR with redshift, spanning log L_IR = 11–12  and z = 0–2, by stacking into the radio and far-IR images. For 1.4-GHz-selected galaxies in GOODS-N, we see tentative evidence of a break in the flux ratio, q_IR, at L_{1.4 GHz} ~ 10^22.7 W Hz^-1, where active galactic nuclei (AGN) are starting to dominate the radio power density, and of weaker correlations with redshift and T_d. From our 250-μm-selected sample we identify a small number of far-IR-bright outliers, and see trends of q_IR with L_{1.4 GHz}, L_IR, T_d and redshift, noting that some of these are inter-related. For our L_IR-matched sample, there is no evidence that q_IR changes significantly as we move back into the epoch of galaxy formation: we find q_IR (1+z)^γ, where γ = -0.04±0.03 at z = 0–2; however, discounting the least reliable data at z < 0.5 we find γ = -0.26±0.07, modest evolution which may be related to the radio background seen by ARCADE 2, perhaps driven by <10-μJy radio activity amongst ordinary star-forming galaxies at z>1.