The far-infrared/radio correlation and radio spectral index of galaxies in the SFR–M_∗ plane up to z~2^⋆

¹ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: magnelli@astro.uni-bonn.de
² Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
³ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁴ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, Giessenbachstraße 1, 85741 Garching, Germany
⁵ Herschel Science Centre, ESAC, Villanueva de la Cañada, 28691 Madrid, Spain
⁶ Department of Physics, Virginia Tech, Blacksburg, VA 24061, USA
⁷ California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
⁸ Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁹ Center for Cosmology, Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA
¹⁰ Instituto de Física y Astronomía, Universidad de Valparaíso, Avda. Gran Bretaña 1111, 5030 Casilla Valparaíso, Chile
¹¹ Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/Service d’Astrophysique, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
¹² School of Physics and Astronomy, The Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, 69978 Tel Aviv, Israel
¹³ Astronomy Centre, Dept. of Physics & Astronomy, University of Sussex, Brighton BN1 9QH, UK
¹⁴ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
¹⁵ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹⁶ Department of Physics, University of Oxford, Keble Road, Oxford, OX1 3RH, UK
¹⁷ RAL Space, Science & Technology Facilities Council, Rutherford Appleton Laboratory, Didcot, OX11 0QX, UK
¹⁸ NASA Ames, Moffett Field, CA 94035, USA
¹⁹ Dipartimento di Astronomia, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy

Received: 6 September 2014
Accepted: 21 October 2014

Abstract

We study the evolution of the radio spectral index and far-infrared/radio correlation (FRC) across the star-formation rate – stellar masse (i.e. SFR–M_∗) plane up to z ~ 2. We start from a stellar-mass-selected sample of galaxies with reliable SFR and redshift estimates. We then grid the SFR–M_∗ plane in several redshift ranges and measure the infrared luminosity, radio luminosity, radio spectral index, and ultimately the FRC index (i.e. q_FIR) of each SFR–M_∗–z bin. The infrared luminosities of our SFR–M_∗–z bins are estimated using their stacked far-infrared flux densities inferred from observations obtained with the Herschel Space Observatory. Their radio luminosities and radio spectral indices (i.e. α, where S_ν ∝ ν^−α) are estimated using their stacked 1.4 GHz and 610 MHz flux densities from the Very Large Array and Giant Metre-wave Radio Telescope, respectively. Our far-infrared and radio observations include the most widely studied blank extragalactic fields – GOODS-N, GOODS-S, ECDFS, and COSMOS – covering a total sky area of ~2.0 deg². Using this methodology, we constrain the radio spectral index and FRC index of star-forming galaxies with M_∗ > 10¹⁰ M_⊙ and 0 <z< 2.3. We find that α^{1.4 GHz}_{610 MHz} does not evolve significantly with redshift or with the distance of a galaxy with respect to the main sequence (MS) of the SFR–M_∗ plane (i.e. Δlog (SSFR)_MS = log  [ SSFR(galaxy) /SSFR_MS(M_∗,z) ]). Instead, star-forming galaxies have a radio spectral index consistent with a canonical value of 0.8, which suggests that their radio spectra are dominated by non-thermal optically thin synchrotron emission. We find that the FRC index, q_FIR,displays a moderate but statistically significant redshift evolution as q_FIR(z) = (2.35 ± 0.08) × (1 + z)^{−0.12 ± 0.04}, consistent with some previous literature. Finally, we find no significant correlation between q_FIR and Δlog (SSFR)_MS, though a weak positive trend, as observed in one of our redshift bins (i.e. Δ [ q_FIR ]/Δ [ Δlog (SSFR)_MS ] = 0.22 ± 0.07 at 0.5 <z< 0.8), cannot be firmly ruled out using our dataset.