Volume 573, January 2015
|Number of page(s)||18|
|Published online||15 December 2014|
The far-infrared/radio correlation and radio spectral index of galaxies in the SFR–M∗ plane up to z~2⋆
Argelander-Institut für Astronomie, Universität Bonn,
Auf dem Hügel 71, 53121
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
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. qFIR) 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 Sν ∝ 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 α1.4 GHz610 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) /SSFRMS(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, qFIR,displays a moderate but statistically significant redshift evolution as qFIR(z) = (2.35 ± 0.08) × (1 + z)−0.12 ± 0.04, consistent with some previous literature. Finally, we find no significant correlation between qFIR and Δlog (SSFR)MS, though a weak positive trend, as observed in one of our redshift bins (i.e. Δ [ qFIR ]/Δ [ Δlog (SSFR)MS ] = 0.22 ± 0.07 at 0.5 <z< 0.8), cannot be firmly ruled out using our dataset.
Key words: galaxies: evolution / galaxies: formation / galaxies: starburst / galaxies: high-redshift / infrared: galaxies
© ESO, 2014
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