Mid- to far-infrared properties of star-forming galaxies and active galactic nuclei^⋆,⋆⋆

¹ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
e-mail: georgios.magdis@astro.ox.ac.uk
² Space Science & Technology Department, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK
³ Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA
⁴ Virginia Polytechnic Institute and State University Department of Physics, MC 0435, 910 Drillfield Drive, Blacksburg, VA 24061, USA
⁵ Astronomy Centre, Department of Physics and Astronomy, University of Sussex, Falmer, Brighton BN1 9QH, UK
⁶ Instituto de Fisica de Cantabria, CSIC-UC, 39005 Santander, Spain
⁷ Augusto G. Linares Senior Research Fellow  
⁸ California Institute of Technology, 1200 E. California Boulevard, Pasadena, CA 91125, USA
⁹ Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹⁰ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille), 13388 Cedex 13 Marseille, France
¹¹ Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada
¹² Department of Physics and ITCP, University of Crete, 71003 Heraklion, Greece
¹³ IESL/Foundation for Research and Technology – Hellas, 71110 Heraklion, Greece
¹⁴ Observatoire de Paris, 75014 Paris, France
¹⁵ Department of Physics & Astronomy, University of California, Irvine, CA 92697, USA
¹⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS65, Cambridge, MA 02138, USA
¹⁷ Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA
¹⁸ Laboratoire AIM-Paris-Saclay, CEA/DSM/Irfu, CNRS, Université Paris Diderot, Saclay, 19190 Gif-sur-Yvette, France
¹⁹ ESO, Karl-Schwarzschild-Str. 2 85748 Garching bei Munchen, Germany
²⁰ Dipartimento di Fisica e Astronomia, Vicolo Osservatorio 2, 35122 Padova, Italy
²¹ National Astronomical Observatories, Chinese Academy of Sciences, 100012 Beijing, PR China
²² Institute for Astronomy, University of Hawaii, Honolulu, HI 968226, USA
²³ Canada-France-Hawaii Telescope, Kamuela, HI 96743, USA
²⁴ Mullard Space Science Laboratory, University College London, Holmbury St Mary Dorking, Surrey RH5 6NT, WCIE6 BT London, UK
²⁵ Department of Physics & Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada
²⁶ School of Astronomy And Space Science, Nanjing University, Jiangsu 210093, PR China

Received: 8 July 2013
Accepted: 23 August 2013

Abstract

We study the mid- to far-IR properties of a 24 μm-selected flux-limited sample (S₂₄> 5 mJy) of 154 intermediate redshift (⟨ z ⟩  ~ 0.15), infrared luminous galaxies, drawn from the 5 Milli-Jansky Unbiased Spitzer Extragalactic Survey. By combining existing mid-IR spectroscopy and new Herschel SPIRE submm photometry from the Herschel Multi-tiered Extragalactic Survey, we derived robust total infrared luminosity (L_IR) and dust mass (M_dust) estimates and infered the relative contribution of the AGN to the infrared energy budget of the sources. We found that the total (8−1000 μm) infrared emission of galaxies with weak 6.2 μm PAH emission (EW_6.2 ≤ 0.2 μm) is dominated by AGN activity, while for galaxies with EW_6.2> 0.2 μm more than 50% of the L_IR arises from star formation. We also found that for galaxies detected in the 250–500 μm Herschel bands an AGN has a statistically insignificant effect on the temperature of the cold dust and the far-IR colours of the host galaxy, which are primarily shaped by star formation activity. For star-forming galaxies we reveal an anti-correlation between the L_IR-to-rest-frame 8 μm luminosity ratio, IR8 ≡ L_IR/L₈  and the strength of PAH features. We found that this anti-correlation is primarily driven by variations in the PAHsemission, and not by variations in the 5−15 μm mid-IR continuum emission. Using the [Ne iii]/[Ne ii] line ratio as a tracer of the hardness of the radiation field, we confirm that galaxies with harder radiation fields tend to exhibit weaker PAH features, and found that they have higher IR8 values and higher dust-mass-weighted luminosities (L_IR/M_dust), the latter being a proxy for the dust temperature (T_d). We argue that these trends originate either from variations in the environment of the star-forming regions or are caused by variations in the age of the starburst. Finally, we provide scaling relations that will allow estimating L_IR, based on single-band observations with the mid-infrared instrument, on board the upcoming James Webb Space Telescope.