Cool dust heating and temperature mixing in nearby star-forming galaxies^{⋆,⋆⋆,⋆⋆⋆}

¹ INAF–Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: hunt@arcetri.astro.it
² Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Sterrenkundig Observatorium, Universiteit Gent, Gent, Belgium
⁵ Institut d’Astrophysique Spatiale, bâtiment 121, Université Paris-Sud 11, CNRS UMR 8617, 91405 Orsay, France
⁶ Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
⁷ Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA
⁸ NASA Herschel Science Center, IPAC, California Institute of Technology, Pasadena, CA 91125, USA
⁹ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
¹⁰ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹¹ Institut d’Astrophysique de Paris, Sorbonne Universités, UPMC Univ. Paris 06, CNRS UMR 7095, 75014 Paris, France
¹² Department of Physics and Astronomy, McMaster University, Hamilton, ON L8S 4M1, Canada
¹³ Department of Astronomy and Laboratory for Millimeter-wave Astronomy, University of Maryland, College Park, MD 20742, USA
¹⁴ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁵ Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus OH 43210, USA
¹⁶ European Southern Observatory, Karl-Schwarzchild-Str. 2, 85748 Garching-bei-Mnchen, Germany
¹⁷ Departamento de Astrofísica, Universidad Complutense de Madrid, Avda. de la Complutense s/n, 28040 Madrid, Spain
¹⁸ Department of Physics and Astronomy, SUNY Stony Brook, Stony Brook NY 11794-3800, USA
¹⁹ European Southern Observatory, Casilla 19001, Santiago 19, Chile
²⁰ Max-Planck-Institut fur Astronomie, Konigstuhl 17, 69117 Heidelberg, Germany
²¹ Steward Observatory, University of Arizona, 933 N. Cherry Ave, Tucson AZ 85721, USA
²² CEA, Laboratoire AIM, Irfu/SAp, Orme des Merisiers, 91191 Gif-sur-Yvette, France
²³ Institut d’Astrophysique de Paris, UMR 7095 CNRS, Université Pierre et Marie Curie, 75014 Paris, France

Received: 1 August 2014
Accepted: 16 September 2014

Abstract

Physical conditions of the interstellar medium in galaxies are closely linked to the ambient radiation field and the heating of dust grains. In order to characterize dust properties in galaxies over a wide range of physical conditions, we present here the radial surface brightness profiles of the entire sample of 61 galaxies from Key Insights into Nearby Galaxies: Far-Infrared Survey with Herschel (KINGFISH). The main goal of our work is the characterization of the grain emissivities, dust temperatures, and interstellar radiation fields (ISRFs) responsible for heating the dust. We first fit the radial profiles with exponential functions in order to compare stellar and cool-dust disk scalelengths, as measured by 3.6 μm and 250 μm surface brightnesses. Our results show thatthe stellar and dust scalelengths are comparable, with a mean ratio of 1.04, although several galaxies show dust-to-stellar scalelength ratios of 1.5 or more. We then fit the far-infrared spectral energy distribution (SED) in each annular region with single-temperature modified blackbodies using both variable (MBBV) and fixed (MBBF) emissivity indices β, as well as with physically motivated dust models. The KINGFISH profiles are well suited to examining trends of dust temperature T_dust and β because they span a factor of ~200 in the ISRF intensity heating the bulk of the dust mass, U_min. Results from fitting the profile SEDs suggest that, on average, T_dust, dust optical depth τ_dust, and U_min decrease with radius. The emissivity index β also decreases with radius in some galaxies, but in others is increasing, or rising in the inner regions and falling in the outer ones. Despite the fixed grain emissivity (average β ~ 2.1) of the physically-motivated models, they are well able to accommodate flat spectral slopes with β ≲ 1. An analysis of the wavelength variations of dust emissivities in both the data and the models shows that flatter slopes (β ≲ 1.5) are associated with cooler temperatures, contrary to what would be expected from the usual T_dust – β degeneracy. This trend is related to variations in U_min since β and U_min are very closely linked over the entire range in U_min sampled by the KINGFISH galaxies: low U_min is associated with flat β ≲ 1. Both these results strongly suggest that the low apparent β values (flat slopes) in MBBV fits are caused by temperature mixing along the line of sight, rather than by intrinsic variations in grain properties. Finally, a comparison of dust models and the data show a slight ~10% excess at 500 μm for low metallicity (12 + log  (O/H) ≲ 8) and low far-infrared surface brightness (Σ₅₀₀).