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Cool dust heating and temperature mixing in nearby star-forming galaxies

Hunt, L. K. and Draine, B. T. and Bianchi, S. and Gordon, K. D. and Aniano, G. and Calzetti, D. and Dale, D. A. and Helou, G. and Hinz, J. L. and Kennicutt, R. C. and Roussel, H. and Wilson, C. D. and Bolatto, A. and Boquien, M. and Croxall, K. V. and Galametz, M. and Gil de Paz, A. and Koda, J. and Muñoz-Mateos, J. C. and Sandstrom, K. M. and Sauvage, M. and Vigroux, L. and Zibetti, S. (2015) Cool dust heating and temperature mixing in nearby star-forming galaxies. Astronomy and Astrophysics, 576 . Art. No. A33. ISSN 0004-6361.

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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 that the 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).

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Hunt, L. K.0000-0001-9162-2371
Draine, B. T.0000-0002-0846-936X
Bianchi, S.0000-0002-4622-4240
Aniano, G.0000-0001-9015-2654
Calzetti, D.0000-0002-5189-8004
Dale, D. A.0000-0002-5782-9093
Helou, G.0000-0003-3367-3415
Kennicutt, R. C.0000-0001-5448-1821
Wilson, C. D.0000-0001-5817-0991
Bolatto, A.0000-0002-5480-5686
Boquien, M.0000-0003-0946-6176
Croxall, K. V.0000-0002-5258-7224
Galametz, M.0000-0002-0283-8689
Gil de Paz, A.0000-0001-6150-2854
Koda, J.0000-0002-8762-7863
Sandstrom, K. M.0000-0002-4378-8534
Additional Information:© 2015 ESO. Received 1 August 2014; Accepted 16 September 2014. We dedicate this paper to the memory of Charles W. Engelbracht, whose excellence and commitment as a scientist were fundamental for this work, and without whom the SPIRE images analyzed here would not have existed. We thank the anonymous referee for concise comments which improved the clarity of the paper. S.B. and L.K.H. acknowledge support from PRIN-INAF 2012/13. B.T.D. was supported in part by NSF grant AST-1408723. Use was made of the NASA/IPAC Extragalactic Database (NED).
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Istituto Nazionale di Astrofisica (INAF)UNSPECIFIED
Subject Keywords:galaxies: ISM; dust, extinction; galaxies: star formation
Record Number:CaltechAUTHORS:20150423-153439902
Persistent URL:
Official Citation:Cool dust heating and temperature mixing in nearby star-forming galaxies L. K. Hunt, B. T. Draine, S. Bianchi, K. D. Gordon, G. Aniano, D. Calzetti, D. A. Dale, G. Helou, J. L. Hinz, R. C. Kennicutt, H. Roussel, C. D. Wilson, A. Bolatto, M. Boquien, K. V. Croxall, M. Galametz, A. Gil de Paz, J. Koda, J. C. Muñoz-Mateos, K. M. Sandstrom, M. Sauvage, L. Vigroux, S. Zibetti A&A 576 A33 (2015) DOI: 10.1051/0004-6361/201424734
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:56933
Deposited By: Joanne McCole
Deposited On:24 Apr 2015 19:41
Last Modified:09 Mar 2020 13:19

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