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

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 mu m and 250 mu m 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 beta, as well as with physically motivated dust models. The KINGFISH profiles are well suited to examining trends of dust temperature T-dust and beta because they span a factor of similar to 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 tau(dust), and U-min decrease with radius. The emissivity index beta 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 beta similar to 2.1) of the physically-motivated models, they are well able to accommodate flat spectral slopes with beta less than or similar to 1. An analysis of the wavelength variations of dust emissivities in both the data and the models shows that flatter slopes (beta less than or similar to 1.5) are associated with cooler temperatures, contrary to what would be expected from the usual T-dust-beta degeneracy. This trend is related to variations in U-min since beta 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 beta less than or similar to 1. Both these results strongly suggest that the low apparent beta 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 similar to 10% excess at 500 mu m for low metallicity (12 + log (O/H) less than or similar to 8) and low far-infrared surface brightness (Sigma(500))