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Description
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 responsible for heating the dust. After fitting the dust and stellar radial profiles with exponential functions, we fit the far-infrared spectral energy distribution (SED) in each annular region with single-temperature modified black bodies using both variable (MBBV) and fixed (MBBF) emissivity indices {beta}, as well as with physically motivated dust models. Results show that while most SED parameters 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}~2.1) of the physically-motivated models, they are well able to accommodate flat spectral slopes with {beta}<=1. We find that flatter slopes ({beta}<=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 Umin 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}<=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.
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