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 black bodies 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 examine trends of dust temperature T _ { dust } and \beta because they span a factor of \sim 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 \sim 2.1 ) of the physically-motivated models , they are well able to accommodate flat spectral slopes with \beta \la 1 .
An analysis of the wavelength variations of dust emissivities in both the data and the models shows that flatter slopes ( \beta \la 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 \la 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 \sim 10 % excess at 500 \mu m for low metallicity ( 12 + log ( O/H ) \la 8 ) and low far-infrared surface brightness ( \Sigma _ { 500 } ) .