Untangling the Nature of Spatial Variations of Cold Dust Properties in Star Forming Galaxies

Abstract: We investigate the far-infrared (IR) dust emission for 20 local star forming galaxies from the Key Insights on Nearby Galaxies: A Far-IR Survey with Herschel (KINGFISH) sample. We model the far-IR/submillimeter spectral energy distribution (SED) using images from Spitzer Space Telescope and Herschel Space Observatory. We calculate the cold dust temperature (T(cold)) and emissivity (beta) on a pixel by pixel basis (where each pixel ranges from 0.1-3 kpc^2) using a two temperature modified blackbody fitting routine. Our fitting method allows us to investigate the resolved nature of temperature and emissivity variations by modeling from the galaxy centers to the outskirts (physical scales of ~15-50 kpc, depending on the size of the galaxy). We fit each SED in two ways: (1) fit T(cold) and beta simultaneously, (2) hold beta constant and fit T(cold). We compare T(cold) and beta with star formation rates (calculated from L(Halpha) and L(24)), the luminosity of the old stellar population (traced through L(3.6), and the dust mass surface density (traced by 500 micron luminosity, L(500)). We find a significant trend between SFR/L(500) and T(cold), implying that the flux of hard UV photons relative to the amount of dust is significantly contributing to the heating of the cold, or diffuse, dust component. We also see a trend between L(3.6)/L(500) and beta, indicating that the old stellar population contributes to the heating at far-IR/submillimeter wavelengths. Finally, we find that when beta is held constant, T(cold) exhibits a strongly decreasing radial trend, illustrating that the shape of the far-IR SED is changing radially through a galaxy, thus confirming on a sample almost double in size the trends observed in Galametz et al. (2012).