Modeling Dust and Starlight in Galaxies Observed by Spitzer and
  Herschel: NGC 628 and NGC 6946
  We characterize the dust in NGC628 and NGC6946, two nearby spiral galaxies in
the KINGFISH sample. With data from 3.6um to 500um, dust models are strongly
constrained. Using the Draine & Li (2007) dust model, (amorphous silicate and
carbonaceous grains), for each pixel in each galaxy we estimate (1) dust mass
surface density, (2) dust mass fraction contributed by polycyclic aromatic
hydrocarbons (PAH)s, (3) distribution of starlight intensities heating the
dust, (4) total infrared (IR) luminosity emitted by the dust, and (5) IR
luminosity originating in regions with high starlight intensity. We obtain maps
for the dust properties, which trace the spiral structure of the galaxies. The
dust models successfully reproduce the observed global and resolved spectral
energy distributions (SEDs). The overall dust/H mass ratio is estimated to be
0.0082+/-0.0017 for NGC628, and 0.0063+/-0.0009 for NGC6946, consistent with
what is expected for galaxies of near-solar metallicity. Our derived dust
masses are larger (by up to a factor 3) than estimates based on
single-temperature modified blackbody fits. We show that the SED fits are
significantly improved if the starlight intensity distribution includes a
(single intensity) "delta function" component. We find no evidence for
significant masses of cold dust T<12K. Discrepancies between PACS and MIPS
photometry in both low and high surface brightness areas result in large
uncertainties when the modeling is done at PACS resolutions, in which case
SPIRE, MIPS70 and MIPS160 data cannot be used. We recommend against attempting
to model dust at the angular resolution of PACS.
