Resolving the Dust-to-Metals Ratio and CO-to-H$_2$ Conversion Factor in
  the Nearby Universe
  We investigate the relationship between the dust-to-metals ratio (D/M) and
the local interstellar medium environment at ~2 kpc resolution in five nearby
galaxies: IC342, M31, M33, M101, and NGC628. A modified blackbody model with a
broken power-law emissivity is used to model the dust emission from 100 to 500
um observed by Herschel. We utilize the metallicity gradient derived from
auroral line measurements in HII regions whenever possible. Both archival and
new CO rotational line and HI 21 cm maps are adopted to calculate gas surface
density, including new wide field CO and HI maps for IC342 from IRAM and the
VLA, respectively. We experiment with several prescriptions of CO-to-H$_2$
conversion factor, and compare the resulting D/M-metallicity and D/M-density
correlations, both of which are expected to be non-negative from depletion
studies. The D/M is sensitive to the choice of the conversion factor. The
conversion factor prescriptions based on metallicity only yield too much
molecular gas in the center of IC342 to obtain the expected correlations. Among
the prescriptions tested, the one that yields the expected correlations depends
on both metallicity and surface density. The 1-$\sigma$ range of the derived
D/M spans 0.40-0.58. Compared to chemical evolution models, our measurements
suggest that the dust growth time scale is much shorter than the dust
destruction time scale. The measured D/M is consistent with D/M in
galaxy-integrated studies derived from infrared dust emission. Meanwhile, the
measured D/M is systematically higher than the D/M derived from absorption,
which likely indicates a systematic offset between the two methods.
