Extinction Maps and Dust-to-Gas Ratios in Nearby Galaxies with LEGUS
  We present a study of the dust-to-gas ratios in five nearby galaxies NGC 628
(M74), NGC 6503, NGC 7793, UGC 5139 (Holmberg I), and UGC 4305 (Holmberg II).
Using Hubble Space Telescope broad band WFC3/UVIS UV and optical images from
the Treasury program LEGUS (Legacy ExtraGalactic UV Survey) combined with
archival HST/ACS data, we correct thousands of individual stars for extinction
across these five galaxies using an isochrone-matching (reddening-free Q)
method. We generate extinction maps for each galaxy from the individual stellar
extinctions using both adaptive and fixed resolution techniques, and correlate
these maps with neutral HI and CO gas maps from literature, including The HI
Nearby Galaxy Survey (THINGS) and the HERA CO-Line Extragalactic Survey
(HERACLES). We calculate dust-to-gas ratios and investigate variations in the
dust-to-gas ratio with galaxy metallicity. We find a power law relationship
between dust-to-gas ratio and metallicity, consistent with other studies of
dust-to-gas ratio compared to metallicity. We find a change in the relation
when H$_2$ is not included. This implies that underestimation of $N_{H_2}$ in
low-metallicity dwarfs from a too-low CO-to-H$_2$ conversion factor $X_{CO}$
could have produced too low a slope in the derived relationship between
dust-to-gas ratio and metallicity. We also compare our extinctions to those
derived from fitting the spectral energy distribution (SED) using the Bayesian
Extinction and Stellar Tool (BEAST) for NGC 7793 and find systematically lower
extinctions from SED-fitting as compared to isochrone matching.
