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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 fromthe Treasury program LEGUS (Legacy ExtraGalactic UV Survey) combined witharchival HST/ACS data, we correct thousands of individual stars for extinctionacross these five galaxies using an isochrone-matching (reddening-free Q)method. We generate extinction maps for each galaxy from the individual stellarextinctions using both adaptive and fixed resolution techniques, and correlatethese maps with neutral HI and CO gas maps from literature, including The HINearby Galaxy Survey (THINGS) and the HERA CO-Line Extragalactic Survey(HERACLES). We calculate dust-to-gas ratios and investigate variations in thedust-to-gas ratio with galaxy metallicity. We find a power law relationshipbetween dust-to-gas ratio and metallicity, consistent with other studies ofdust-to-gas ratio compared to metallicity. We find a change in the relationwhen H$_2$ is not included. This implies that underestimation of $N_{H_2}$ inlow-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 betweendust-to-gas ratio and metallicity. We also compare our extinctions to thosederived from fitting the spectral energy distribution (SED) using the BayesianExtinction and Stellar Tool (BEAST) for NGC 7793 and find systematically lowerextinctions from SED-fitting as compared to isochrone matching.

Extinction Maps and Dust-to-Gas Ratios in Nearby Galaxies with LEGUS