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# Extinction maps and dust-to-gas ratios in nearby galaxies with LEGUS.

Kahre, L. and Walterbos, R. A. and Kim, H. and Thilker, D. and Calzetti, D. and Lee, J. C. and Sabbi, E. and Ubeda, L. and Aloisi, A. and Cignoni, M. and Cook, D. O. and Dale, D. A. and Elmegreen, B. G. and Elmegreen, D. M. and Fumagalli, M. and Gallagher, J. S. III and Gouliermis, D. A. and Grasha, K. and Grebel, E. K. and Hunter, D. A. and Sacchi, E. and Smith, L. J. and Tosi, M. and Adamo, A. and Andrews, J. E. and Ashworth, G. and Bright, S. N. and Brown, T. M. and Chandar, R. and Christian, C. and de Mink, S. E. and Dobbs, C. and Evans, A. S. and Herrero, A. and Johnson, K. E. and Kennicutt, R. C. and Krumholz, M. R. and Messa, M. and Nair, P. and Nota, A. and Pellerin, A. and Ryon, J. E. and Schaerer, D. and Shabani, F. and Van Dyk, S. D. and Whitmore, B. C. and Wofford, A. (2018) 'Extinction maps and dust-to-gas ratios in nearby galaxies with LEGUS.', Astrophysical journal., 855 (2). p. 133.

## Abstract

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 broadband WFC3/UVIS UV and optical images from the Treasury program Legacy ExtraGalactic UV Survey (LEGUS) combined with archival HST/Advanced Camera for Surveys 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 H i and CO gas maps from the literature, including the H i Nearby Galaxy Survey and the HERA CO-Line Extragalactic Survey. 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 H2 is not included. This implies that underestimation of ${N}_{{{\rm{H}}}_{2}}$ in low-metallicity dwarfs from a too-low CO-to-H2 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 for NGC 7793 and find systematically lower extinctions from SED fitting as compared to isochrone matching.