The Quest for the Missing Dust. II. Two Orders of Magnitude of Evolution in the Dust-to-gas Ratio Resolved within Local Group Galaxies

Abstract We explore evolution in the dust-to-gas ratio with density within four well-resolved Local Group galaxies—the LMC, SMC, M31, and M33. do this using new Herschel maps, which restore extended emission that was missed by previous reductions. Combining sensitivity to diffuse dust excellent physical resolution allows us probe across 2.5 orders of magnitude interstellar medium (ISM) surface density. find a significant increase density, varying each galaxy up factor 22.4, as changes. several possible reasons for this, our favored explanation is it being driven grain growth denser regions ISM. ISM very similar between M31 M33, despite their large differences mass, metallicity, star formation rate; conversely, we M33 LMC have different profiles, close similarity those properties. Our ratios address disagreement UV- far-IR-based estimates Magellanic Clouds, removing considerably reducing SMC—with measurements factors 2.4 2.0 greater than far-IR estimates, respectively. also observe appears fall at highest densities M33; unlikely be an actual phenomenon, posit may due combined effect dark gas, changing mass opacity.