The quest for the missing dust. II. two orders of magnitude of evolution in the dust-to-gas ratio resolved within local group galaxies

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