Two Orders of Magnitude of Evolution in the Dust-to-Gas Ratio Resolved Within Local Group Galaxies

We have explored the evolution of the dust-to-gas ratio (D/G) with density for four well-resolved galaxies in the local group – the LMC, SMC, M31, and M33. We do this using new Herschel maps of these galaxies, which restore extended dust emission that had been missed by previous Herschel reductions. Combining this sensitivity to diffuse dust emission with excellent angular – and hence physical – resolution, allows us to probe D/G across 2.5 orders of magnitude in ISM surface density. We find very strong evolution in D/G, finding that D/G can vary with density by up to a factor of 22 within a galaxy (for the SMC). This is strong evidence for very high levels of dust grain growth in the denser regions of the ISM. We find M31 and M33 have very similar D/G evolution profiles, despite their large differences in mass, metallicity, and star formation rate; conversely, we find M33 and the LMC to have very different D/G evolution profiles, despite their close similarity in those same traits. Our D/G address previous disagreement between UV- and FIR- based D/G estimates for the Magellanic Clouds, removing the disagreement for the LMC, and considerably reducing it for the SMC – with our new D/G measurements being factors of 2.4 and 2.0 greater than the previous far-infrared estimates, respectively. This indicates how insufficient density and spatial resolution and lead to D/G ratios being underestimated, especially at lower metallicities, with implications for the vast majority of low-metallicity galaxies for which available far-infrared data is poorly- or un-resolved.