Abstract

The spatial variations of the gas-to-dust ratio (GDR) provide constraints on the chemical evolution and lifecycle of dust in galaxies. We examine the relation between dust and gas at 10-50 pc resolution in the Large and Small Magellanic Clouds (LMC and SMC) based on Herschel far-infrared (FIR), Hi 21 cm, CO, and Ha observations. In the diffuse atomic interstellar medium (ISM), we derive the GDR as the slope of the dust-gas relation and find GDRs of 380(-130)(+250) +/- 3 in the LMC, and 1200(-420)(+1600) +/- 120 in the SMC, not including helium. The atomicto- molecular transition is located at dust surface densities of 0.05M(circle dot) pc(-2) in the LMC and 0.03M(circle dot) pc(-2) in the SMC, corresponding to A(V) similar to 0.4 and 0.2, respectively. We investigate the range of CO-to-H-2 conversion factor to best account for all the molecular gas in the beam of the observations, and find upper limits on X-CO to be 6 x 10(20) cm(-2) K-1 km(-1) s in the LMC (Z = 0.5Z(circle dot)) at 15 pc resolution, and 4 x 10(21) cm(-2) K-1 km(-1) s in the SMC (Z = 0.2Z(circle dot)) at 45 pc resolution. In the LMC, the slope of the dust-gas relation in the dense ISM is lower than in the diffuse ISM by a factor similar to 2, even after accounting for the effects of CO-dark H-2 in the translucent envelopes of molecular clouds. Coagulation of dust grains and the subsequent dust emissivity increase in molecular clouds, and/or accretion of gas-phase metals onto dust grains, and the subsequent dust abundance (dust-to-gas ratio) increase in molecular clouds could explain the observations. In the SMC, variations in the dust-gas slope caused by coagulation or accretion are degenerate with the effects of CO-dark H-2. Within the expected 5-20 times Galactic X-CO range, the dust-gas slope can be either constant or decrease by a factor of several across ISM phases. Further modeling and observations are required to break the degeneracy between dust grain coagulation, accretion, and CO-dark H-2. Our analysis demonstrates that obtaining robust ISM masses remains a non-trivial endeavor even in the local Universe using state-of-the-art maps of thermal dust emission.