2014ApJ...797...86R

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), H I 21 cm, CO, and Hα 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 atomic-to-molecular transition is located at dust surface densities of 0.05 M_☉/pc2 in the LMC and 0.03 M_☉/pc2 in the SMC, corresponding to A_V∼ 0.4 and 0.2, respectively. We investigate the range of CO-to-H₂ conversion factor to best account for all the molecular gas in the beam of the observations, and find upper limits on X_COto be 6x10²⁰/cm2.K^–1.km^–1 s in the LMC (Z = 0.5 Z_☉) at 15 pc resolution, and 4x10²¹/cm2.K^–1.km^–1 s in the SMC (Z = 0.2 Z_☉) 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 ∼2, even after accounting for the effects of CO-dark H₂ 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 H₂. Within the expected 5-20 times Galactic X_COrange, 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 H₂. 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.