Dust and gas in the Magellanic Clouds from the heritage Herschel Key Project. II. gas-to-dust ratio variations across interstellar medium phases
Résumé
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 ISM, we derive the gas-to-dust ratio as the slope of the dust-gas relation and find gas-to-dust ratios of 380$^{+250}_{−130}$±3 in the LMC, and 1200$^{+1600}_{−420}$±120 in the SMC, not including helium. The atomic-to-molecular transition is located at dust surface densities of 0.05 $M_⊙$ pc$^{−2}$ in the LMC and 0.03 $M_⊙$ pc$^{−2}$ in the SMC, corresponding to A$_V$∼ 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 6×10$^{20}$ cm$^{−2}$ K$^{−1}$ km$^{−1}$ s in the LMC (Z=0.5Z$_⊙$) at 15 pc resolution, and 4×10$^{21}$ cm$^{−2}$ K$^{−1}$ km$^{−1}$ s in the SMC (Z=0.2Z$_⊙$) 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 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.
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