Dust and Gas in the Magellanic Clouds from the HERITAGE Herschel Key Project. II. Gas-to-dust Ratio Variations across Interstellar Medium Phases

Roman-Duval, Julia, Gordon, Karl D., Meixner, Margaret, Bot, Caroline, Bolatto, Alberto, Hughes, Annie, Wong, Tony, Babler, Brian, Bernard, Jean-Philippe, Clayton, Geoffrey C., Fukui, Yasuo, Galametz, Maud, Galliano, Frederic, Glover, Simon, Hony, Sacha, Israel, Frank, Jameson, Katherine, Lebouteiller, Vianney, Lee, Min-Young, Li, Aigen, Madden, Suzanne, Misselt, Karl, Montiel, Edward, Okumura, Koryo, Onishi, Toshikazu, Panuzzo, Pasquale, Reach, William, Remy-Ruyer, Aurelie, Robitaille, Thomas, Rubio, Monica, Sauvage, Marc, Seale, Jonathan, Sewilo, Marta, Staveley-Smith, Lister, & Zhukovska, Svitlana
2014, The Astrophysical Journal, 797, 86

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+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-H2 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 × 1020 cm-2 K-1 km-1 s in the LMC (Z = 0.5 Z ⊙) at 15 pc resolution, and 4 × 1021 cm-2 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 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 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.


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