Dust and gas in the magellanic clouds from the heritage herschel key project. II. Gas-to-dust ratio variations across interstellar medium phases

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

Research output: Contribution to journalArticlepeer-review

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 Hiα 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 pc-2 in the LMC and 0.03 M pc-2 in the SMC, corresponding to AV ∼ 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 XCO 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 XCO 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.

Original languageEnglish (US)
Article number86
JournalAstrophysical Journal
Volume797
Issue number2
DOIs
StatePublished - Dec 20 2014

Keywords

  • Dust, extinction
  • Ism: Clouds
  • Ism: Molecules
  • Ism: Structure

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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