Title

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

Authors

Julia Roman-Duval, Space Telescope Science Institute
Karl D. Gordon, Space Telescope Science Institute
Margaret Meixner, Space Telescope Science Institute
Caroline Bot, Université de Strasbourg
Alberto Bolatto, University of Maryland, College Park
Annie Hughes, Max Planck Institute for Astronomy
Tony Wong, University of Illinois Urbana-Champaign
Brian Babler, University of Wisconsin-Madison
Jean Philippe Bernard, Institut de Recherche en Astrophysique et Planétologie (IRAP)
Geoffrey C. Clayton, Louisiana State University
Yasuo Fukui, Nagoya University
Maud Galametz, European Southern Observatory
Frederic Galliano, Astrophysique, Instrumentation et Modélisation de Paris-Saclay
Simon Glover, Universität Heidelberg
Sacha Hony, Max Planck Institute for Astronomy
Frank Israel, Leiden Observatory Research Institute
Katherine Jameson, University of Maryland, College Park
Vianney Lebouteiller, Astrophysique, Instrumentation et Modélisation de Paris-Saclay
L. E.E. Min-Young, Astrophysique, Instrumentation et Modélisation de Paris-Saclay
Aigen Li, University of Missouri
Suzanne Madden, Astrophysique, Instrumentation et Modélisation de Paris-Saclay
Karl Misselt, The University of Arizona
Edward Montiel, Louisiana State University
Koryo Okumura, Astrophysique, Instrumentation et Modélisation de Paris-Saclay
Toshikazu Onishi, Osaka Prefecture University
Pasquale Panuzzo, L'Observatoire de Paris
William Reach, Universities Space Research Association
Aurelie Remy-Ruyer, Astrophysique, Instrumentation et Modélisation de Paris-Saclay
Thomas Robitaille, Max Planck Institute for Astronomy
Monica Rubio, Universidad de Chile
Marc Sauvage, Astrophysique, Instrumentation et Modélisation de Paris-Saclay
Jonathan Seale, Johns Hopkins University
Marta Sewilo, Johns Hopkins University

Document Type

Article

Publication Date

12-20-2014

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.

Publication Source (Journal or Book title)

Astrophysical Journal

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