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Where is OH and does it trace the dark molecular gas (DMG)?

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Li, D, Tang, N, Nguyen, H, Dawson, JR, Heiles, C, Xu, D, Pan, Z, Goldsmith, PF, Gibson, SJ, Murray, CE, Robishaw, T, McClure-Griffiths, NM, Dickey, J ORCID: 0000-0002-6300-7459, Pineda, J, Stanimirovic, S, Bronfman, L and Troland, T 2018 , 'Where is OH and does it trace the dark molecular gas (DMG)?' , Astrophysical Journal Supplement Series, vol. 235, no. 1 , pp. 1-15 , doi: 10.3847/1538-4365/aaa762.

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Abstract

Hydroxyl (OH) is expected to be abundant in diffuse interstellar molecular gas because it forms along with H2 under similar conditions and forms within a similar extinction range. We have analyzed absorption measurements of OH at 1665 MHz and 1667 MHz toward 44 extragalactic continuum sources, together with the J = 1–0 transitions of 12CO, 13CO, and C18O, and the J = 2–1 transition of 12CO. The excitation temperatures of OH were found to follow a modified lognormal distribution the peak of which is close to the temperature of the Galactic emission background (CMB+synchrotron). In fact, 90% of the OH has excitation temperatures within 2 K of the Galactic background at the same location, providing a plausible explanation for the apparent difficulty of mapping this abundant molecule in emission. The opacities of OH were found to be small and to peak around 0.01. For gas at intermediate extinctions (AV ~ 0.05–2 mag), the detection rate of OH with a detection limit N(OH)  1012 cm−2 is approximately independent of AV. We conclude that OH is abundant in the diffuse molecular gas and OH absorption is a good tracer of "dark molecular gas (DMG)." The measured fraction of DMG depends on the assumed detection threshold of the CO data set. The next generation of highly sensitive low-frequency radio telescopes, such as FAST and SKA, will make feasible the systematic inventory of diffuse molecular gas through decomposing, in velocity, the molecular (e.g., OH and CH) absorption profiles toward background continuum sources with numbers exceeding what is currently available by orders of magnitude.

Item Type: Article
Authors/Creators:Li, D and Tang, N and Nguyen, H and Dawson, JR and Heiles, C and Xu, D and Pan, Z and Goldsmith, PF and Gibson, SJ and Murray, CE and Robishaw, T and McClure-Griffiths, NM and Dickey, J and Pineda, J and Stanimirovic, S and Bronfman, L and Troland, T
Keywords: radio astronomy, gas clouds, evolution, molecules
Journal or Publication Title: Astrophysical Journal Supplement Series
Publisher: Univ Chicago Press
ISSN: 0067-0049
DOI / ID Number: 10.3847/1538-4365/aaa762
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Copyright 2018 The American Astronomical Society

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