Revealing the CO X-factor in Dark Molecular Gas through Sensitive ALMA Absorption Observations
Luo,Gan1,2; Li,Di1,2,3; Tang,Ningyu1; Dawson,J. R.4; Dickey,John M.5; Bronfman,L.6; Qin,Sheng-Li7; Gibson,Steven J.8; Plambeck,Richard9; Finger,Ricardo6; Green,Anne10; Mardones,Diego11; Koo,Bon-Chul12; Lo,Nadia6
Source PublicationThe Astrophysical Journal Letters
AbstractAbstract Carbon-bearing molecules, particularly CO, have been widely used as tracers of molecular gas in the interstellar medium (ISM). In this work, we aim to study the properties of molecules in diffuse, cold environments, where CO tends to be underabundant and/or subthermally excited. We performed one of the most sensitive (down to and ) submillimeter molecular absorption line observations toward 13 continuum sources with the ALMA. CO absorption was detected in diffuse ISM down to and HCO+ was detected down to , where atomic gas and dark molecular gas start to dominate. Multiple transitions measured in absorption toward 3C454.3 allow for a direct determination of excitation temperatures Tex of 4.1 and 2.7 K, for CO and for HCO+, respectively, which are close to the cosmic microwave background and explain their being undercounted in emission surveys. A stronger linear correlation was found between and (Pearson correlation coefficient P ~ 0.93) than that of NCO and (P ~ 0.33), suggesting HCO+ is a better tracer of H2 than CO in diffuse gas. The derived CO-to-H2?conversion factor (the CO X-factor) of (14 ± 3)?×?1020 cm?2 (K )?1 is approximately six times larger than the average value found in the Milky Way.
KeywordInterstellar medium Interstellar absorption Interstellar molecules
WOS IDIOP:2041-8205-889-1-ab6337
PublisherThe American Astronomical Society
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Document Type期刊论文
Affiliation1.CAS Key Laboratory of FAST, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, People’s Republic of China,,
2.University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
3.NAOC-UKZN Computational Astrophysics Centre, University of KwaZulu-Natal, Durban 4000, South Africa
4.Department of Physics and Astronomy and MQ Research Centre in Astronomy, Astrophysics and Astrophotonics, Macquarie University, NSW 2109, Australia
5.University of Tasmania, School of Maths and Physics, Hobart, TAS 7001, Australia
6.Astronomy Department, Universidad de Chile, Casilla 36-D, Santiago, Chile
7.Department of Astronomy, Yunnan University, and Key Laboratory of Astroparticle Physics of Yunnan Province, Kunming, 650091, People’s Republic of China
8.Western Kentucky University, Dept. of Physics and Astronomy, 1906 College Heights Boulevard, Bowling Green, KY 42101, USA
9.Radio Astronomy Laboratory, University of California, Berkeley, CA 94720, USA
10.Sydney Institute for Astronomy (SIfA), School of Physics, University of Sydney, NSW 2006, Australia
11.Departamento de Astronomía, Universidad de Chile, Casilla 36, Santiago de Chile, Chile
12.Department of Physics and Astronomy, Seoul National University Seoul 151–747, Republic of Korea
First Author AffilicationNational Astronomical Observatories, Chinese Academy of Sciences
Recommended Citation
GB/T 7714
Luo,Gan,Li,Di,Tang,Ningyu,et al. Revealing the CO X-factor in Dark Molecular Gas through Sensitive ALMA Absorption Observations[J]. The Astrophysical Journal Letters,2020,889(1).
APA Luo,Gan.,Li,Di.,Tang,Ningyu.,Dawson,J. R..,Dickey,John M..,...&Lo,Nadia.(2020).Revealing the CO X-factor in Dark Molecular Gas through Sensitive ALMA Absorption Observations.The Astrophysical Journal Letters,889(1).
MLA Luo,Gan,et al."Revealing the CO X-factor in Dark Molecular Gas through Sensitive ALMA Absorption Observations".The Astrophysical Journal Letters 889.1(2020).
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