| Issue |
A&A
Volume 685, May 2024
|
|
|---|---|---|
| Article Number | L12 | |
| Number of page(s) | 7 | |
| Section | Letters to the Editor | |
| DOI | https://doi.org/10.1051/0004-6361/202450067 | |
| Published online | 20 May 2024 | |
Letter to the Editor
The CO-dark molecular gas in the cold H I arc
1
Institut de Radioastronomie Millimetrique, 300 Rue de la Piscine, 38400 Saint-Martin-d’Hères, France
e-mail: luo@iram.fr
2
CAS Key Laboratory of FAST, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
3
University of Chinese Academy of Sciences, Beijing 100049, PR China
4
School of Astronomy and Space Science, Nanjing University, Nanjing 210093, PR China
5
Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, PR China
6
Research Center for Astronomical Computing, Zhejiang Laboratory, Hangzhou 311100, PR China
7
Department of Physics, Anhui Normal University, Wuhu, Anhui 241002, PR China
Received:
22
March
2024
Accepted:
3
May
2024
The CO-dark molecular gas (DMG), which refers to the molecular gas not traced by CO emission, is crucial for the evolution of the interstellar medium (ISM). While the gas properties of DMG have been widely explored in the Solar neighborhood, whether or not they are similar in the outer disk regions of the Milky Way is still not well understood. In this Letter, we confirm the existence of DMG toward a cold H I arc structure at 13 kpc away from the Galactic center with both OH emission and H I narrow self-absorption (HINSA). This is the first detection of HINSA in the outer disk region, in which the HINSA fraction (NHINSA/NH2 = 0.022 ± 0.011) is an order of magnitude higher than the average value observed in nearby evolved dark clouds, but is consistent with that of the early evolutionary stage of dark clouds. The inferred H2 column density from both extinction and OH emission (NH2 ≈ 1020 cm−2) is an order of magnitude higher than previously estimated. Although the ISM environmental parameters are expected to be different between the outer Galactic disk regions and the Solar neighborhood, we find that the visual extinction (AV = 0.19 ± 0.03 mag), H2-gas density (nH2 = 91 ± 46 cm−3), and molecular fraction (58% ± 28%) of the DMG are rather similar to those of nearby diffuse molecular clouds. The existence of DMG associated with the expanding H I supershell supports a scenario where the expansion of supershells may trigger the formation of molecular clouds within a crossing timescale of the shock wave (∼106 yr).
Key words: ISM: abundances / ISM: clouds / evolution / ISM: molecules
© The Authors 2024
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