Issue |
A&A
Volume 698, May 2025
|
|
---|---|---|
Article Number | A54 | |
Number of page(s) | 14 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/202453007 | |
Published online | 03 June 2025 |
Molecular clouds at the edge of the Galaxy
I. Variation in the CO J = 2–1/1–0 line ratio
1
Xinjiang Astronomical Observatory, Chinese Academy of Sciences,
Urumqi
830011,
PR China
2
University of the Chinese Academy of Sciences,
Beijing
100080,
PR China
3
Key Laboratory of Radio Astronomy and Technology, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District,
Beijing
100101,
PR China
4
Xinjiang Key Laboratory of Radio Astrophysics,
Urumqi
830011,
PR China
5
Max-Planck-Institut für Radioastronomie,
Auf dem Hügel 69,
53121
Bonn,
Germany
6
Purple Mountain Observatory, Chinese Academy of Sciences,
Nanjing
210008,
PR China
7
School of Astronomy and Space Science, Nanjing University,
Nanjing
210093,
PR China
8
Shanghai Astronomical Observatory, Chinese Academy of Sciences,
80 Nandan Road,
Shanghai
200030,
PR China
9
Kavli Institute for Astronomy and Astrophysics, Peking University,
Beijing
100871,
PR China
10
National Astronomical Observatories, Chinese Academy of Sciences,
Beijing
100101,
PR China
11
School of Mathmatics and Physics, Jinggangshan University,
Ji’an
343009,
PR China
★ Corresponding author: tangxindi@xao.ac.cn
Received:
15
November
2024
Accepted:
17
April
2025
The Galactic edge, at Galactocentric distances of 14–22 kpc, provides an ideal laboratory for studying molecular clouds in an environ-ment that is different from the solar neighborhood, due to its lower gas density, lower metallicity, and little or no perturbation from the spiral arms. Observations of CO(J = 2–1) spectral lines were carried out toward 72 molecular clouds located at the Galactic edge using the IRAM 30 m telescope. With these observations combined with CO(J = 1–0) data from the MWISP project, we investigated the variations in R21 across these Galactic edge clouds, with R21 representing CO(2–1)/CO(1–0) integrated intensity ratios. They are found to range from 0.3 to 3.0 with a mean of 1.0 ± 0.1 in the Galactic edge clouds. The proportions of very low-ratio gas (R21 < 0.4), low-ratio gas (0.4 ≤ R21 < 0.7), high-ratio gas (HRG; 0.7 ≤ R21 < 1.0), and very high-ratio gas (VHRG; R21 ≥ 1.0) are 6.9%, 29.2%, 26.4%, and 37.5%, respectively, indicating a significant presence of high R21 ratio molecular gas within these regions. In our Galaxy, the gradient of the R21 ratio exhibits an initial radial decline followed by a high dispersion with increasing Galactocentric distance and a prevalence for VHRG. There is no apparent systematic variation within the Galactocentric distance range of 14 to 22 kpc. A substantial proportion of HRG and VHRG is found to be associated with compact clouds and regions of star-forming activity, suggesting that the high R21 ratios stem from dense gas concentrations and recent episodes of star formation.
Key words: stars: formation / ISM: clouds / ISM: molecules
© The Authors 2025
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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