分子雲が銀河外縁の過酷環境で生き残る仕組みを解明（Astronomers Reveal How Molecular Clouds Survive in Harsh Outskirts of the Galaxy）

2025-08-01 中国科学院(CAS)

Left: Distribution of molecular clouds (cyan circles) at Galactic edge; Right: Variation of the R₂₁ ratio with the Galactocentric distance. (Image by XAO)

＜関連情報＞

• https://english.cas.cn/newsroom/research_news/phys/202508/t20250801_1048956.shtml
• https://www.aanda.org/articles/aa/full_html/2025/06/aa53007-24/aa53007-24.html

銀河の縁にある分子雲 I. CO J = 2–1/1–0線の比の変動 Molecular clouds at the edge of the Galaxy I. Variation in the CO J = 2–1/1–0 line ratio

C. S. Luo, X. D. Tang, C. Henkel, K. M. Menten, Y. Sun, Y. Gong, X. W. Zheng, D. L. Li, Y. X. He, X. Lu, Y. P. Ao, X. P. Chen, T. Liu, K. Wang, J. W. Wu, J. Esimbek, J. J. Zhou, J. J. Qiu, X. Zhao, J. S. Li, Q. Zhao^, and L. D. Liu
Astronomy & Astrophysics  Published:03 June 2025
DOI:https://doi.org/10.1051/0004-6361/202453007

Abstract

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.