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

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.