2025A&A...698A..54L

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.