The Milky Way Scroll team at the Purple Mountain Observatory reveals the basic relationship between dense gas radiation in large samples of molecular clouds

The factors that determine the content and distribution of dense gases inside molecular clouds are a fundamental problem in the field of molecular cloud research. Based on the large samples of 18,190 12CO molecular clouds in the second quadrant of the Milky Way, researchers such as Yuan Lixia, Yang Ji, and Du Fujun of the Milky Way Scroll Survey Team of the Purple Mountain Observatory of the Chinese Academy of Sciences measured the dense gas content represented by 13CO radiation in the molecular cloud, and completed the statistical analysis of the physical parameters of the dense gas in the sample molecular cloud. The findings were published in the August issue of the international astrophysical journal ApJS (The Astrophysical Journal Supplement Series).

The researchers found that molecular clouds, which account for 15 percent of the total number of 12CO clouds, were able to detect significant 13CO radiation. Molecular clouds with 13CO radiation tend to have larger scales, higher masses, and wider velocity dispersions than samples that have not detected 13CO, and the total mass of these molecular clouds accounts for about 93% of the total mass of all samples. Of the dense gases in the molecular cloud, about 60% present only a single 13CO gas substructure, about 15% have two separate 13CO substructures, and the remaining 25% have multiple 13CO substructures.

Comparing the morphology of the molecular cloud of the entire sample with the detected 13CO gas, 13CO emission was mainly detected in the fibrous molecular cloud. The 13CO radiation of the molecular cloud is clearly correlated with the 12CO radiation parameter. Although in the direction of both 12CO and 13CO radiation, the velocity integral intensity of 12CO and 13CO shows a strong correlation. However, the total amount of 12CO radiation of a molecular cloud determines the upper limit of its 13CO radiation, and the radiation area of 13CO usually does not exceed 70% of its 12CO radiation area, and the integrating flow of 13CO does not exceed 20% of the 12CO integration flow (as shown in Figure 1). These results suggest that the overall properties of a molecular cloud define the upper limit of the dense gas content in it. The study provides systematic observational evidence for understanding the aggregation and structural formation of dense gases inside molecular clouds.

Figure 1: The image above is a composite image of three typical molecular clouds in a sample. Among them, blue represents 12CO radiation and red represents 13CO radiation; The figure below shows the correlation between the radiation parameters of 12CO and the radiation parameters of 13CO in molecular cloud samples. Radiation parameters include the angular area (bottom left) and the integral flow value (bottom right). In the following figure, each point represents a molecular cloud, and the color represents the density of the number of clouds on the plot.

This work has been funded by the National Natural Science Foundation of China, the National Key Research and Development Program of china and the Frontier Science Key Research Program of the Chinese Academy of Sciences. (Source: Purple Mountain Observatory, Chinese Academy of Sciences)

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