Molecular Gas Structures Traced by 13 CO Emission in the 18,190 12 CO Molecular Clouds from the MWISP Survey

Abstract

After morphological classification of 18,190 12 CO molecular clouds, we further investigate the properties of their internal molecular gas structures traced by the 13 CO ( J = 1−0) line emissions. Using three different methods to extract the 13 CO gas structures within each 12 CO cloud, we find that ∼15% of the 12 CO clouds (2851) have 13 CO gas structures and these 12 CO clouds contribute about 93% of the total integrated flux of 12 CO emission. In each of the 2851 12 CO clouds with 13 CO gas structures, the 13 CO emission area generally does not exceed 70% of the 12 CO emission area, and the 13 CO integrated flux does not exceed 20% of the 12 CO integrated flux. We reveal a strong correlation between the velocity-integrated intensities of 12 CO lines and those of 13 CO lines in both 12 CO and 13 CO emission regions. This indicates the H 2 column densities of molecular clouds are crucial for the 13 CO line emission. After linking the 13 CO structure detection rates of the 18,190 12 CO molecular clouds to their morphologies, i.e., nonfilaments and filaments, we find that the 13 CO gas structures are primarily detected in 12 CO clouds with filamentary morphologies. Moreover, these filaments tend to harbor more than one 13 CO structure. That demonstrates filaments not only have larger spatial scales, but also have more molecular gas structures traced by 13 CO lines, i.e., local gas density enhancements. Our results favor the turbulent compression scenario for filament formation, in which dynamical compression of turbulent flows induces local density enhancements. The nonfilaments tend to be in the low-pressure and quiescent turbulent environments of the diffuse interstellar medium.