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

After morphological classification of 18,190 ¹²CO molecular clouds, we further investigate the properties of their internal molecular gas structures traced by the ¹³CO (J = 1-0) line emissions. Using three different methods to extract the ¹³CO gas structures within each ¹²CO cloud, we find that ~15% of the ¹²CO clouds (2851) have ¹³CO gas structures and these ¹²CO clouds contribute about 93% of the total integrated flux of ¹²CO emission. In each of the 2851 ¹²CO clouds with ¹³CO gas structures, the ¹³CO emission area generally does not exceed 70% of the ¹²CO emission area, and the ¹³CO integrated flux does not exceed 20% of the ¹²CO integrated flux. We reveal a strong correlation between the velocity-integrated intensities of ¹²CO lines and those of ¹³CO lines in both ¹²CO and ¹³CO emission regions. This indicates the H₂ column densities of molecular clouds are crucial for the ¹³CO line emission. After linking the ¹³CO structure detection rates of the 18,190 ¹²CO molecular clouds to their morphologies, i.e., nonfilaments and filaments, we find that the ¹³CO gas structures are primarily detected in ¹²CO clouds with filamentary morphologies. Moreover, these filaments tend to harbor more than one ¹³CO structure. That demonstrates filaments not only have larger spatial scales, but also have more molecular gas structures traced by ¹³CO 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.