2022ApJS..262...16M

We have obtained column density maps for an unbiased sample of 120 molecular clouds in the third quadrant of the Milky Way midplane (b ≤ ∣5∣°) within the Galactic longitude range from 195° to 225°, using the high-sensitivity ¹²CO and ¹³CO (J = 1 - 0) data from the Milky Way Imaging Scroll Painting (MWISP) project. The probability density functions of the molecular hydrogen column density of the clouds, N-pdfs, are fitted with both a lognormal (LN) function and a lognormal plus power-law (LN+PL) function. The molecular clouds are classified into three categories according to their shapes of N-pdfs, i.e., LN, LN+PL, and UN (unclear). About 72% of the molecular clouds fall into the LN category, while 18% and 10% fall into the LN+PL and UN categories, respectively. A PL scaling relation, ${\sigma }_{s}\propto {N}_{{{\rm{H}}}_{2}}^{0.44}$, exists between the width of the N-pdf, σ_s, and the average column density, ${N}_{{{\rm{H}}}_{2}}$, of the molecular clouds. However, σ_s shows no correlation with the mass of the clouds. A correlation is found between the dispersion of normalized column density, σ_N/〈N〉, and the sonic Mach number, ${ \mathcal M }$, of molecular clouds. Overall, as predicted by numerical simulations, the N-pdfs of the molecular clouds with active star formation activity tend to have N-pdfs with PL high-density tails.