We studied the probability distribution function of the column density (N-PDF) of molecular clouds based on a fit with a multi-log-normal function using the Nobeyama 45-m Cygnus X CO survey data. We identified 124 molecular clouds in 13CO data using the DENDROGRAM and SCIMES algorithms. The N-PDF was constructed for 11 extended (≥ 0.4 deg2) molecular clouds of these identified clouds. We found that every N-PDF is well-fitted with one or two log-normal (LN) distributions. We investigated the distributions of the column density, C18O dense cores, and radio continuum source in each cloud and found that the N-PDF was less correlated with the star-forming activity. The LN N-PDF parameters showed two impressive features. First, the LN distribution at the low-density part had the same mean column density (∼1021.5 cm−2 ) for almost all the molecular clouds. Second, the wider LN distribution tended to show the lower mean density of the structures.

“Gas Density Histogram (GDH)” is an observational counterpart of the probability density function (PDF) of the gas density of interstellar medium (ISM). We used 12CO data in (l, b) = (29°, 0) region from “FOREST unbiased galactic imaging survey with Nobeyama 45-m telescope (FUGIN)”, which is a large coverage survey in three CO (1-0) lines. Using the kinetic distance, we estimated the volume density of the voxel from the observed column density. The resultant GDHs of the inter-arm regions show lognormal or lognormal-like, but those in the spiral arm regions show flat-top shape.

We report a recent result of the FUGIN project, a Galactic plane CO survey using the Nobeyama 45-m Telescope and the FOREST receiver. In the third galactic quadrant, 42 square degrees are observed and 4752 molecular clouds are detected. Among them, 12 clouds are located at R (distance from the Galactic center) > 16 kpc. Molecular clouds at R < 16 kpc trace the Local, Perseus, and Outer arms.