Sub kpc-scale gas density histogram: a new statistical method to characterize galactic-scale gas structures.

To understand the physical properties of the interstellar medium (ISM) in various scales, we should investigate it with pc-scale resolution over kpc scale coverage. Here, we report the sub-kpc scale Gas Density Histogram (GDH) of the Milky Way. GDH is a histogram of averaged density and corresponds to the probability density distribution (PDF) of gas volume density. We use galactic plain survey data (l =10∘− 50∘) at 12CO and 13CO (J = 1 − 0) obtained as a part of the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45m telescope (FUGIN). With this method and data, we are free from spatial structure and molecular cloud identification. GDH can be well fitted with single or double log-normal distribution; which we call as the low-density log-normal (L-LN) and high-density log-normal (H-LN) components. We found both the H-LN fraction (f H) and L-LN width (σL) along the gas density axis show a coherent structure on the longitude-velocity diagram. It suggests that there is a relationship between the ISM property and kpc scale structure in the Milky Way. [ABSTRACT FROM AUTHOR]