Physical properties of molecular clouds for the entire Milky Way disk

TL;DR

This paper provides a comprehensive catalog of over 8,000 molecular clouds across the entire Milky Way disk, analyzing their properties, distributions, and dynamics to improve understanding of galactic structure and cloud evolution.

Contribution

It introduces a new, extensive catalog of molecular clouds with detailed property measurements, covering the entire Galactic plane and applying a hierarchical clustering method to CO emission data.

Findings

The total H2 mass in the catalog is 1.2×10^9 solar masses.

Cloud size distribution peaks at approximately 30 parsecs.

A strong variation in turbulent energy injection rate is observed across the Galaxy.

Abstract

This study presents a catalog of 8107 molecular clouds that covers the entire Galactic plane and includes 98% of the $^{12}$CO emission observed within $b\pm5^\circ$. The catalog was produced using a hierarchical cluster identification method applied to the result of a Gaussian decomposition of the Dame et al. data. The total H$_2$ mass in the catalog is $1.2\times10^9 M_\odot$, in agreement with previous estimates. We find that 30% of the sight lines intersect only a single cloud, with another 25% intersecting only two clouds. The most probable cloud size is $R\sim30$ pc. We find that $M\propto R^{2.2\pm0.2}$, with no correlation between the cloud surface density, $\Sigma$, and $R$. In contrast with the general idea, we find a rather large range of values of $\Sigma$, from 2 to $300 M_\odot$ pc$^{-2}$, and a systematic decrease with increasing Galactic radius, $R_{\rm gal}$. The cloud velocity dispersion and the normalization $\sigma_0=\sigma_v/R^{1/2}$ both decrease systematically with $R_{\rm gal}$. When studied over the whole Galactic disk, there is a large dispersion in the line width-size relation, and a significantly better correlation between $\sigma_v$ and $\Sigma\,R$. The normalization of this correlation is constant to better than a factor of two for $R_{\rm gal}<20$ kpc. This relation is used to disentangle the ambiguity between near and far kinematic distances. We report a strong variation of the turbulent energy injection rate. In the outer Galaxy it may be maintained by accretion through the disk and/or onto the clouds, but neither source can drive the 100 times higher cloud-averaged injection rate in the inner Galaxy.