Physical properties of molecular clouds for the entire Milky Way disk

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.