Physical Properties of Molecular Clouds for the Entire Milky Way Disk

This study presents a catalog of 8107 molecular clouds that covers the entire Galactic plane and includes 98% of the ¹²CO emission observed within b+/- 5^\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 H₂ mass in the catalog is 1.2× {10}⁹ {M}_⊙ , 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∼ 30 pc. We find that M\propto {R}^{2.2+/- 0.2}, with no correlation between the cloud surface density, Σ, and R. In contrast with the general idea, we find a rather large range of values of Σ, from 2 to 300 M_⊙ pc^-2, and a systematic decrease with increasing Galactic radius, {R}_{gal}. The cloud velocity dispersion and the normalization {σ }₀={σ }_v/{R}^1/2 both decrease systematically with {R}_{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 {σ }_v and {{Σ }} R. The normalization of this correlation is constant to better than a factor of two for {R}_{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.