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 ± 5• . 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 × 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 ∝ 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.

Molecular Clouds in the Extreme Outer Galaxy between l = 34.°75 to 45.°25

“…The fact that the sizes and masses of these clouds range from 1.6 pc to 14.8 pc, and 1.3×10 2 M ⊙ to 1.4×10 4 M ⊙ implies the lack of large and massive clouds in the EOG regions. The median gas surface density of these clouds obtained here is similar to that of MCs at similar Galactocentric radius (Miville-Deschênes et al 2017), however, is about half value of the Outer arm (Su et al 2016). In addition, it appears that the viral mass is much larger than the mass obtained by X factor in the EOG region.…”

“…It is worthwhile to note that the two known EOG clouds identified by Dame & Thaddeus (2011) are also detected by us. Condering the beam size of the CfA 1.2 m telescope and the size of the MCs, three out of the four EOG clouds identified by Miville-Deschênes et al (2017) are also detected by our new observations, the other one is marginally detected by us.…”

“…The white-dashed lines (V LSR = −1.6 l ± 13.2 km s −1 ) indicate the assumed velocity range of the Outer Scu-Cen arm. Only six EOG clouds indicated by red crosses (Dame & Thaddeus 2011) and black crosses (Miville-Deschênes et al 2017) in Figure 1 are known MCs in this velocity range. However, our high-sensitivity CO data reveal much more features distributed beyond the Outer arm at a maximum negative velocity.…”

“…13). The observed variation of the power index of different samples of MCs simply reflects plausible differences of the amplitude of the turbulent motions between clouds owing to Galactic environments (Miville-Deschênes et al 2017).…”

“…Using a Dendrogram technique and a hierarchical cluster identification method applied to the Dame et al (2001) CfA 1.2 m data, Rice et al (2016) and Miville-Deschênes et al (2017) recently obtained a catalog of 1064 massive MCs and a catalog of 8107 MCs, respectively. These catalogs provide the most complete distribution of Milky Way MCs.…”