Molecular Gas Distribution Perpendicular to the Galactic Plane

Abstract

We use the ∼370 deg 2 data from the MWISP CO survey to study the vertical distribution of the molecular clouds (MCs) toward the tangent points in the region of l = [+16°, +52°] and ∣ b ∣ < 5.°1. We find that the molecular disk consists of two components with a layer thickness (FWHM) of ∼85 pc and ∼280 pc, respectively. In the inner Galaxy, the molecular mass in the thin disk is dominant, while the molecular mass traced by the discrete MCs with weak CO emission in the thick disk is probably ≲10% of the whole molecular disk. For the CO gas in the thick disk, we identified 1055 high- z MCs that are ≳100 pc from the Galactic plane. However, only a few samples (i.e., 32 MCs or 3%) are located in the ∣ z ∣ ≳ 360 pc region. Typically, the discrete MCs of the thick-disk population have a median peak temperature of 2.1 K, a median velocity dispersion of 0.8 km s −1 , and a median effective radius of 2.5 pc. Assuming a constant value of X CO = 2 × 10 20 cm −2 (K km s −1 ) −1 , the median surface density of these MCs is 6.8 M ⊙ pc −2 , indicating very faint CO emission for the high- z gas. The cloud–cloud velocity dispersion is 4.9 ± 1.3 km s −1 and a linear variation with a slope of ∼−0.4 km s −1 kpc −1 is obtained in the region of R GC = 2.2–6.4 kpc. Assuming that these clouds are supported by their turbulent motions against the gravitational pull of the disk, a model of ρ 0 ( R ) = 1.28 M ⊙ pc −3 can be used to describe the distribution of the total mass density in the Galactic midplane.