Abstract Molecular clouds (MCs) are cradles of star and planet formation, thereby playing an important role in the evolution of galaxies. Based on the unbiased Milky Way Imaging Scroll Painting survey data of 12 CO, 13 CO, and C 18 O ( J = 1–0) line emission in two regions toward the inner and outer Galaxy, i.e., the G50 (44 . ° 75 ≤ l ≤ 60 . ° 25) and G120 (119 . ° 75 ≤ l ≤ 130 . ° 25) regions, the distribution of molecular gas is studied. Both regions have Galactic latitudes of ∣ b ∣≤ 5 . ° 25. A catalog containing 24,724 MCs is constructed from the data. In our proximity, several molecular structures with large angular scales and small velocity dispersions are discovered, resembling curtains of mist. Beyond the nearby molecular gas, a clear aggregation of MCs along coherent structures in the Galactic plane is visible, sketching spiral arm structures. Nevertheless, the aggregation of MCs is also detected in the inter-arm region between the Perseus and Outer arms in the G50 region. The Galactic molecular disk in this inter-arm region is found to be thinner than that in the adjacent spiral arm region. In addition, the thickness of the Galactic molecular disk examined here is found to be correlated with the warp of it, indicating their homologous origins. The molecular disk has a typical thickness of ∼220 pc in the inner Galaxy. Moreover, the dispersion of the MC systemic velocity decreases with increasing galactocentric radius, resulting in lower kinematic distance uncertainties at larger radii. However, the Perseus arm segment in the G120 region exhibits a relatively large cloud-to-cloud velocity dispersion and split components in its MC velocity distribution.
Zhou et al. (Fri,) studied this question.