Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: II. The Molecular Gas Disk

Abstract We created a three-dimensional distribution map of molecular gas throughout the Milky Way galaxy using the latest ${{12\atop} \mathrm{CO}}$ ($J = 1 \hbox{--} 0$) survey data cube and the rotation curve based on the kinematic distance. The radial distribution of the molecular gas shows a central peak and a second peak around $0.5 \,R_0$ ($R_0$: the solar Galactocentric distance). The thickness of the molecular disk slightly increases from 48 pc to 160 pc with the Galactocentric distance within a radius range of 0–11 kpc. We were able to trace the Outer, the Perseus, the Sagittarius–Carina, the Scutum–Crux, and the Norma arms as logarithmic spiral arms with pitch angles of $11 {{}^{\circ }} \hbox{--} 15^\circ$. Considering that the pitch angles of the spiral arms are within this range, the Norma and the Outer arms seem to be identified as the same spiral arm. We could also trace a midplane displacement, whose amplitude is nearly constant inside a 10 kpc radius and increases beyond this radius. The ridges of the midplane displacement form the leading spiral arms.