Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: II.   The Molecular Gas Disk
  We created a three-dimensional distribution map of molecular gas throughout the Milky Way Galaxy using the latest $^{12}$CO($J=$1--0) survey data cube and 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 $R_0$. 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 $11\arcdeg - 15\arcdeg$. 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 midplane displacement form leading spiral arms. 
Hiroyuki Nakanishi and Yoshiaki Sofue