Three-dimensional distribution of the ISM in the Milky Way galaxy. III. The total neutral gas disk

We present newly obtained three-dimensional gaseous maps of the Milky Way Galaxy: H I, H₂, and total-gas (H I plus H₂) maps, which were derived from the H I and ¹²CO(J = 1-0) survey data and rotation curves based on the kinematic distance. The H I and H₂ face-on maps show that the H I disk is extended to a radius of 15-20 kpc and its outskirts are asymmetric to the Galactic center, while most of the H₂ gas is distributed inside the solar circle. The total gas mass within a radius of 30 kpc amounts to 8.0 × 10⁹ M_⊙, 89% and 11% of which are H I and H₂, respectively. The vertical slices show that the outer H I disk is strongly warped and the inner H I and H₂ disks are corrugated. The total gas map is advantageous for tracing spiral structures from the inner to outer disk. Spiral structures such as the Norma-Cygnus, the Perseus, the Sagittarius-Carina, the Scutum-Crux, and the Orion arms are more clearly traced in the total gas map than ever. All the spiral arms are well explained by logarithmic spiral arms with pitch angles of 11°-15°. The molecular fraction of the total gas is high near the Galactic center and decreases with Galactocentric distance. The molecular fraction is also locally enhanced at the spiral arms compared with the inter-arm regions.