Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: III. The Total Neutral Gas Disk

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We present newly obtained three-dimensional gaseous maps of the Milky Way Galaxy; HI, H₂ and total-gas (HI plus H₂) maps, which were derived from the HI and ¹²CO(J=1--0) survey data and rotation curves based on the kinematic distance. The HI and H₂ face-on maps show that the HI disk is extended to the radius of 15--20 kpc and its outskirt is asymmetric to the Galactic center, while most of the H₂ gas is distributed inside the solar circle. The total gas mass within radius 30 kpc amounts to 8.0× 10⁹ M, 89% and 11% of which are HI and H₂, respectively. The vertical slices show that the outer HI disk is strongly warped and the inner HI and H₂ disks are corrugated. The total gas map is advantageous to trace spiral structure 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 with logarithmic spiral arms with pitch angle of 11° -- 15°. The molecular fraction to the total gas is high near the Galactic center and decreases with the Galactocentric distance. The molecular fraction also locally enhanced at the spiral arms compared with the inter-arm regions.