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

TL;DR

This paper presents detailed three-dimensional maps of the Milky Way's neutral gas, revealing the structure, distribution, and spiral arm features of HI and H$_2$ gas, and analyzing their implications for Galactic structure.

Contribution

It provides new 3D maps of HI, H$_2$, and total gas in the Milky Way, with detailed analysis of spiral arms and gas distribution patterns, based on survey data and rotation curves.

Findings

HI disk extends to 15-20 kpc with asymmetry

Most H$_2$ is within the solar circle

Total gas mass within 30 kpc is 8.0×10^9 M$_\odot$

Abstract

We present newly obtained three-dimensional gaseous maps of the Milky Way Galaxy; HI, H$_2$ and total-gas (HI plus H$_2$) maps, which were derived from the HI and $^{12}$CO($J=1$--0) survey data and rotation curves based on the kinematic distance. The HI and H$_2$ 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 H$_2$ gas is distributed inside the solar circle. The total gas mass within radius 30 kpc amounts to $8.0\times 10^9$ M$_\odot$, 89\% and 11\% of which are HI and H$_2$, {respectively}. The vertical slices show that the outer HI disk is strongly warped and the inner HI and H$_2$ 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 $11\degree$ -- $15\degree$. 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.