Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: I. The HI Disk

Abstract

We derived the three-dimensional distribution of HI gas in the Milky Way Galaxy using the latest HI survey data cubes and rotation curves. The distance of the HI gas was determined by the kinematic distance using a rotation curve. We solved the near-far problem in the inner Galaxy by a fitting method which involves introducing a model of vertical HI distribution. In our resultant maps we could trace three prominent arms: the Sagittarius-Carina arm, the Perseus arm, and the Outer arm. These three arms were found to be logarithmic spiral arms. The pitch angles of the Sagittarius-Carina, Perseus, and Outer arms were estimated to be about $11\arcdeg$, $18\arcdeg$, and $7\arcdeg$, respectively. The Sun is located in a region rich in HI gas between the Sagittarius-Carina arm and the Perseus arm. The HI disk shows large and asymmetric warping in the outer disk: the HI disk goes up to about 1.5 kpc above the Galactic plane in the northern hemisphere, and down to about 1 kpc in the southern hemisphere, which means asymmetric warping. The inner HI disk is also found to be tilting. The radius of the HI disk is about 17 kpc and the HI mass within this radius is estimated to be $2.5 \times 10^9$ M$_\odot$, which corresponds to 1.5 % of the dynamical mass predicted from the rotation curve. We also found that the HI outskirt is largely swelling in the fourth quadrant, and hence the Galaxy is significantly lopsided. The scale-height of the HI layer increases with the radius, and is correlated with the HI volume density at the centroid of the HI layer.