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

Abstract

Abstract We derived the three-dimensional distribution of H i gas in the Milky Way Galaxy using the latest H i survey data cubes and rotation curves. The distance of the H i 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 H i 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 ${11{}^{\mathrm {\circ }}}$,${18{}^{\mathrm {\circ }}}$, and ${7{}^{\mathrm {\circ }}}$, respectively. The Sun is located in a region rich in H i gas between the Sagittarius–Carina arm and the Perseus arm. The H i disk shows large and asymmetric warping in the outer disk: the H i 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 H i disk is also found to be tilting. The radius of the H i disk is about 17 kpc and the H i 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 H i outskirt is largely swelling in the fourth quadrant, and hence the Galaxy is significantly lopsided. The scale-height of the H i layer increases with the radius, and is correlated with the H i volume density at the centroid of the H i layer.