Gas Streams and Spiral Structure in the Milky Way

TL;DR

This study models the gas flow in the Milky Way using a mass distribution based on near-IR observations, successfully reproducing the galaxy's spiral arms and gas dynamics, including the influence of the central bar.

Contribution

The paper presents a new model of the Milky Way's stellar mass distribution that explains observed gas flow patterns and spiral structure, integrating near-IR data with analytical components.

Findings

The model reproduces the 4-arm spiral pattern observed in molecular gas and HII regions.

It explains non-circular motions and forbidden velocities in the galactic center.

A gas disk near the center matches observations of CS emission.

Abstract

The observed gas dynamics in the Milky Way can only be explained by a bar in the galactic center. Such a bar is directly visible in the near-IR maps of the bulge, where it causes a distinctive asymmetric light distribution pattern. Another large-scale structure is the grand-design 4-arm spiral pattern, most clearly observed in the spatial distribution of molecular gas and HII-regions. In order to model the observed gas flow structure, we constructed a model for the stellar mass distribution. For the inner 5 $\kpc$ we used the 3D deprojected near-IR light distribution, as observed by the COBE/DIRBE experiment, and added an analytical disk model outside the box as well as a halo model. With this frozen mass distribution, we computed the stationary gas flow for various deprojection parameters and pattern speeds. For all reasonable parameter choices, we obtain a 4-armed spiral pattern, which can be matched to the observed spiral arms. In the bar region, our model can explain the non-circular motion visible in the terminal velocity curve as well as part of the forbidden velocities. Inside the corotation, we also find 4 spiral arms, the nearest arm corresponds to the 3-kpc-arm, although only qualitatively. The missing southern 3-kpc-arm at the far end of the galaxy is explained by running parallel to another arm. Close to the center, we find gas on circular orbits forming a disk. Such a disk has been observed in emission of the CS molecule, however only part of the disk appears to be occupied by dense enough gas to be traced by CS. Further we compare our model to the distribution of OH/IR stars in the inner galaxy.