Gas dynamics in the Milky Way: the nuclear bar and the 3-kpc arms

TL;DR

This paper presents a gas dynamics model of the Milky Way with two bars, successfully reproducing observed features like the 3-kpc arms and the Central Molecular Zone, based on a potential derived from star counts.

Contribution

It introduces the first gas dynamics model of the Milky Way including both primary and secondary bars, matching many observed features with a single pattern speed.

Findings

The model reproduces the 3-kpc arm and its far-side counterpart.

The Galactic Molecular Ring is part of the spiral arms, not a true ring.

The secondary bar's orientation and mass are constrained by the model.

Abstract

We discuss the results of the first model of the gas dynamics in the Milky Way in the presence of two bars: the large scale primary bar or boxy bulge and a secondary bar in the Galactic center region. We have obtained an accurate potential by modeling 2MASS star counts and we have used this potential to simulate the gas dynamics. As a first approximation we have used one single pattern speed \Omega_p. The models with Omega_p=30-40 \kmskpc and a primary bar orientation of 20-35 deg reproduce successfully many characteristics of the observed longitude-velocity diagrams as the terminal velocity curve or the spiral arm tangent points. The Galactic Molecular Ring is not an actual ring but the inner part of the spiral arms, within corotation. The model reproduces quantitatively the "3-kpc arm" and the recently found far-side counterpart, which are the lateral arms that contour the bar. In the Galactic center region, the model reproduces the 1-kpc HI ring and the Central Molecular Zone (CMZ), which is the gas response to the secondary bar. In order to reproduce the observed parallelogram shape of the CO longitude velocity diagram of the CMZ, the secondary bar should be oriented by and angle of 60-70 deg with respect to the Sun-GC line. The mass of the secondary bar amounts to (2-5.5)10^9 Msun, which is 10-25 % of the mass of the primary bar.