3D self-consistent N-body barred models of the Milky Way: II. Gas dynamics

Abstract

The gas dynamics in the Galactic disc is modeled by releasing an initially axisymmetric SPH component in a completely self-consistent and symmetry-free 3D N-body simulation of the Milky Way in which the stellar components display a COBE-like bar. The density centre of the stellar bar wanders around the centre of mass and the resulting gas flow is asymmetric and non-stationary, reproducing the HI and CO l-V diagrams only at specific times and thus suggesting a transient nature of the observed inner gas kinematics. The best matching models allow a new and coherent interpretation of the main features standing out of the l-V data within the bar region. In particular, the l-V traces of the prominent offset dustlanes leading the bar major axis in early-type barred spirals can be unambiguously identified, and the 3-kpc arm and its non-symmetric galactocentric opposite counterarm are the inner prolongations of disc spiral arms passing round the bar and joining the dustlanes at very different galactocentric distances. Bania's clump 1 and 2, and another velocity-elongated feature near l= 5.5 deg, are interpreted as gas lumps crossing the dustlane shocks. The terminal velocity peaks near l=+/- 2.5 deg are produced by gas along the dustlanes and not by the trace of the cusped x1 orbit, which passes farther away from the Galactic centre. According to these models and to related geometrical constraints, the Galactic bar must have an inclination angle of 25+/-4 deg, a corotation radius of 4.0-4.5 kpc and a face-on axis ratio b/a=0.6.