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

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 = 5.57deg;, are interpreted as gas lumps crossing the dustlane shocks. The terminal velocity peaks near ℓ = ±2.5° 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°, a corotation radius of 4.0-4.5 kpc and a face-on axis ratio b/a ≈ 0.6.