Gas Dynamics in the Galactic Bar Region

Many early type barred spiral galaxies exhibit strong dustlanes at the leading edges of their bar (e.g. NGC 1300, NGC 1433), which from gas velocity field measurements and numerical hydrodynamical simulations are known to be the loci of shearing shocks with velocity gaps up to 200 km s −1 across. As an SBbc type galaxy, the Milky Way is expected to present similar dustlanes. Self-consistent high resolution 3D N-body and SPH simulations allow us to reliably identify their gaseous traces in the Galactic CO and HI ℓ − V distributions. The nearside branch of these dustlanes corresponds to the connecting arm feature and lies below (b < 0) the Galactic plane, while the far side branch, seen nearly end-on, traces a velocity-elongated feature near ℓ = −4 • and is located above the plane. The 3-kpc and 135-km s −1 arms are lateral arms, i.e. inner prolongations of disc spiral arms passing round the bar and finally intersecting the dustlanes at their respective latitudes (Figs. 1a-b). Considering that the latter intersections occur close to the major axis of the bar and within the bar ends, their spatial location can provide new constraints on the inclination angle ϕ and corotation radius R CR of the Galactic bar. Geometrically , if ℓ 1 and ℓ 2 are the longitudes where the 3-kpc and the 135-km s −1 arms respectively intersect the dustlanes, and s 1 and s 2 the galactocentric distances of these intersections (Fig. 1c), then: s i R • = sin ℓ i sin (ϕ + ℓ i) (i = 1, 2), ϕ = − 1 + q cot ℓ 2 + q cot ℓ 1 , where q ≡ s 1 /s 2 is an unknown asymmetry parameter. The 135-km s −1 arm indeed involves higher forbidden velocities than the 3-kpc arm which, according to our models, means that the former arm is closer to the Galactic centre than the latter, i.e. q > 1. The CO observations suggest ℓ 1 = 14.5 • and ℓ 2 = −4.5 • , yielding the constraints shown in Fig. 1d. The length s 1 increases for smaller inclination angles. If ϕ ≤ 30 • , then s 1 ≥ 2.9 kpc, q ≤ 2 and, since s 1 < a and R CR ≈ 1.2a according to numerical simulations and early type galaxy analyses, where a is the semi major axis …