Gas Dynamics and Large–Scale Morphology of the Milky Way Galaxy

We present a new model for the gas dynamics in the galactic disk inside the Sun’s orbit. Quasi–equilibrium flow solutions are determined in the gravitational potential of the deprojected COBE NIR bar and disk, complemented by a central cusp and, in some models, an outer halo. These models generically lead to four–armed spiral structure between corotation of the bar and the solar circle; their large–scale morphology is not sensitive to the precise value of the bar’s pattern speed, to the orientation of the bar with respect to the observer, and to whether or not the spiral arms carry mass. Our best model provides a coherent interpretation of many observed gas dynamical features. Its four-armed spiral structure outside corotation reproduces quantitatively the directions to the five main spiral arm tangents at |l| ≤ 60◦ observed in a variety of tracers. The 3-kpc-arm is identified with one of the model arms emanating from the ends of the bar, extending into the corotation region. The model features an inner gas disk with a cusped orbit shock transition to an x2 orbit disk of radius R ∼ 150 pc. The bar’s corotation radius is fairly well–constrained at Rc ≃ 3.5 ± 0.5 kpc. The best value for the orientation angle of the bar is probably 20−25◦, but the uncertainty is large since no detailed quantitative fit to all features in the observed (l, v) diagrams is yet possible. The Galactic terminal velocity curve from HI and CO observations out to l ≃ ±45◦ (∼ 5 kpc) is approximately described by a maximal disk model with constant mass–to–light ratio for the NIR bulge and disk.