Gas dynamics and large-scale morphology of the Milky Way galaxy

We present a new model for the gas dynamics in the galactic disc inside the orbit of the Sun. Quasi-equilibrium flow solutions are determined in the g ravitational potential of the deprojected COBE near-infrared bar and disc, complemented by a central cusp and, in some models, an outer halo. These mo dels 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 pattern speed of the bar, to the orientation o f the bar with respect tc, the observer, or to whether or not the spiral ar ms carry mass. Our best model provides a coherent interpretation of many observed gas dyna mical features. Its four-armed spiral structure outside corotation reproduc es quantitatively the directions to the live main spiral arm tangents at \l \ less than or equal to 60 degrees observed in a variety of tracers. The 3- kpc arm is identified with one of the model arms emanating fi om the ends o f the bar, extending into the corotation region. The model features an inne r gas disc with a cusped orbit shock transition to an x(2) orbit disc of ra dius R similar to 150 pc. The corotation radius of the bar is fairly well constrained at R-c similar or equal to 3.5 +/- 0.5 kpc, The best value for the orientation angle of th e bar is probably 20-25 degrees, but the uncertainty is large since no deta iled quantitative fit to all features in the observed (l, v) diagrams is ye t possible. The Galactic terminal velocity curve from H I and CO observatio ns out to l similar or equal to +/-45 degrees (similar to 5 kpc) is approxi mately described by a maximal disc model with constant mass-to-light ratio for the near-infrared bulge and disc.