ELLIPTIC STREAMLINES IN THE INNER GALAXY AND THEIR LARGE-SCALE ORGANIZATION

Interstellar gas moving in elliptical streamlines is used to model the observed terminal velocities of neutral hydrogen gas in the I. and IV galactic quadrant. The eccentricity of the orbits decreases with incr easing distance from the galactic center r, and they are well aligned with a position angle of 35-degrees +/- 10-degrees (as seen from the l ine sun-galactic center) for r less than or similar to 3 kpc indicatin g the existence of a bar in that region. Plotting the orbits in the ga lactic plane spiral-arm-like features are marked by orbital crowding e ffects. This spiral arm system is 4-armed for 3 less-than-or-similar-t o r less-than-or-similar-to 6 kpc, and two-armed outside of this and i t agrees reasonably well in position with the spiral structure outline d by Georgelin & Georgelin (1976). The strong peak of the galactic rot ation curve found amongst others by Rohlfs & Kreitschmann (1987) near r = 500 pc is seen to be caused by the geometry of the elliptical orbi ts and not to be a feature of the dynamics. For the orbital velocity o f the gas in the apogalactic position a value of about 200 km-1 is fou nd that varies remarkably little with the galactic radius. From the ca lculated velocity field it is possible to compute model line-profiles which can reproduce some of the features of the measured profile bette r than models with circular geometry do.