Galactic elliptical streamlines 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 excentricity of the orbits decreases with increasing r, and they are well aligned with a position angle of 40° for r≤3 kpc indicating the existence of a bar in that region. Plotting the orbits in the galactic plane spiral‐arm‐like features are marked by orbital crowding effects. This spiral arm system is 4‐armed for 3≤r≤6 kpc, and two‐armed outside of this and it agrees reasonably well in position with the spiral structure outlined by Georgelin and Georgelin (1976). The strong peak of the galactic rotation curve found amongst others by Rohlfs and Kreitschmann (1987) near r=500 pc is seen to be caused by the alignment of the elliptical orbits and not to be a feature of the dynamics.