Stellar streaming motions in triaxial objects: the case of the galactic bulge

Dynamical consequences are studied of the hypothesis that the gas ring, at about Σ = 1.5 kpc, which is seen in HI emission to be tilted by 220 to the galactic plane, is in steady state, i.e., that it circulates in closed, stable, non-self-intersecting orbits tilted with respect to the galactic disk. The author shows this implies that the galactic bulge is triaxial and that its figure is rotating in the direction opposite to that of the disk material. He determines the rotation period of the bulge by requiring that the potential has an anomalous orbit of the correct size at the observed tilt. He models the bulge as a triaxial object with axial ratios 0.4:0.7:1, and rotation period T/sub rho/ = 7.8 x 107 yr. It is suggested that these values are consistent with those needed to explain an analogous feature detected in 13CO and suggest a relation of the model with observations of the inner pc. Two self-consistent models for a bulge immersed in the galactic potential modeled by Bahcall et al. (1982) are constructed. In one of the models, the stellar streaming motions are maximized in the direction of the disk rotation, i.e. opposite to the bulge figure rotation. When seen from an inertial frame, the matter streaming and the figure rotation must occur in the same sense. The two models, with existing velocity observations of extra galactic bulges, and of galactic planetary nebulae and OH/IR stars are compared, it is shown that, even though improbable, the existence of a bulge counter-rotating with respect to the galactic disk is not immediately inconsistent with observations