The peculiar shape of the inner galactic rotation curve

The apparent galactic rotation curve as inferred from HI and CO terminal velocities shows a narrow peak ($$v_{max}\simeq250-260\text {kms}^{-1}$$) at $$r\simeq500$$ pc, and a steep decline for 600 $$\text {pc}\le r \le1.5$$ kpc down to a rather broad minimum ($$v_{min}\simeq195\text{kms}^{-1}$$) around 2.8 kpc. In this paper, we show that this morphology can be reconciled with the bulge density distribution determined by infrared observations and the local density of spheroid stars only if the bulge of our Galaxy is non-axisymmetric and the resulting potential triaxial. In order to reproduce the galactic rotation curve satisfactorily, we require that the Sun be within 20° of the short axis of the bulge in the equatorial plane (the long axis of the closed loop orbits), and that the mean axial ratio of the bulge in the disc plane be 0.6 inside the knee in the density distribution at 800 pc. From such a viewing direction this simultaneously moves the peak in the apparent rotation curve inwards to 500 pc. and explains the near-Keplerian fall-off in terms of the rapidly decreasing quadrupole moment of the potential. For the same peak rotation velocity, our best-fitting bulge model has only 2/3 of the mass of a corresponding axisymmetric bulge. In several external galaxies, the inner rotation curves show anomalies in the sense expected if the bulges of these systems were triaxial. In NGC 2708 and NGC 3054 we appear to observe the bulge from a direction near the plane containing the major and minor axes, and in NGC 3200 and UGC 2885 from near the plane containing the intermediate and minor axes. The apparent mass-to-light ratios of the bulge components inferred from the inner rotation curves will depend considerably on the viewing geometry in cases where the bulge is triaxial.