The incorrect rotation curve of the Milky Way

Abstract

In the fundamental quest of the rotation curve of the Milky Way, the tangent-point (TP) method has long been the simplest way to infer velocities for the inner, low latitude regions of the Galactic disk from observations of the gas component. We test the validity of the method on realistic gas distribution and kinematics of the Milky Way, using a numerical simulation of the Galaxy. We show that the resulting velocity profile strongly deviates from the true rotation curve of the simulation, as it overstimates it in the central regions, and underestimates it around the bar corotation. Also, its shape strongly depends on the orientation of the stellar bar. The discrepancies are caused by highly non-uniform azimuthal velocities, and the systematic selection by the TP method of high-velocity gas along the bar and spiral arms, or low-velocity gas in less dense regions. The velocity profile is in good agreement with the rotation curve only beyond corotation, far from massive asymmetric structures. Therefore the observed velocity profile of the Milky Way inferred by the TP method is expected to be very close to the true Galactic rotation curve for 4.5<R<8 kpc. Another consequence is that the Galactic velocity profile for R<4-4.5 kpc is very likely flawed by the non-uniform azimuthal velocities, and does not represent the true Galactic rotation curve, but instead local motions. The real shape of the innermost rotation curve is probably shallower than previously thought. Using a wrong rotation curve has a dramatic impact on the modelling of the mass distribution, in particular for the bulge component of which derived enclosed mass within the central kpc and scale radius are, respectively, twice and half of the actual values. We thus strongly argue against using terminal velocities or the velocity curve from the TP method for modelling the mass distribution of the Milky Way. (abridged)Comment: Accepted for publication in Astronomy & Astrophysics, 8 pages, 10 figures, revised version after A&A language editin