The Case for a Leaner Milky Way

The shape of the Milky Way's dark halo has been determined using two independent methods. First, we have compared the amount of dark matter close to the Galactic plane as inferred from stellar kinematics to the predictions of the different dark halo models. Second, we have compared the thickness of the Galaxy's gas layer to the predictions of dark halos models with a wide range in halo shape. Combining these two techniques, we find that the Milky Way has a relatively-round dark halo with shortest-to-longest axial ratio of q=0.75 +/- 0.25. The inferred halo flattening depends on the Sun's distance from the Galactic centre (R_0), the Galactic rotation speed at the Solar circle (\Theta_0). and the stellar column density in the Solar neighbourhood. We find that an internally consistent Milky Way model can only be constructed for a very limited range in parameters. If the dark halo is oblate, we infer \Theta_0 \la 190 km/s. Models which use the IAU-sanctioned Galactic constants of 8.5 kpc and 220 km/s REQUIRE a substantially prolate dark matter halo. The distance to the Galactic centre can be determined in several ways, but all yield small similarly values for R_0, 7.1 - 7.5 kpc. Low values for R_0 also imply a low Galactic rotation speed: \Theta_0 ~187 km/s. If the Milky Way's rotation speed is larger than ~170 km/s, two dark matter candidates which require a highly flattened dark matter halo are ruled out: 1) decaying massive neutrinos; and 2) a disk of cold molecular hydrogen. These results imply that the luminous Milky Way is smaller than hitherto assumed. Such a leaner Galaxy is entirely consistent with ALL observational kinematical constraints, including the determination of R_0 from water maser proper motions (7.2 +/- 0.7 kpc), the reflex motion of the Galactic centre, Cepheid velocities, the HIPPARCOS based value of the Oort limit, and the kinematics of the Local group members. The total mass estimate for the Milky Way is unaffected.