Broken degeneracies: the rotation curve and velocity anisotropy of the Milky Way halo

We use distant blue horizontal branch stars with Galactocentric distances 16 < r < 48 kpc as kinematic tracers of the Milky Way dark halo. We model the tracer density as an oblate, power law embedded within a spherical power-law potential. Using a distribution function method, we estimate the overall power-law potential and the velocity anisotropy of the halo tracers. We measure the slope of the potential to be gamma similar to 0.4, and the overall mass within 50 kpc is similar to 4 x 10(11)M(circle dot). The tracer velocity anisotropy is radially biased with beta similar to 0.5, which is in good agreement with local solar neighbourhood studies. Our results provide an accurate outer circular velocity profile for the Milky Way and suggest a relatively high-concentration dark matter halo (c(vir) similar to 20).