2012MNRAS.424L..44D

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 γ ∼ 0.4, and the overall mass within 50 kpc is ∼ 4 {x} 10¹¹ M_☉. The tracer velocity anisotropy is radially biased with β ∼ 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 ∼ 20).