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

Abstract

We use distant Blue Horizontal Branch stars with Galactocentric distances 16 < r/kpc < 48 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 ~ 0.4 and the overall mass within 50 kpc is ~ 4 x 10^11 M_sol. The tracer velocity anisotropy is radially biased with beta ~ 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).

5 pages, 4 figures. Accepted for publication by MNRAS Letters

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