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 < 48 kpc as kinematic tracers of the MilkyWay 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 × 10 11M. 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). © 2012 The Authors, MNRAS 424, L44-L48 Monthly Notices of the Royal Astronomical Society. © 2012 RAS.