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Keywords:

  • stars;
  • Galaxy: halo;
  • Galaxy: kinematics and dynamics;
  • dark matter

ABSTRACT

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 × 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 (cvir∼ 20).