Kinematics of the Stellar Halo and the Mass Distribution of the Milky Way Using Blue Horizontal Branch Stars

Here, we present a kinematic study of the Galactic halo out to a radius of ~60 kpc, using 4664 blue horizontal branch stars selected from the SDSS/SEGUE survey to determine key dynamical properties. Using a maximum likelihood analysis, we determine the velocity dispersion profiles in spherical coordinates (σ _r , σ_θ, σ_phi) and the anisotropy profile (β). The radial velocity dispersion profile (σ _r ) is measured out to a galactocentric radius of r ~ 60 kpc, but due to the lack of proper-motion information, σ_θ, σ_phi, and β could only be derived directly out to r ~ 25 kpc. From a starting value of β ≈ 0.5 in the inner parts (9 < r/kpc < 12), the profile falls sharply in the range r ≈ 13-18 kpc, with a minimum value of β = -1.2 at r = 17 kpc, rising sharply at larger radius. In the outer parts, in the range 25 < r/kpc < 56, we predict the profile to be roughly constant with a value of β ≈ 0.5. The newly discovered kinematic anomalies are shown not to arise from halo substructures. We also studied the anisotropy profile of simulated stellar halos formed purely by accretion and found that they cannot reproduce the sharp dip seen in the data. From the Jeans equation, we compute the stellar rotation curve (v _circ) of the Galaxy out to r ~ 25 kpc. The mass of the Galaxy within r <~ 25 kpc is determined to be 2.1 × 10¹¹ M _⊙, and with a three-component fit to v _circ(r), we determine the virial mass of the Milky Way dark matter halo to be M _vir = 0.9^+0.4 _-0.3 × 10¹² M _⊙ (R _vir = 249⁺³⁴ _-31 kpc).