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 similar to 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 (sigma(r), sigma(theta), sigma(phi)) and the anisotropy profile (beta). The radial velocity dispersion profile (sigma(r)) is measured out to a galactocentric radius of r similar to 60 kpc, but due to the lack of proper-motion information, sigma(theta), sigma(phi), and beta could only be derived directly out to r similar to 25 kpc. From a starting value of beta approximate to 0.5 in the inner parts (9 < r/kpc < 12), the profile falls sharply in the range r approximate to 13-18 kpc, with a minimum value of beta = -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 beta approximate to 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 similar to 25 kpc. The mass of the Galaxy within r less than or similar to 25 kpc is determined to be 2.1 x 10(11) M-circle dot, 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(-0.3)(+0.4) x 10(12) M-circle dot (R-vir = 249(-31)(+34) kpc).