KINEMATICS OF THE STELLAR HALO AND THE MASS DISTRIBUTION OF THE MILKY WAY USING BLUE HORIZONTAL BRANCH STARS

Abstract

Here we present a kinematic study of the Galactic halo out to a radius of $\sim$ 60 kpc, using 4664 blue horizontal branch (BHB) 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 $r \sim 60$ kpc, but due to the lack of proper-motion information, $\sigma_{\theta}$, $\sigma_{\phi}$ and $\beta$ could only be derived directly out to $r \sim25$ kpc. From a starting value of $\beta\approx 0.5$ in the inner parts ($9<r/\kpc<12$), the profile falls sharply in the range $r \approx 13-18$ 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 $\beta\approx 0.5$. 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 ($v_{\rm circ}$) of the Galaxy out to $r \sim 25$ kpc. The mass of the Galaxy within $r \lesssim 25$ kpc is determined to be $2.1 \times 10^{11}$ $M_{\sun}$, and with a 3-component fit to $v_{\rm circ}(r)$, we determine the virial mass of the Milky Way dark matter halo to be $M_{\rm vir} = 0.9 ^{+0.4}_{-0.3} \times 10^{12}$ $M_{\sun}$ ($R_{\rm vir} = 249^{+34}_{-31}$ kpc).