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We derive new constraints on the mass of the Milky Way's dark matter halo,based on a set of halo stars from SDSS as kinematic tracers. Our samplecomprises 2401 rigorously selected Blue Horizontal-Branch (BHB) halo starsdrawn from SDSS DR-6. To interpret these distributions, we compare them tomatched mock observations drawn from two different cosmological galaxyformation simulations designed to resemble the Milky Way, which we presume tohave an appropriate orbital distribution of halo stars. We then determine whichvalue of $\rm V_{cir}(r)$ brings the observed distribution into agreement withthe corresponding distributions from the simulations. This procedure results inan estimate of the Milky Way's circular velocity curve to $\sim 60$ kpc, whichis found to be slightly falling from the adopted value of $\rm 220 km s^{-1}$at the Sun's location, and implies M$(<60 \rm kpc) = 4.0\pm 0.7\times10^{11}$M$_\odot$. The radial dependence of $\rm V_{cir}(r)$, derived instatistically independent bins, is found to be consistent with the expectationsfrom an NFW dark matter halo with the established stellar mass components atits center. If we assume an NFW halo profile of characteristic concentrationholds, we can use the observations to estimate the virial mass of the MilkyWay's dark matter halo, M$_{\rm vir}=1.0^{+0.3}_{-0.2} \times10^{12}$M$_\odot$, which is lower than many previous estimates. This estimateimplies that nearly 40% of the baryons within the virial radius of the MilkyWay's dark matter halo reside in the stellar components of our Galaxy. A valuefor M$_{\rm vir}$ of only $\sim 1\times10^{12}$M$_\odot$ also (re-)opens thequestion of whether all of the Milky Way's satellite galaxies are on boundorbits.

astrophysical journal/the astrophysical journal

The Milky Way's Circular Velocity Curve to 60 kpc and an Estimate of the  Dark Matter Halo Mass from Kinematics of ~2400 SDSS Blue Horizontal Branch  Stars