The Mass Profile of the Galaxy to 80 kpc
  The Hypervelocity Star survey presents the currently largest sample of radial
velocity measurements of halo stars out to 80 kpc. We apply spherical Jeans
modeling to these data in order to derive the mass profile of the Galaxy. We
restrict the analysis to distances larger than 25 kpc from the Galactic center,
where the density profile of halo stars is well approximated by a single power
law with logarithmic slope between -3.5 and -4.5. With this restriction, we
also avoid the complication of modeling a flattened Galactic disk. In the range
25 < r < 80 kpc, the radial velocity dispersion declines remarkably little; a
robust measure of its logarithmic slope is between -0.05 and -0.1. The circular
velocity profile also declines remarkably little with radius. The allowed range
of V_c(80 kpc) lies between 175 and 231 km/s, with the most likely value 193
km/s. Compared with the value at the solar location, the Galactic circular
velocity declines by less than 20% over an order of magnitude in radius. Such a
flat profile requires a massive and extended dark matter halo. The mass
enclosed within 80 kpc is 6.9(+3.0-1.2) 10^11 Msun. Our sample of radial
velocities is large enough that the biggest uncertainty in the mass is not
statistical but systematic, dominated by the density slope and anisotropy of
the tracer population. Further progress requires modeling observed datasets
within realistic simulations of galaxy formation.
