A "Light," Centrally-Concentrated Milky Way Halo?
  We discuss a novel approach to "weighing" the Milky Way dark matter halo, one
that combines the latest samples of halo stars selected from the Sloan Digital
Sky Survey (SDSS) with state-of-the-art numerical simulations of Milky Way
analogs. The fully cosmological runs employed in the present study include
"Eris", one of the highest-resolution hydrodynamical simulations of the
formation of a M_vir=8e11 M_sun late-type spiral, and the dark-matter only
M_vir=1.7e12 M_sun "Via Lactea II" simulation. Eris provides an excellent
laboratory for creating mock SDSS samples of tracer halo stars, and we
successfully compare their density, velocity anisotropy, and radial velocity
dispersion profiles with the observational data. Most mock SDSS realizations
show the same "cold veil" recently observed in the distant stellar halo of the
Milky Way, with tracers as cold as sigma_los ~ 50 km/s between 100-150 kpc.
Controlled experiments based on the integration of the spherical Jeans equation
as well as a particle tagging technique applied to Via Lactea II show that a
"heavy" M_vir 2e12 M_sun realistic host produces a poor fit to the kinematic
SDSS data. We argue that these results offer added evidence for a "light,"
centrally-concentrated Milky Way halo.
