Haloes light and dark: dynamical models of the stellar halo and
  constraints on the mass of the Galaxy
  We develop a flexible set of action-based distribution functions (DFs) for
stellar haloes. The DFs have five free parameters, controlling the inner and
outer density slope, break radius, flattening and anisotropy respectively. The
DFs generate flattened stellar haloes with a rapidly varying logarithmic slope
in density, as well as a spherically aligned velocity ellipsoid with a long
axis that points towards the Galactic centre - all attributes possessed by the
stellar halo of the Milky Way. We use our action-based distribution function to
model the blue horizontal branch stars extracted from the Sloan Digital Sky
Survey as stellar halo tracers in a spherical Galactic potential. As the
selection function is hard to model, we fix the density law from earlier
studies and solve for the anisotropy and gravitational potential parameters.
Our best fit model has a velocity anisotropy that becomes more radially
anisotropic on moving outwards. It changes from $\beta \approx 0.4$ at
Galactocentric radius of 15 kpc to $\approx 0.7$ at 60 kpc. This is a gentler
increase than is typically found in simulations of stellar haloes built from
the mutiple accretion of smaller systems. We find the potential corresponds to
an almost flat rotation curve with amplitude of $\approx 200$ kms$^{-1}$ at
these distances. This implies an enclosed mass of $\approx 4.5 \times 10^{11}
M_\odot$ within a spherical shell of radius 50 kpc.
