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We develop a flexible set of action-based distribution functions (DFs) forstellar haloes. The DFs have five free parameters, controlling the inner andouter density slope, break radius, flattening and anisotropy respectively. TheDFs generate flattened stellar haloes with a rapidly varying logarithmic slopein density, as well as a spherically aligned velocity ellipsoid with a longaxis that points towards the Galactic centre - all attributes possessed by thestellar halo of the Milky Way. We use our action-based distribution function tomodel the blue horizontal branch stars extracted from the Sloan Digital SkySurvey as stellar halo tracers in a spherical Galactic potential. As theselection function is hard to model, we fix the density law from earlierstudies and solve for the anisotropy and gravitational potential parameters.Our best fit model has a velocity anisotropy that becomes more radiallyanisotropic on moving outwards. It changes from $\beta \approx 0.4$ atGalactocentric radius of 15 kpc to $\approx 0.7$ at 60 kpc. This is a gentlerincrease than is typically found in simulations of stellar haloes built fromthe mutiple accretion of smaller systems. We find the potential corresponds toan almost flat rotation curve with amplitude of $\approx 200$ kms$^{-1}$ atthese distances. This implies an enclosed mass of $\approx 4.5 \times 10^{11}M_\odot$ within a spherical shell of radius 50 kpc.

Haloes light and dark: dynamical models of the stellar halo and constraints on the mass of the Galaxy