The Halo's Ancient Metal-Rich Progenitor Revealed with BHB Stars
  Using the data from the Sloan Digital Sky Survey and the Gaia satellite, we
assemble a pure sample of $\sim$3000 Blue Horizontal Branch (BHB) stars with
7-D information, including positions, velocities and metallicities. We
demonstrate that, as traced with BHBs, the Milky Way's stellar halo is largely
unmixed and can not be well represented with a conventional Gaussian velocity
distribution. A single-component model fails because the inner portions of the
halo are swamped with metal-rich tidal debris from an ancient, head-on
collision, known as the "Gaia Sausage". Motivated by the data, we build a
flexible mixture model which allows us to track the evolution of the halo
make-up across a wide range of radii. It is built from two components, one
representing the radially anisotropic Sausage stars with their lobed velocity
distribution, the other representing a more metal-poor and more isotropic
component built up from minor mergers. We show that inside 25 kpc the "Sausage"
contributes at least 50 % of the Galactic halo. The fraction of "Sausage" stars
diminishes sharply beyond 30 kpc, which is the long-established break radius of
the classical stellar halo.
