BHB stars reveal halo's metal-rich progenitor

We demonstrate that, as traced with BHBs, the Milky Way's stellar halo is largely unmixed and cannot 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 that 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, and the other representing a more metal-poor and more isotropic component built up from minor mergers. We show that inside 25kpc the 'Sausage' contributes at least 50 per cent of the Galactic halo.