Apocenter Pile-Up: Origin of the Stellar Halo Density Break
  We measure the orbital properties of halo stars using 7-dimensional
information provided by Gaia and the Sloan Digital Sky Survey. A metal-rich
population of stars, present in both local main sequence stars and more distant
blue horizontal branch stars, have very radial orbits (eccentricity ~ 0.9) and
apocenters that coincide with the stellar halo "break radius" at galactocentric
distance r ~ 20 kpc. Previous work has shown that the stellar halo density
falls off much more rapidly beyond this break radius. We argue that the
correspondence between the apocenters of high metallicity, high eccentricity
stars and the broken density profile is caused by the build-up of stars at the
apocenter of a common dwarf progenitor. Although the radially biased stars are
likely present down to metallicities of [Fe/H] ~ -2 the increasing dominance at
higher metallicities suggests a massive dwarf progenitor, which is at least as
massive as the Fornax and Sagittarius dwarf galaxies, and is likely the
dominant progenitor of the inner stellar halo.
