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We measure the orbital properties of halo stars using 7-dimensionalinformation provided by Gaia and the Sloan Digital Sky Survey. A metal-richpopulation of stars, present in both local main sequence stars and more distantblue horizontal branch stars, have very radial orbits (eccentricity ~ 0.9) andapocenters that coincide with the stellar halo "break radius" at galactocentricdistance r ~ 20 kpc. Previous work has shown that the stellar halo densityfalls off much more rapidly beyond this break radius. We argue that thecorrespondence between the apocenters of high metallicity, high eccentricitystars and the broken density profile is caused by the build-up of stars at theapocenter of a common dwarf progenitor. Although the radially biased stars arelikely present down to metallicities of [Fe/H] ~ -2 the increasing dominance athigher metallicities suggests a massive dwarf progenitor, which is at least asmassive as the Fornax and Sagittarius dwarf galaxies, and is likely thedominant progenitor of the inner stellar halo.

Apocenter Pile-Up: Origin of the Stellar Halo Density Break