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

We measure the orbital properties of halo stars using seven-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 similar to 0.9) and apocenters that coincide with the stellar halo break radius at galactocentric distance r similar to 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] similar to - 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.