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 \sim 0.9 ) and apocenters that coincide with the stellar halo “ break radius ” at galactocentric distance r \sim 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 ] \sim - 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 .