such a core formation mechanism: during a merger, the two central SMBHs of the progenitors sink to the center of the remnant by dynamical friction and form a binary. The binary then loses angular momentum to stars in the center via three-body interactions, scouring out the core while becoming progressively more tightly bound until it eventually merges [@Begelman80].

Although this model currently represents the only convincing scenario for core formation, only indirect evidence for this process had been presented previously: the size of the core and the amount of starlight that is “missing” in the centers of core galaxies have been found to scale approximately with the mass of the central black hole, as predicted by theory [@Graham04; @KormendyBender09; @KormendyHo13]. By their very nature, however, missing-light measurements depend strongly on model-dependent assumptions about the original central light profiles [@Hopkins10].

Core scouring by a black hole binary proceeds after the phase of violent randomization of stellar orbits by a merger. This is crucial because stellar orbits then become an important “archeological” diagnostic for core formation. The two-body relaxation time for stars in galaxies exceeds the age of the universe. Galaxies are therefore collisionless systems and once the stars have settled on 