The two phases of core formation -- orbital evolution in the centres of
  ellipticals with supermassive black hole binaries
  The flat stellar density cores of massive elliptical galaxies form rapidly
due to sinking supermassive black holes (SMBH) in gas-poor galaxy mergers.
After the SMBHs form a bound binary, gravitational slingshot interactions with
nearby stars drive the core regions towards a tangentially biased stellar
velocity distribution. We use collisionless galaxy merger simulations with
accurate collisional orbit integration around the central SMBHs to demonstrate
that the removal of stars from the centre by slingshot kicks accounts for the
entire change in velocity anisotropy. The rate of strong (unbinding) kicks is
constant over several hundred Myr at $\sim 3 \ M_\odot \rm yr^{-1}$ for our
most massive SMBH binary ($M_{\rm BH} = 1.7 \times 10^{10} M_\odot$). Using a
frequency-based orbit classification scheme (box, x-tube, z-tube, rosette) we
demonstrate that slingshot kicks mostly affect box orbits with small pericentre
distances, leading to a velocity anisotropy of $\beta \lesssim -0.6$ within
several hundred Myr as observed in massive ellipticals with large cores. We
show how different SMBH masses affect the orbital structure of the merger
remnants and present a kinematic tomography connecting orbit families to
integral field kinematic features. Our direct orbit classification agrees
remarkably well with a modern triaxial Schwarzschild analysis applied to
simulated mock kinematic maps.
