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The flat stellar density cores of massive elliptical galaxies form rapidlydue to sinking supermassive black holes (SMBH) in gas-poor galaxy mergers.After the SMBHs form a bound binary, gravitational slingshot interactions withnearby stars drive the core regions towards a tangentially biased stellarvelocity distribution. We use collisionless galaxy merger simulations withaccurate collisional orbit integration around the central SMBHs to demonstratethat the removal of stars from the centre by slingshot kicks accounts for theentire change in velocity anisotropy. The rate of strong (unbinding) kicks isconstant over several hundred Myr at $\sim 3 \ M_\odot \rm yr^{-1}$ for ourmost massive SMBH binary ($M_{\rm BH} = 1.7 \times 10^{10} M_\odot$). Using afrequency-based orbit classification scheme (box, x-tube, z-tube, rosette) wedemonstrate that slingshot kicks mostly affect box orbits with small pericentredistances, leading to a velocity anisotropy of $\beta \lesssim -0.6$ withinseveral hundred Myr as observed in massive ellipticals with large cores. Weshow how different SMBH masses affect the orbital structure of the mergerremnants and present a kinematic tomography connecting orbit families tointegral field kinematic features. Our direct orbit classification agreesremarkably well with a modern triaxial Schwarzschild analysis applied tosimulated mock kinematic maps.

The two phases of core formation -- orbital evolution in the centres of ellipticals with supermassive black hole binaries