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

Abstract

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 $\sim 3M_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$ for our most massive SMBH binary ($M_{\mathrm{BH}}=1.7\times 10^{10}{M}_{\odot }$). 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.

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Cited by in corpus (12)

• Supermassive black hole mass in the massive elliptical galaxy M87 from integral-field stellar dynamics using OASIS and MUSE with adaptive optics: assessing systematic uncertainties
• KETJU -- resolving small-scale supermassive black hole dynamics in GADGET-4
• Modelling the accretion and feedback of supermassive black hole binaries in gas-rich galaxy mergers
• RABBITS -- I. The crucial role of nuclear star formation in driving the coalescence of supermassive black hole binaries
• Accuracy and precision of triaxial orbit models I: SMBH mass, stellar mass and dark-matter halo
• The isotropic center of NGC 5419 -- A core in formation?
• Detailed shapes of the line-of-sight velocity distributions in massive early-type galaxies from non-parametric spectral models
• Accuracy and precision of triaxial orbit models II: Viewing angles, shape and orbital structure
• The supermassive black hole merger driven evolution of high-redshift red nuggets into present-day cored early-type galaxies
• Triaxial Schwarzschild Models of NGC 708: a 10-billion solar mass black hole in a low dispersion galaxy with a Kroupa IMF
• RABBITS -- II. The impact of AGN feedback on coalescing supermassive black holes in disc and elliptical galaxy mergers
• Identifying supermassive black hole recoil in elliptical galaxies