SUPERMASSIVE BLACK HOLE BINARY EVOLUTION IN AXISYMMETRIC GALAXIES: THE FINAL PARSEC PROBLEM IS NOT A PROBLEM

Abstract

During a galaxy merger, the supermassive black hole (SMBH) in each galaxy is\nthought to sink to the center of the potential and form a supermassive black\nhole binary; this binary can eject stars via 3-body scattering, bringing the\nSMBHs ever closer. In a static spherical galaxy model, the binary stalls at a\nseparation of about a parsec after ejecting all the stars in its loss cone --\nthis is the well-known final parsec problem. Earlier work has shown that the\ncentrophilic orbits in triaxial galaxy models are key in refilling the loss\ncone at a high enough rate to prevent the black holes from stalling. However,\nthe evolution of binary SMBHs has never been explored in axisymmetric galaxies,\nso it is not clear if the final parsec problem persists in these systems. Here\nwe use a suite of direct N-body simulations to follow SMBH binary evolution in\ngalaxy models with a range of ellipticity. For the first time, we show that\nmere axisymmetry can solve the final parsec problem; we find the the SMBH\nevolution is independent of N for an axis ratio of c/a=0.8, and that the SMBH\nbinary separation reaches the gravitational radiation regime for c/a=0.75.\n