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