Galaxy Rotation and Rapid Supermassive Black Hole Binary Coalescence
  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. However it has been shown that
SMBH binaries in non-spherical galactic nuclei harden at a nearly constant rate
until reaching the gravitational wave regime. Here we use a suite of direct
N-body simulations to follow SMBH binary evolution in both corotating and
counterrotating flattened galaxy models. For N larger than 500K, we find that
the evolution of the SMBH binary is convergent, and is independent of the
particle number. Rotation in general increases the hardening rate of SMBH
binaries even more effectively than galaxy geometry alone. SMBH binary
hardening rates are similar for co- and counterrotating galaxies. In the
corotating case, the center of mass of SMBH binary settles into an orbit that
is in a corotation resonance with the background rotating model, and the
coalescence time is roughly few hundred Myr faster than a non-rotating
flattened model. We find that counterrotation drives SMBHs to coalesce on a
nearly radial orbit promptly after forming a hard binary. We discuss the
implications for gravitational wave astronomy, hypervelocity star production,
and the effect on the structure of the host galaxy.
