Misaligned gas discs around eccentric black-hole binaries and
  implications for the final-parsec problem
  We investigate the evolution of low mass (Md /Mb = 0.005) misaligned gaseous
discs around eccentric supermassive black hole (SMBH) binaries. These are
expected to form from randomly oriented accretion events onto a SMBH binary
formed in a galaxy merger. When expanding the interaction terms between the
binary and a circular ring to quadrupole order and averaging over the binary
orbit, we expect four non-precessing disc orientations: aligned or
counter-aligned with the binary, or polar orbits around the binary eccentricity
vector with either sense of rotation. All other orientations precess around
either of these, with the polar precession dominating for high eccentricity.
These expectations are borne out by smoothed particle hydrodynamics simulations
of initially misaligned viscous circumbinary discs, resulting in the formation
of polar rings around highly eccentric binaries in contrast to the co-planar
discs around circular binaries. Moreover, we observe disc tearing and violent
interactions between differentially precessing rings in the disc significantly
disrupting the disc structure and causing gas to fall onto the binary with
little angular momentum. While accretion from a polar disc may not promote SMBH
binary coalescence (solving the `final-parsec problem'), ejection of this
infalling low-angular momentum material via gravitational slingshot is a
possible mechanism to reduce the binary separation. Moreover, this process acts
on dynamical rather than viscous time scales, and so is much faster.
