Formation of counter-rotating and highly eccentric massive black hole
  binaries in galaxy mergers
  Supermassive black hole (SMBH) binaries represent the main target for
missions such as the Laser Interferometer Space Antenna and Pulsar Timing
Arrays. The understanding of their dynamical evolution prior to coalescence is
therefore crucial to improving detection strategies and for the astrophysical
interpretation of the gravitational wave data. In this paper, we use
high-resolution $N$-body simulations to model the merger of two equal-mass
galaxies hosting a central SMBH. In our models, all binaries are initially
prograde with respect to the galaxy sense of rotation. But, binaries that form
with a high eccentricity, $e\gtrsim 0.7$, quickly reverse their sense of
rotation and become almost perfectly retrograde at the moment of binary
formation. The evolution of these binaries proceeds towards larger
eccentricities, as expected for a binary hardening in a counter-rotating
stellar distribution. Binaries that form with lower eccentricities remain
prograde and at comparatively low eccentricities. We study the origin of the
orbital flip by using an analytical model that describes the early stages of
binary evolution. This model indicates that the orbital plane flip is due to
the torque from the triaxial background mass distribution that naturally arises
from the galactic merger process. Our results imply the existence of a
population of SMBH binaries with a high eccentricity and could have significant
implications for the detection of the gravitational wave signal emitted by
these systems.
