Mergers of Unequal Mass Galaxies: Supermassive Black Hole Binary
  Evolution and Structure of Merger Remnants
  Galaxy centers are residing places for Super Massive Black Holes (SMBHs).
Galaxy mergers bring SMBHs close together to form gravitationally bound binary
systems which, if able to coalesce in less than a Hubble time, would be one of
the most promising sources of gravitational waves for the Laser Interferometer
Space Antenna (LISA). In spherical galaxy models, SMBH binaries stall at a
separation of approximately one parsec, leading to the "final parsec problem"
(FPP). On the other hand, it has been shown that merger-induced triaxiality of
the remnant in equal-mass mergers is capable of supporting a constant supply of
stars on so-called centrophilic orbits that interact with the binary and thus
avoid the FPP. In this paper, using a set of direct N-body simulations of
mergers of initially spherically symmetric galaxies with different mass ratios,
we show that the merger-induced triaxiality is able to drive unequal-mass SMBH
binaries to coalescence. The binary hardening rates are high and depend only
weakly on the mass ratios of SMBHs for a wide range of mass ratios q. The
hardening rates are significantly higher for galaxies having steep cusps in
comparison with those having shallow cups at centers. The evolution of the
binary SMBH leads to relatively shallower inner slopes at the centers of the
merger remnants. The stellar mass displaced by the SMBH binary on its way to
coalescence is ~ 1-5 times the combined mass of binary SMBHs. The coalescence
times for SMBH binary with mass ~ million solar masses are less than 1 Gyr and
for those at the upper end of SMBH masses (~ billion solar masses) are 1-2 Gyr
for less eccentric binaries whereas less than 1 Gyr for highly eccentric
binaries. SMBH binaries are thus expected to be promising sources of
gravitational waves at low and high redshifts.
