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 also 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 .
There is , however , an abrupt transition in the hardening rates for mergers with mass ratios somewhere between q \sim 0.05 and 0.1 , resulting from the monotonic decrease of merger-induced triaxiality with mass ratio q , as the secondary galaxy becomes too small and light to significantly perturb the primary , i.e. , the more massive one .
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 \sim 1 - 5 times the combined mass of binary SMBHs .
The coalescence time scales for SMBH binary with mass \sim 10 ^ { 6 } M _ { \odot } are less than 1 Gyr and for those at the upper end of SMBH masses 10 ^ { 9 } M _ { \odot } 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 .