MERGERS OF UNEQUAL-MASS GALAXIES: SUPERMASSIVE BLACK HOLE BINARY EVOLUTION AND STRUCTURE OF MERGER REMNANTS

Abstract

Galaxy centers are residing places for Super Massive Black Holes (SMBHs).\nGalaxy mergers bring SMBHs close together to form gravitationally bound binary\nsystems which, if able to coalesce in less than a Hubble time, would be one of\nthe most promising sources of gravitational waves for the Laser Interferometer\nSpace Antenna (LISA). In spherical galaxy models, SMBH binaries stall at a\nseparation of approximately one parsec, leading to the "final parsec problem"\n(FPP). On the other hand, it has been shown that merger-induced triaxiality of\nthe remnant in equal-mass mergers is capable of supporting a constant supply of\nstars on so-called centrophilic orbits that interact with the binary and thus\navoid the FPP. In this paper, using a set of direct N-body simulations of\nmergers of initially spherically symmetric galaxies with different mass ratios,\nwe show that the merger-induced triaxiality is able to drive unequal-mass SMBH\nbinaries to coalescence. The binary hardening rates are high and depend only\nweakly on the mass ratios of SMBHs for a wide range of mass ratios q. The\nhardening rates are significantly higher for galaxies having steep cusps in\ncomparison with those having shallow cups at centers. The evolution of the\nbinary SMBH leads to relatively shallower inner slopes at the centers of the\nmerger remnants. The stellar mass displaced by the SMBH binary on its way to\ncoalescence is ~ 1-5 times the combined mass of binary SMBHs. The coalescence\ntimes for SMBH binary with mass ~ million solar masses are less than 1 Gyr and\nfor those at the upper end of SMBH masses (~ billion solar masses) are 1-2 Gyr\nfor less eccentric binaries whereas less than 1 Gyr for highly eccentric\nbinaries. SMBH binaries are thus expected to be promising sources of\ngravitational waves at low and high redshifts.\n