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). 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 ∼ 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 ∼ 1 − 5 times the combined mass of binary SMBHs. The coalescence time scales for SMBH binary with mass ∼ 10 6 M ⊙ are less than 1 Gyr and for those at the upper end of SMBH masses 10 9 M ⊙ 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. Subject headings: Stellar dynamics -black hole physics -Galaxies: kinematics and dynamics -Galaxy: center.

Gravitational-Wave Limits from Pulsar Timing Constrain Supermassive Black Hole Evolution

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The puzzling case of the radio-loud QSO 3C 186: a gravitational wave recoiling black hole in a young radio source?

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