The Role of Gravitational Recoil in the Assembly of Massive Black Hole Seeds

Abstract

When two black holes merge, the asymmetric emission of gravitational waves provides an impulse to the merged system; this gravitational-wave recoil velocity can be up to 4000 km s −1 , easily fast enough for the black hole to escape its host galaxy. We combine semianalytic modeling with cosmological zoom-in simulations of a Milky Way-type galaxy to investigate the role of black hole spin and gravitational recoil in the epoch of massive black hole (MBH) seeding. We sample four different spin distributions (random, aligned, antialigned, and zero spin), and compare the resulting merger rates, occupation fractions, and MBH-host relations with what is expected by excluding the effect of recoil. The inclusion of gravitational recoil and MBH spin in the assembly of MBH seeds can reduce the final z  = 5 MBH mass by up to an order of magnitude. The MBH occupation fraction, however, remains effectively unaltered due to episodes of black hole formation following a recoil event. While electromagnetic detections of these events are unlikely, the Laser Interferometer Space Antenna is ideally suited to detect gravitational-wave signals from such events.