The role of primordial kicks on black hole merger rates

Abstract

Primordial stars are likely to be very massive (≥ 30 M⊙), form in isolation, and will likely leave black holes as remnants in the centres of their host dark matter haloes. We expect primordial stars to form in haloes in the mass range 106–1010 M⊙. Some of these early black holes, formed at redshifts z≳ 10, could be the seed black holes for a significant fraction of the supermassive black holes (SMBHs) found in galaxies in the local Universe. If the black hole descendants of the primordial stars exist, their merger with nearby SMBHs may be a prime candidate for long wavelength gravitational wave detectors. We simulate formation and evolution of dark matter haloes in a Λ cold dark matter universe. We seed high‐redshift dark matter haloes with early black holes, and explore the merger history of the host haloes and the implications of black hole kick velocities arising from their coalescence. The central concentration of low‐mass early black holes in present‐day galaxies is reduced if they experience even moderate kicks of tens of km s−1. Even such modest kicks allow the black holes to leave their parent halo, which consequently leads to dynamical friction being less effective on the low‐mass black holes that were ejected, compared to those still embedded in their parent haloes. Therefore, merger rates with central SMBHs in the largest halo may be reduced by more than an order of magnitude. Using analytical and illustrative cosmological N‐body simulations, we quantify the role of kicks on the merger rates of black holes formed from massive metal‐free stars with SMBHs in present‐day galaxies.

Recoil velocity at second post-Newtonian order for spinning black hole binaries

“…There has been a lot of effort devoted in quantifying the impact of gravitational recoil on stellar mass black hole population and supermassive black hole (SMBH) growth scenarios [2,3,4,5,6,7,8,9], as well as on formation of galactic cores [10,11]. In addition the observation of a candidate black hole ejected from its host galaxy after a merger has recently been reported [12].…”

Recoiling from a kick in the head-on collision of spinning black holes

“…Both the spin scaling and the orthogonality of spin-kicks and mass-ratio-kicks are consistent with leading-order PN predictions for these effects, as given by Eqs. (7,8).…”

“…The final merger of such systems will produce an intense burst of gravitational radiation; if this radiation is emitted asymmetrically, as in the case of unequal masses and spins, the resulting remnant black hole will experience a recoil kick. The magnitude of this kick is important in a variety of astrophysical situations, such as the cosmological evolution of supermassive black holes [1,2,3,4,5,6,7,8] and the growth and retention of intermediate-mass black holes in dense stellar clusters [9,10,11,12,13,14,15], and it also affects the expected rates of black hole mergers for gravitational wave detectors [16]. Given the importance of recoil kicks in astrophysics, there have been numerous analytic studies of this phenomenon [17,18,19,20,21,22,23,24,25].…”

Gravitational Recoil Velocities from Eccentric Binary Black Hole Mergers

“…The knowledge of the magnitude of the recoil velocity is crucial to understanding the demography of SMBHs at the centers of galaxies and in the interstellar and intergalactic media, and their apparent absence in dwarf galaxies and stellar clusters (Madau & Quataert 2004;Merritt et al 2004). An estimate of the recoil can also be used to constrain theories in which SMBHs grow at the center of dark matter halos (Haiman 2004) and to estimate SMBH merger rates (Micic et al 2006).…”