The Role of Primordial Kicks on Black Hole Merger Rates

Department of Physics, Stanford University, Palo Alto, California, United States
Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.11). 10/2006; 372(4). DOI: 10.1111/j.1365-2966.2006.11013.x
Source: arXiv


Primordial stars are likely to be very massive >30 Msun, form in isolation, and will likely leave black holes as remnants in the centers of their host dark matter halos. We expect primordial stars to form in halos in the mass range 10^6-10^10 Msun. Some of these early black holes, formed at redshifts z>10, could be the seed black hole for a significant fraction of the supermassive black holes found in galaxies in the local universe. If the black hole descendants of the primordial stars exist, their mergers with nearby supermassive black holes may be a prime candidate for long wavelength gravitational wave detectors. We simulate formation and evolution of dark matter halos in LambdaCDM universe. We seed high-redshift dark matter halos with early black holes, and explore the merger history of the host halos and the implications of black hole's 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. 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 halos. Therefore, merger rates with central supermassive black holes in the largest halos 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 supermassive black holes in present day galaxies.

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    • "Small kicks could still eject black holes from galaxies in the early universe when halos were much less massive, though the presence of supermassive black holes at the centers of most galaxies might be a more robust consequence of structure formation than naively thought in light of kicks. One study examined the effect of natal kicks in a scenario where supermassive black holes are formed in a primary halo via capture of intermediate mass black holes from surrounding secondary halos, and found that kick velocities imparted to mergers of the intermediate mass black holes had little affect on the growth of the supermassive black hole [301]. Another study following black hole formation through a simplified merger tree model found that even when a high probability of ejection was assigned to merger events, still more than 50% of galaxies today retain their supermassive black holes [309]. "
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