Quasars atz = 6: The Survival of the Fittest

The Sloan Digital Sky survey detected luminous quasars at very high redshift,z>6. Follow-up observations indicated that at least some of these quasars arepowered by supermassive black holes (SMBHs) with masses in excess of billionsolar masses. SMBHs, therefore, seem to have already existed when the Universewas less than 1 Gyr old, and the bulk of galaxy formation still has to takeplace. We investigate in this paper to which extent accretion and dynamicalprocesses influence the early growth of SMBHs. We assess the impact of (i)black hole mergers, (ii) the influence of the merging efficiency and (iii) thenegative contribution due to dynamical effects which can kick black holes outof their host halos (gravitational recoil). We find that if accretion is alwayslimited by the Eddington rate via a thin disc, the maximum radiative efficiencyallowed to reproduce the LF at z=6 is of order 12%, when the adverse effect ofthe gravitational recoil is taken into consideration. Dynamical effects cannotbe neglected in studies of high-redshift SMBHs. If black holes can accrete atsuper-critical rate during an early phase, reproducing the observed SMBH massvalues is not an issue, even in the case that the recoil velocity is in theupper limits range, as the mass ratios of merging binaries are skewed towardslow values, where the gravitational recoil effect is very mild. We propose thatSMBH growth at early times is very selective, and efficient only for blackholes hosted in high density peak halos.Comment: Accepted for publication in the ApJ. 9 pages, 6 b/w figure