Quasars at z = 6: The Survival of the Fittest

The Sloan Digital Sky Survey has detected luminous quasars at very high redshift, z > 6. Follow-up observations indicate that at least some of these quasars are powered by supermassive black holes (SMBHs), with masses in excess of 109 M☉. SMBHs, therefore, seem to have already existed when the universe was less than 1 Gyr old and the bulk of galaxy formation had yet to take place. Here we investigate the extent to which accretion and dynamical processes influence the early growth of SMBHs. We assess the impact of (1) black hole mergers, (2) the influence of the merger efficiency, and (3) the negative contribution due to dynamical effects, which can kick black holes out of their host halos (gravitational recoil). We find that if accretion is always limited by the Eddington rate via a thin disk, the maximum allowed radiative efficiency (or spin) to reproduce the luminosity function at z = 6 is ϵ = 0.12 (or = 0.8), against the adverse effect of the gravitational recoil. Dynamical effects unquestionably cannot be neglected in studies of high-redshift SMBHs. If black holes can accrete at a supercritical rate during an early phase, reproducing the observed SMBH mass values is not an issue, even in the case that the recoil velocity is in the upper limit range, as the mass ratios of merging binaries are skewed toward low values, where the gravitational recoil effect is very mild. We propose that SMBH growth at early times is very selective, and efficient only for black holes hosted in high density peak halos.

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