Driving the growth of the earliest supermassive black holes with major mergers of host galaxies

The formation mechanism of supermassive black holes (SMBHs) in general, and of similar to 10(9) M-circle dot SMBHs observed as luminous quasars at redshifts z > 6 in particular, remains an open fundamental question. The presence of such massive BHs at such early times, when the Universe was less than a billion years old, implies that they grew via either super-Eddington accretion, or nearly uninterrupted gas accretion near the Eddington limit; the latter, at first glance, is at odds with empirical trends at lower redshifts, where quasar episodes associated with rapid BH growth are rare and brief. In this work, I examine whether and to what extent the growth of the z > 6 quasar SMBHs can be explained within the standard quasar paradigm, in which major mergers of host galaxies trigger episodes of rapid gas accretion below or near the Eddington limit. Using a suite of Monte Carlo merger tree simulations of the assembly histories of 40 likely z > 6 quasar host halos, I investigate (i) their growth and major merger rates out to z similar to 40, and (ii) how long the feeding episodes induced by host mergers must last in order to explain the observed z greater than or similar to 6 quasar population without super-Eddington accretion. The halo major merger rate scales roughly as alpha (1 + z)(5/2), consistent with cosmological simulations at lower redshifts, with quasar hosts typically experiencing greater than or similar to 10 major mergers between 15 > z > 6 (approximate to 650 Myr), compared to similar to 1 for typical massive galaxies at 3 > z > 0 (approximate to 11 Gyr). The high rate of major mergers allows for nearly continuous SMBH growth if (for example) a merger triggers feeding for a duration comparable to the halo dynamical time. These findings suggest that the growth mechanisms of the earliest quasar SMBHs need not have been drastically different from their counterparts at lower redshifts.