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The formation mechanism of supermassive black holes (SMBHs) in general, andof $\sim 10^9\,{\rm M}_{\odot}$ SMBHs observed as luminous quasars at redshifts$z> 6$ in particular, remains an open fundamental question. The presence ofsuch massive BHs at such early times, when the Universe was less than a billionyears old, implies that they grew via either super-Eddington accretion, ornearly uninterrupted gas accretion near the Eddington limit; the latter, atfirst glance, is at odds with empirical trends at lower redshifts, where quasarepisodes associated with rapid BH growth are rare and brief. In this work, Iexamine whether and to what extent the growth of the $z> 6$ quasar SMBHs can beexplained within the standard quasar paradigm, in which major mergers of hostgalaxies trigger episodes of rapid gas accretion below or near the Eddingtonlimit. Using a suite of Monte Carlo merger tree simulations of the assemblyhistories of the likely hosts of the $z> 6$ quasars, I investigate (i) theirgrowth and major merger rates out to $z\sim 40$, and (ii) how long the feedingepisodes induced by host mergers must last in order to explain the observed $z>6$ quasar population without super-Eddington accretion. The halo major mergerrate scales roughly as $\propto (1+z)^{5/2}$, with quasar hosts typicallyexperiencing $> 10$ major mergers between $15> z > 6$ ($\approx 650\,{\rmMyr}$), compared to $\sim 1$ for typical massive galaxies at $3>z > 0$($\approx 11 \,{\rm Gyr}$). An example of a viable sub-Eddington SMBH growthmodel is one where a host merger triggers feeding for a duration comparable tothe halo dynamical time. These findings suggest that the growth mechanisms ofthe earliest quasar SMBHs need not have been drastically different from theircounterparts at lower redshifts.

Driving the Growth of the Earliest Supermassive Black Holes with Major Mergers of Host Galaxies