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 $\sim 10^9\,{\rm M}_{\odot}$ 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 the likely hosts of the $z> 6$ quasars, I investigate (i) their
growth and major merger rates out to $z\sim 40$, and (ii) how long the feeding
episodes induced by host mergers must last in order to explain the observed $z>
6$ quasar population without super-Eddington accretion. The halo major merger
rate scales roughly as $\propto (1+z)^{5/2}$, with quasar hosts typically
experiencing $> 10$ major mergers between $15> z > 6$ ($\approx 650\,{\rm
Myr}$), compared to $\sim 1$ for typical massive galaxies at $3>z > 0$
($\approx 11 \,{\rm Gyr}$). An example of a viable sub-Eddington SMBH growth
model is one where a host 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.
