  We use zoom-in techniques to re-simulate three high-redshift (z > 5.5) halos
which host 10^9 solar mass blackholes from the ~ Gpc volume, MassiveBlack
cosmological hydrodynamic simulation. We examine a number of factors
potentially affecting supermassive blackhole growth at high redshift in
cosmological simulations. These include numerical resolution, feedback
prescriptions and formulation of smoothed particle hydrodynamics. We find that
varying the size of the region over which feedback energy is deposited
directly, either for fixed number of neighbours or fixed volume makes very
little difference to the accretion history of blackholes. Changing mass
resolution by factors of up to 64 also does not change the blackhole growth
history significantly. We find that switching from the density-entropy
formulation to the pressure-entropy formulation of smoothed particle
hydrodynamics slightly increases the accretion rate onto blackholes. In general
numerical details appear to have small effects on the main fueling mechanism
for blackholes at these high redshifts. We examine the fashion by which this
occurs, finding that the insensitivity to simulation technique seems to be a
hallmark of the cold flow feeding picture of these high-z supermassive
blackholes. We show that the gas that participates in critical accretion
phases, in these massive objects at z > 6~7 is in all cases colder, denser, and
forms more coherent streams than the average gas in the halo. This is also
mostly the case when the blackhole accretion is feedback regulated (z < 6),
however the distinction is less prominent. For our resimulated halos, cold
flows appear to be a viable mechanism for forming the most massive blackholes
in the early universe, occurring naturally in LambdaCDM models of structure
formation. Not requiring fine tuning of numerical parameters, they seem to be
physically inevitable in these objects.
