  We employ cosmological hydrodynamical simulations to investigate models in
which the supermassive black holes (BHs) powering luminous z ~ 6 QSOs grow from
massive seeds. We simulate at high resolution 18 fields sampling regions with
densities ranging from the mean cosmic density all the way to the highest sigma
peaks in the Millennium simulation volume. Only in the most massive halos, BHs
can grow to masses up to ~ 10^9 Msun by z ~ 6 without invoking super-Eddington
accretion. Accretion onto the most massive BHs becomes limited by thermal AGN
feedback by z ~ 9-8 with further BH growth proceeding in short Eddington
limited bursts. Our modelling suggests that current flux-limited surveys of
QSOs at high redshift preferentially detect objects at their peak luminosity
and therefore miss a substantial population of QSOs powered by similarly
massive BHs but with low accretion rates. To test whether the required host
halo masses are consistent with the observed galaxy environment of z ~ 6 QSOs,
we produce realistic rest-frame UV images of our simulated galaxies. Without
strong stellar feedback, our simulations predict numbers of bright galaxies
larger than observed by a factor ten or more. Supernova-driven galactic winds
reduce the predicted numbers to a level consistent with observations indicating
that stellar feedback was already very efficient at high redshifts. We have
further investigated the effect of thermal AGN feedback on the surrounding gas.
Our adopted AGN feedback prescription drives mostly energy-driven highly
anisotropic outflows with gas speeds of >= 1000 km/s to distances of >= 10 kpc
consistent with observations. The spatially extended thermal X-ray emission
around bright QSOs powered by these outflows can exceed by large factors the
emission expected without AGN feedback and is an important diagnostic of the
mechanism whereby AGN feedback energy couples to surrounding gas.
