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We employ cosmological hydrodynamical simulations to investigate models inwhich the supermassive black holes (BHs) powering luminous z ~ 6 QSOs grow frommassive seeds. We simulate at high resolution 18 fields sampling regions withdensities ranging from the mean cosmic density all the way to the highest sigmapeaks in the Millennium simulation volume. Only in the most massive halos, BHscan grow to masses up to ~ 10^9 Msun by z ~ 6 without invoking super-Eddingtonaccretion. Accretion onto the most massive BHs becomes limited by thermal AGNfeedback by z ~ 9-8 with further BH growth proceeding in short Eddingtonlimited bursts. Our modelling suggests that current flux-limited surveys ofQSOs at high redshift preferentially detect objects at their peak luminosityand therefore miss a substantial population of QSOs powered by similarlymassive BHs but with low accretion rates. To test whether the required hosthalo masses are consistent with the observed galaxy environment of z ~ 6 QSOs,we produce realistic rest-frame UV images of our simulated galaxies. Withoutstrong stellar feedback, our simulations predict numbers of bright galaxieslarger than observed by a factor ten or more. Supernova-driven galactic windsreduce the predicted numbers to a level consistent with observations indicatingthat stellar feedback was already very efficient at high redshifts. We havefurther investigated the effect of thermal AGN feedback on the surrounding gas.Our adopted AGN feedback prescription drives mostly energy-driven highlyanisotropic outflows with gas speeds of >= 1000 km/s to distances of >= 10 kpcconsistent with observations. The spatially extended thermal X-ray emissionaround bright QSOs powered by these outflows can exceed by large factors theemission expected without AGN feedback and is an important diagnostic of themechanism whereby AGN feedback energy couples to surrounding gas.

The environment of bright QSOs at z ~ 6: Star forming galaxies and X-ray emission