The Dark Side of QSO Formation at High Redshifts
  Observed high-redshift QSOs, at z~6, may reside in massive dark matter (DM)
halos of more than 10^{12} Msun and are thus expected to be surrounded by
overdense regions. In a series of 10 constrained simulations, we have tested
the environment of such QSOs. Comparing the computed overdensities with respect
to the unconstrained simulations of regions empty of QSOs, assuming there is no
bias between the DM and baryon distributions, and invoking an
observationally-constrained duty-cycle for Lyman Break Galaxies, we have
obtained the galaxy count number for the QSO environment. We find that a clear
discrepancy exists between the computed and observed galaxy counts in the Kim
et al. (2009) samples. Our simulations predict that on average eight z~6
galaxies per QSO field should have been observed, while Kim et al. detect on
average four galaxies per QSO field compared to an average of three galaxies in
a control sample (GOODS fields). While we cannot rule out a small number
statistics for the observed fields to high confidence, the discrepancy suggests
that galaxy formation in the QSO neighborhood proceeds differently than in the
field. We also find that QSO halos are the most massive of the simulated volume
at z~6 but this is no longer true at z~3. This implies that QSO halos, even in
the case they are the most massive ones at high redshifts, do not evolve into
most massive galaxy clusters at z=0.
