  High-redshift quasars are believed to reside in highly biased regions of the
Universe, where black hole (BH) growth is sustained by an enhanced number of
mergers and by being at the intersection of filaments bringing fresh gas. This
assumption should be supported by an enhancement of the number counts of
galaxies in the field of view of quasars. While the current observations of
quasar environments do not lead to a consensus on a possible excess of
galaxies, the future missions JWST, WFIRST, and Euclid will provide new
insights on quasar environments, and will substantially increase the number of
study-cases. We are in a crucial period, where we need to both understand the
current observations and predict how upcoming missions will improve our
understanding of BH environments. Using the large-scale simulation Horizon-AGN,
we find that statistically the most massive BHs reside in environments with the
largest number counts of galaxies. However, we find a large variance in galaxy
number counts, and some massive BHs do not show enhanced counts in their
neighborhood. Interestingly, some massive BHs have a very close galaxy
companion but no further enhancement of the galaxy number counts at larger
scales, in agreement with recent observations. We find that AGN feedback in the
surrounding galaxies is able to decrease their luminosity and stellar mass, and
therefore to make them un-observable when using restrictive galaxy selection
criteria. Radiation from the quasars can spread over large distances, which
could affect the formation history of surrounding galaxies, but a careful
analysis of these processes requires radiative transfer simulations.
