4MOST Consortium Survey 6: Active Galactic Nuclei
  X-ray and mid-infrared emission are signposts of the accretion of matter onto
the supermassive black holes that reside at the centres of most galaxies. As a
major step towards understanding accreting supermassive black holes and their
role in the evolution of galaxies, we will use the 4MOST multi-object
spectrograph to provide a highly complete census of active galactic nuclei over
a large fraction of the extragalactic sky observed in X-rays by eROSITA that is
visible to 4MOST. We will systematically follow up all eROSITA point-like
extragalactic X-ray sources (mostly active galactic nuclei), and complement
them with a heavily obscured active galactic nuclei selection approach using
mid-infrared data from the Wide-field Infrared Survey Explorer (WISE). The
X-ray and mid-infrared flux limits of eROSITA and WISE are well matched to the
spectroscopic capabilities of a 4-metre-class telescope, allowing us to reach
completeness levels of ~80-90% for all X-ray selected active galactic nuclei
with fluxes $f_{0.5-2\ {\rm keV}} > 10^{-14}$ erg s$^{-1}$ cm$^{-2}$; this is
about a factor of 30 deeper than the ROSAT all-sky survey. With these data we
will determine the physical properties (redshift, luminosity, line emission
strength, masses, etc.) of up to one million supermassive black holes,
constrain their cosmic evolution and clustering properties, and explore the
connection between active galactic nuclei and large-scale structure over
redshifts $0 \le z \le 6$.
