Dual AGN in the Horizon-AGN simulation and their link to galaxy and
  massive black hole mergers, with an excursus on multiple AGN
  The occurrence of dual active galactic nuclei (AGN) on scales of a few tens
of kpc can be used to study merger-induced accretion on massive black holes
(MBHs) and to derive clues on MBH mergers, using dual AGN as a parent
population of precursors. We investigate the properties of dual AGN in the
cosmological simulation Horizon-AGN. We create catalogs of dual AGN selected
with distance and luminosity criteria, plus sub-catalogs where further mass
cuts are applied. We divide the sample into dual AGN hosted in different
galaxies, on the way to a merger, and into those hosed in one galaxy, after the
galaxy merger has happened. We find that the most luminous AGN in a pair has
higher Eddington ratio and mass than the general AGN population, but that the
relation between MBH and galaxy mass is similar to that of all AGN. The typical
mass ratio of galaxy mergers associated to dual AGN is 0.2, with mass loss in
the smaller galaxy decreasing the mass ratio as the merger progresses. The
connection between dual AGN and MBH mergers is weaker. Between 30 and 80 per
cent of dual AGN with separations between 4 and 30 kpc can be matched to an
ensuing MBH merger. The dual AGN fraction increases with redshift and with
separation threshold, although above 50 kpc the increase of multiple AGN limits
that of duals. Multiple AGN are generally associated with massive halos, and
mass loss of satellites shapes the galaxy-halo relation.
