Observability of Dual Active Galactic Nuclei in Merging Galaxies
  Supermassive black holes (SMBHs) have been detected in the centers of most
nearby massive galaxies. Galaxies today are the products of billions of years
of galaxy mergers, but also billions of years of SMBH activity as active
galactic nuclei (AGNs) that is connected to galaxy mergers. In this context,
detection of AGN pairs should be relatively common. Observationally, however,
dual AGN are scant, being just a few percent of all AGN. In this Letter we
investigate the triggering of AGN activity in merging galaxies via a suite of
high resolution hydrodynamical simulations. We follow the dynamics and
accretion onto the SMBHs as they move from separations of tens of kiloparsecs
to tens of parsecs. Our resolution, cooling and star formation implementation
produce an inhomogeneous, multi-phase interstellar medium, allowing us to
accurately trace star formation and accretion onto the SMBHs. We study the
impact of gas content, morphology, and mass ratio, allowing us to study AGN
activity and dynamics across a wide range of relevant conditions. We test when
the two AGN are simultaneously detectable, for how long and at which
separations. We find that strong dual AGN activity occurs during the late
phases of the mergers, at small separations (<1-10 kpc) below the resolution
limit of most surveys. Much of the SMBH accretion is not simultaneous, limiting
the dual AGN fraction detectable through imaging and spectroscopy to a few
percent, in agreement with observational samples.
