Adaptive Optics Imaging of QSOs with Double-Peaked Narrow Lines: Are
  they Dual AGNs?
  Active galaxies hosting two accreting and merging super-massive black holes
(SMBHs) -- dual Active Galactic Nuclei (AGN) -- are predicted by many current
and popular models of black hole-galaxy co-evolution. We present here the
results of a program that has identified a set of probable dual AGN candidates
based on near Infra-red (NIR) Laser Guide-Star Adaptive Optics (LGS AO) imaging
with the Keck II telescope. These candidates are selected from a complete
sample of radio-quiet Quasi-stellar Objects (QSOs) drawn from the Sloan Digital
Sky Survey (SDSS), which show double-peaked narrow AGN emission lines. Of the
twelve AGNs imaged, we find six with double galaxy structure, of which four are
in galaxy mergers. We measure the ionization of the two velocity components in
the narrow AGN lines to test the hypothesis that both velocity components come
from an active nucleus. The combination of a well-defined parent sample and
high-quality imaging allows us to place constraints on the fraction of SDSS
QSOs that host dual accreting black holes separated on kiloparsec (kpc) scales:
~0.3%-0.65%. We derive from this fraction the time spent in a QSO phase during
a typical merger and find a value that is much lower than estimates that arise
from QSO space densities and galaxy merger statistics. We discuss possible
reasons for this difference. Finally, we compare the SMBH mass distributions of
single and dual AGN and find little difference between the two within the
limited statistics of our program, hinting that most SMBH growth happens in the
later stages of a merger process.
