Mergers in Double-Peaked [O III] Active Galactic Nuclei
  As a natural consequence of galaxy mergers, binary active galactic nuclei
(AGNs) should be commonplace. Nevertheless, observational confirmations are
rare, especially for binaries with separations less than ten kpc. Such a system
may show two sets of narrow emission lines in a single spectrum owing to the
orbital motion of the binary. We have obtained high-resolution near-infrared
images of 50 double-peaked [O III] 5007 AGNs with the Keck II laser guide star
adaptive optics system. The Sloan Digital Sky Survey sample is compiled from
the literature and consists of 17 type-1 AGNs between 0.18 < z < 0.56 and 33
type-2 AGNs between 0.03 < z < 0.24. The new images reveal eight type-1 and
eight type-2 sources that are apparently undergoing mergers. These are strong
candidates of kpc-scale binary AGNs, because they show multiple components
separated between 0.6 and 12 kpc and often disturbed morphologies. Because most
of the type-1s are at higher redshifts than the type-2s, the higher merger
fraction of type-1s (47+/-20%) compared to that of type-2s (24+/-10%) can be
attributed to the general evolution of galaxy merger fraction with redshift.
Furthermore, we show that AGN mergers are outliers of the M_BH-sigma relation
because of over-estimated stellar velocity dispersions, illustrating the
importance of removing mergers from the samples defining the M_BH-sigma
relations. Finally, we find that the emission-line properties are
indistinguishable for spatially resolved and unresolved sources, emphasizing
that scenarios involving a single AGN can produce the same double-peaked line
profiles and they account for at least 70% of the double-peaked [O III] AGNs.
