Lines: Narrow Line Region Kinematics or Merging Supermassive Black Hole Pairs?

We present a sample of 167 type 2 AGNs with double-peaked [O III]λλ4959,5007 narrow emission lines, selected from the seventh data release of the Sloan Digital Sky Survey. The double-peaked profiles can be well modeled by two velocity components, blueshifted and redshifted from the systemic velocity. Half of these objects have a more prominent redshifted component. In cases where the Hβ emission line is strong, it also shows two velocity components whose line-of-sight (LOS) velocity offsets are consistent with those of [O III]. The relative LOS velocity offset between the two components is typically a few hundred kms−1, larger by a factor of ∼ 1.5 than the line full width at half maximum of each component. The offset correlates with the host stellar velocity dispersion σ∗. The host galaxies of this sample show systematically larger σ∗, stellar masses, and concentrations, and older luminosity-weighted mean stellar ages than a regular type 2 AGN sample matched in redshift, [O III]λ5007 equivalent width and luminosity; they show no significant difference in radio properties. These double-peaked features could be due to narrow-line region kinematics, or binary black holes. We suggest that a considerable fraction of the sample could be binaries, but spatially resolved optical imaging, spectroscopy, radio or X-ray follow up are needed to draw firm conclusions. Subject headings: black hole physics – cosmology: observations – galaxies: active – quasars: general – surveys