Type 2 active galactic nuclei with double-peaked [O III] Lines: Narrow-line region kinematics or merging supermassive black hole pairs?

Abstract

We present a sample of 167 type 2 active galactic nuclei (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 km s-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. The statistical properties do not show strong preference for or against either scenario, and spatially resolved optical imaging, spectroscopy, radio or X-ray follow-up are needed to draw firm conclusions. © 2010. The American Astronomical Society.