Type 2 AGNs with Double-Peaked [O III] Lines: Narrow Line Region Kinematics or Merging Supermassive Black Hole Pairs?

TL;DR

This study analyzes a sample of 167 type 2 AGNs with double-peaked [O III] emission lines to investigate whether these features are caused by narrow-line region kinematics or merging supermassive black hole pairs, highlighting their host galaxy properties.

Contribution

The paper provides a large, well-characterized sample of double-peaked AGNs and compares their properties to typical AGNs, offering insights into their possible origins and the need for follow-up observations.

Findings

Double-peaked lines are modeled by two velocity components with offsets of a few hundred km/s.

Host galaxies of these AGNs have larger stellar velocity dispersions and masses than typical AGNs.

No definitive conclusion on whether the features are due to kinematics or black hole mergers; further imaging and spectroscopy needed.

Abstract

We present a sample of 167 type 2 AGNs with double-peaked [O III] 4959,5007 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 H-beta 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, 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 sigma*. The host galaxies of this sample show systematically larger sigma*, stellar masses, and concentrations, and older luminosity-weighted mean stellar ages than a regular type 2 AGN sample matched in redshift, [O III] 5007 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 followup are needed to draw firm conclusions.