Type 2 Active Galactic Nuclei with Double-Peaked [OIII] Lines. II. Single AGNs with Complex Narrow-Line Region Kinematics are More Common than Binary AGNs

TL;DR

This study investigates the origins of double-peaked [OIII] emission lines in type 2 AGNs, finding that most are due to complex kinematics around single AGNs rather than binary systems, with only a small fraction being true binary AGNs.

Contribution

The paper provides a detailed analysis combining imaging and spectroscopy to distinguish between binary AGNs and complex kinematic effects in double-peaked [OIII] AGNs, revealing that single AGN kinematics are more common.

Findings

Approximately 10% are binary AGNs at kpc-scale separations.

About 50% show velocity offsets without double stellar components.

The majority are due to kinematic effects in single AGNs.

Abstract

(Abridged) Approximately 1% of low redshift (z<0.3) optically-selected type 2 AGNs show a double-peaked [OIII] narrow emission line profile in their spatially-integrated spectra. Such features are usually interpreted as due either to kinematics, such as biconical outflows and/or disk rotation of the narrow line region (NLR) around single black holes, or to the relative motion of two distinct NLRs in a merging pair of AGNs. Here we report follow-up near infrared (NIR) imaging and optical slit spectroscopy of 31 double-peaked [OIII] type 2 AGNs drawn from the SDSS parent sample presented in Liu et al (2010). These data reveal a mixture of origins for the double-peaked feature. Roughly 10% of our objects are best explained by binary AGNs at (projected) kpc-scale separations, where two stellar components with spatially coincident NLRs are seen. ~ 50% of our objects have [OIII] emission offset by a few kpc, corresponding to the two velocity components seen in the SDSS spectra, but there are no corresponding double stellar components seen in the NIR imaging. For those objects with sufficiently high quality slit spectra, we see velocity and/or velocity dispersion gradients in [OIII] emission, suggestive of the kinematic signatures of a single NLR. The remaining ~40% of our objects are ambiguous, and will need higher spatial resolution observations to distinguish between the two scenarios. Our observations therefore favor the kinematics scenario with a single AGN for the majority of these double-peaked [OIII] type 2 AGNs. We emphasize the importance of combining imaging and slit spectroscopy in identifying kpc binary AGNs, i.e., in no cases does one of these alone allow an unambiguous identification. We estimate that ~ 0.5-2.5% of the z<0.3 type 2 AGNs are kpc-scale binary AGNs of comparable luminosities, with a relative orbital velocity >~150 km/s.