Active Galactic Nuclei with Double-peaked Narrow Lines: Are They Dual AGNs?

TL;DR

This study identifies a sample of 87 type 2 AGNs with double-peaked [O III] profiles, providing evidence that most are dual AGNs at ~1 kpc separation, which helps explain the scarcity of observed binary supermassive black holes.

Contribution

The paper presents a statistically supported method to identify dual AGNs using double-peaked [O III] profiles, favoring the dual AGN interpretation over other models.

Findings

Most double-peaked sources are dual AGNs.

Dual AGNs are typically separated by ~1 kpc.

The occurrence rate of dual AGNs is about 1%.

Abstract

Double-peaked [O III]5007, profiles in active galactic nuclei (AGNs) may provide evidence for the existence of dual AGNs, but a good diagnostic for selecting them is currently lacking. Starting from $\sim$ 7000 active galaxies in SDSS DR7, we assemble a sample of 87 type 2 AGNs with double-peaked [O III]5007, profiles. The nuclear obscuration in the type 2 AGNs allows us to determine redshifts of host galaxies through stellar absorption lines. We typically find that one peak is redshifted and another is blueshifted relative to the host galaxy. We find a strong correlation between the ratios of the shifts and the double peak fluxes. The correlation can be naturally explained by the Keplerian relation predicted by models of co-rotating dual AGNs. The current sample statistically favors that most of the [O III] double-peaked sources are dual AGNs and disfavors other explanations, such as rotating disk and outflows. These dual AGNs have a separation distance at $\sim 1$ kpc scale, showing an intermediate phase of merging systems. The appearance of dual AGNs is about $\sim 10^{-2}$, impacting on the current observational deficit of binary supermassive black holes with a probability of $\sim 10^{-4}$ (Boroson & Lauer).