A High Fraction of Double-peaked Narrow Emission Lines in Powerful Active Galactic Nuclei

TL;DR

This study finds that the occurrence of double-peaked narrow emission lines in Type 2 AGNs increases with AGN luminosity, indicating more prevalent outflows or SMBH mergers in more powerful AGNs.

Contribution

It establishes a significant correlation between AGN luminosity and the fraction of double-peaked narrow emission lines, expanding understanding of AGN outflows and mergers across luminosities.

Findings

Significant (~4.2σ) correlation between [O III] luminosity and double-peaked line fraction.

Higher luminosity AGNs show increased prevalence of outflows or SMBH mergers.

Sample includes 178 Type 2 AGNs at z~0.5 from SDSS-III/BOSS survey.

Abstract

One percent of redshift z~0.1 Active Galactic Nuclei (AGNs) show velocity splitting of a few hundred km/s in the narrow emission lines in spatially integrated spectra. Such line profiles have been found to arise from the bulk motion of ionized gas clouds associated with galactic-scale outflows, merging pairs of galaxies each harboring a supermassive black hole (SMBH), and/or galactic-scale disk rotation. It remains unclear, however, how the frequency of narrow-line velocity splitting may depend on AGN luminosity. Here we study the correlation between the fraction of Type 2 AGNs with double-peaked narrow emission lines and AGN luminosity as indicated by [O III]5007 emission-line luminosity L_[O III]. We combine the sample of Liu et al. (2010) at z~0.1 with a new sample of 178 Type 2 AGNs with double-peaked [O III] emission lines at z~0.5. We select the new sample from a parent sample of 2089 Type 2 AGNs from the SDSS-III/Baryon Oscillation Spectroscopic Survey. We find a statistically significant (~4.2\sigma) correlation between L_[O III] and the fraction of objects that exhibit double-peaked narrow emission lines among all Type 2 AGNs, corrected for selection bias and incompleteness due to [O III] line width, equivalent width, splitting velocity, and/or equivalent width ratio between the two velocity components. Our result suggests that galactic-scale outflows and/or merging pairs of SMBHs are more prevalent in more powerful AGNs, although spatially resolved follow up observations are needed to resolve the origin(s) for the narrow-line velocity splitting for individual AGNs.