The Prevalence of Narrow Optical Fe II Emission Lines in Type 1 Active Galactic Nuclei

TL;DR

This study reveals that narrow optical Fe II emission lines are common in type 1 AGNs but absent in type 2 AGNs, indicating a specific location within the inner regions of active galactic nuclei.

Contribution

It provides the first statistical evidence of the prevalence of narrow optical Fe II lines in type 1 AGNs and suggests their origin in a disk-like structure near the nucleus.

Findings

Narrow optical Fe II lines are prevalent in type 1 AGNs.

These lines are absent in type 2 AGNs across various luminosities.

The Fe II-emitting gas is likely confined to a small, disk-like region near the nucleus.

Abstract

From detailed spectral analysis of a large sample of low-redshift active galactic nuclei (AGNs) selected from the Sloan Digital Sky Survey, we demonstrate---statistically for the first time---that narrow optical Fe II emission lines, both permitted and forbidden, are prevalent in type 1 AGNs. Remarkably, these optical lines are completely absent in type 2 AGNs, across a wide luminosity range, from Seyfert 2 galaxies to type 2 quasars. We suggest that the narrow FeII-emitting gas is confined to a disk-like geometry in the innermost regions of the narrow-line region on physical scales smaller than the obscuring torus.