The First Detection of Forbidden Emission Lines at the Outskirts of the AGN Broad Line Region?

TL;DR

This study presents the first detection of forbidden emission lines with double-peaked profiles at the outskirts of the AGN broad-line region, suggesting forbidden lines can originate from lower-density regions farther from the core.

Contribution

It demonstrates that forbidden emission lines can exhibit double-peaked profiles and originate from the outer regions of the BLR, expanding understanding of AGN structure.

Findings

Forbidden lines show double-peaked profiles similar to recombination lines.

Forbidden lines are produced in lower-density regions near the BLR outskirts.

Disk-like BLR model successfully fits the observed profiles.

Abstract

Double-peaked (DP) broad emission line profiles in active galactic nuclei (AGNs) are often interpreted as signatures of rotating disk-like structures in the broad-line region (BLR) and are commonly observed in low-luminosity AGNs using recombination lines. We use optical spectroscopy to investigate the origin of double-peaked broad emission line profiles observed not only in hydrogen recombination lines but also in forbidden transitions in the LINER galaxy IC 1459. We detected DP emission in all strong optical lines except for the [S II] doublet, which has the lowest critical density among all the lines. We successfully fitted the DP broad profiles using a disk-like BLR model, assuming a circular accretion disk with an inclination of approximately 35 degrees and internal turbulence of about 500 km/s, confined within a maximum radius of 9.6 (+4.8, -1.1) light-years. We estimate a full width at half maximum of the DP profiles of about 3300 km/s. Our results provide new insights into the structure of the BLR, indicating that forbidden emission lines can be produced in lower-density regions near the outskirts of the BLR.