Narrow double-peaked emission lines of SDSS J131642.90+175332.5: signature of a single or a binary AGN in a merger, jet-cloud interaction, or unusual narrow-line region geometry

TL;DR

This paper analyzes the complex emission line features of SDSS J131642.90+175332.5, exploring whether they indicate a binary AGN, jet-cloud interactions, or unique NLR structures, and discusses various models and future observations.

Contribution

It provides a comprehensive analysis of double-peaked emission lines in SDSS J131642.90+175332.5, evaluating multiple scenarios and proposing future observations to distinguish between them.

Findings

Double-peaked narrow emission lines observed in all lines.

Unusual broad component at intermediate velocity.

Single galaxy or merger more likely than binary AGN.

Abstract

We present an analysis of the active galaxy SDSS J131642.90+175332.5, which is remarkable because all of its narrow emission lines are double-peaked, and because it additionally shows an extra broad component (FHWM ~ 1400 km/s) in most of its forbidden lines, peaking in between the two narrow systems. The peaks of the two narrow systems are separated by 400--500 km/s in velocity space. The spectral characteristics of double-peaked [O III] emission have previously been interpreted as a signature of dual or binary active galactic nuclei (AGNs), among other models. In the context of the binary scenario, SDSS J131642.90+175332.5 is a particularly good candidate because not just one line but all of its emission lines are double-peaked. However, we also discuss a number of other scenarios which can potentially account for double-peaked narrow emission lines, including projection effects, a two-sided outflow, jet-cloud interactions, special narrow-line region (NLR) geometries (disks, bars, or inner spirals), and a galaxy merger with only one AGN illuminating two NLRs. We argue that the similarity of the emission-line ratios in both systems, and the presence of the very unusual broad component at intermediate velocity, makes a close pair of unrelated AGNs unlikely, and rather argues for processes in a single galaxy or merger. We describe future observations which can distinguish between these remaining possibilities.