Coronal-Line Forest AGN: the best view of the inner edge of the AGN torus?

TL;DR

This paper introduces Coronal-Line Forest AGN, revealing their emission lines originate near the inner edge of the torus, with implications for understanding AGN structure and viewing angles.

Contribution

It identifies the physical location of CLiF emission regions as near the dust sublimation zone, linking spectral features to the AGN torus geometry.

Findings

CLiF regions have higher densities than FLIL regions.

The radial distance of CLiF regions is comparable to dust sublimation distances.

Unusual FHIL strength is due to specific viewing angles.

Abstract

We introduce Coronal-Line Forest Active Galactic Nuclei (CLiF AGN), AGN which have a rich spectrum of forbidden high-ionization lines (FHILs, e.g. [FeVII], [FeX] and [NeV]), as well as relatively strong narrow ($\sim$300 km s$^{-1}$) H$\alpha$ emission when compared to the other Balmer transition lines. We find that the kinematics of the CLiF emitting region are similar to those of the forbidden low-ionization emission-line (FLIL) region. We compare emission line strengths of both FHILs and FLILs to CLOUDY photoionization results and find that the CLiF emitting region has higher densities (10$^{4.5}$ $<$ n$_H$ $<$ 10$^{7.5}$ cm$^{-3}$) when compared to the FLIL emitting region (10$^{3.0}$ $<$ n$_H$ $<$ 10$^{4.5}$ cm$^{-3}$). We use the photoionization results to calculate the CLiF regions radial distances (0.04 $<$ R$_{CLiF}$ $<$ 32.5 pc) and find that they are comparable to the dust grain sublimation distances (0.10 $<$ R$_{SUB}$ $<$ 4.3 pc). As a result we suggest that the inner torus wall is the most likely location of the CLiF region, and the unusual strength of the FHILs is due to a specific viewing angle giving a maximal view of the far wall of the torus without the continuum being revealed.