The Coronal Line Region of Active Galactic Nuclei

TL;DR

This paper reviews the properties of coronal line regions in active galactic nuclei, highlighting their extended emission, diagnostic uses for black hole mass and feedback, and recent advances enabled by improved instrumentation.

Contribution

It synthesizes recent developments in understanding coronal lines, emphasizing their extended nature and applications in black hole and feedback studies.

Findings

Coronal lines are extended up to a few kiloparsecs.

They can be used to estimate supermassive black hole masses.

Coronal lines trace energetic feedback in AGN.

Abstract

Forbidden coronal lines has traditionally called the attention due to the high-energy photons required for their production (IP $>$100~eV, where IP is the ionisation potential of the transitions that originate the line). As such, they are regarded as the most highly ionised component of Active Galactic Nuclei (AGN). For decades, it was thought that they were only formed in the inner portions of the narrow line region (NLR). Nowadays, due to the larger sensitivity of the detectors and the availability of integral field unit (IFU) spectrographs, that emission in addition to the nuclear component is found to be extended up to a few kiloparsecs away from the active centre. In this review, we highlight the most important aspects of the coronal emission and discuss the recent developments in the field. In particular, we emphasize the discovery that they can be used to determine the mass of the central supermassive black hole, to reconstruct the SED, as well as to trace the most energetic feedback component of the ionised gas in AGN.