Highly Ionised Gas as a Diagnostic of the Inner NLR

TL;DR

This paper reviews the properties of highly ionised emission lines in AGN spectra, discusses recent studies and case examples, and highlights future high-resolution observations to understand the ionised gas in the inner narrow-line region.

Contribution

It provides a synthesis of recent findings on highly ionised lines in AGN and proposes future observational strategies for detailed study.

Findings

Highly ionised lines span a wide ionisation range in AGN spectra.

Case studies illustrate the properties of these lines in specific objects.

Future IFU observations will enhance understanding of gas excitation and kinematics.

Abstract

The spectra of AGN from the ultraviolet to the near infrared, exhibit emission lines covering a wide range of ionisation states, from neutral species such as [O I] 6300A, up to [Fe XIV] 5303A. Here we report on some recent studies of the properties of highly ionised lines (HILs), plus two case studies of individual objects. Future IFU observations at high spatial and good spectral resolution, will probe the excitation and kinematics of the gas in the zone between the extended NLR and unresolved BLR. Multi-component SED fitting can be used to link the source of photoionisation with the strengths and ratios of the HILs.