Theoretical Diagnostics for Narrow Line Regions of Active Galactic Nuclei

TL;DR

This paper develops and tests new theoretical diagnostics for gas metallicity, ionization, and pressure in AGN narrow line regions, filling a gap in existing methods and enabling better metallicity studies of AGN-hosting galaxies.

Contribution

It introduces a suite of theoretical diagnostics for AGN narrow line regions based on a new AGN model, and evaluates their robustness and consistency with observations.

Findings

Diagnostics are consistent within ~0.3 dex across methods.

HII and AGN diagnostics should not be used interchangeably.

New diagnostics enable metallicity studies of AGN-dominated galaxies.

Abstract

Gas metallicity, ionization parameter, and gas pressure can affect the observed ratios of specific strong emission lines within galaxies. While the theoretical strong lines diagnostics for gas metallicity, ionization parameters, and gas pressure in star-forming regions are well-established, theoretical diagnostics for active galactic nuclei (AGNs) narrow line regions are still lacking. In Zhu et al. (2023), we presented a new AGN model that provides the best predictions for observations spanning the UV, optical, and infrared wavelengths. This paper presents a suite of theoretical diagnostics for the gas metallicity, ionization parameter, gas pressure, and the peak energy in AGN ionizing radiation field $E_{peak}$ for AGN narrow-line regions spanning the UV and optical wavelengths. We investigate the model dependency on the ionization parameter, gas pressure, $E_{peak}$, and the nitrogen scaling relation and make recommendations on metallicity diagnostics that are most robust against these parameters. We test our new AGN metallicity diagnostics using optical galaxy spectra from Sloan Digital Sky Survey DR16. These tests show that the metallicities measured from different diagnostics in this paper are consistent within $\sim0.3$ dex. We compare consistent HII and AGN diagnostics and demonstrate that HII and AGN diagnostics should not be used interchangeably. With a wide wavelength coverage, we anticipate that these AGN diagnostics will enable new metallicity studies of galaxies dominated by AGN.