Work

Identifying AGNs from X-ray detections – II. Metallicity calibrations for the N2O2 and N2S2 diagnostics

Year: 2026

Type: article

Abstract: ABSTRACT Emission-line ratios from ions with nearly identical ionization potentials offer a robust solution to the degeneracies inherent to traditional active galactic nuclei (AGNs) metallicity diagnostics. We introduce new semi-empirical metallicity calibrations for the $\mathrm{N_2 O_2}$ and $\mathrm{N_2 S_2}$ diagnostics, explicitly designed to isolate the chemical abundance from the incident radiation field. By coupling an extensive grid of cloudy photoionization simulations directly to the intrinsic 2–10 keV X-ray luminosity ($L_{\mathrm{X}}$) and benchmarking against Seyfert 2 nuclei from the Burst Alert Telescope AGN Spectroscopic Survey, we establish robust relations valid across the metallicity regime of $8.0 \lesssim 12+\log ({\rm O/H}) \lesssim 9.1$ ($0.2 \lesssim Z/Z_{\odot } \lesssim 2.6$). The $\mathrm{N_2 O_2}$ and $\mathrm{N_2 S_2}$ indices trace co-spatial emitting volumes within the narrow-line regions, enabling this framework to resolve the significant $L_{\mathrm{X}}$-driven systematic biases we previously identified in the standard $\mathrm{N_2}$ and $\mathrm{O_3 N_2}$ indices. While the $\mathrm{N_2 O_2}$ ratio proves to be virtually independent of nebular structural variations, the $\mathrm{N_2 S_2}$ index exhibits a subtle, yet discernible, electron density ($N_{\rm e}$) susceptibility due to the low critical density ($N_{\rm c}$) of the $\mathrm{[S\, {\small II}]}\lambda \lambda 6716,6731$ doublet. Nevertheless, both diagnostics yield highly precise metallicity constraints with tight rms residual dispersions of ${\sim} 0.081$ dex for $\mathrm{N_2 O_2}$ and ${\sim} 0.121$ dex for $\mathrm{N_2 S_2}$. We propose the $\mathrm{N_2 O_2}$ and $\mathrm{N_2 S_2}$ calibrations as highly optimized, unbiased metallicity tracers for AGNs.

Source: Monthly Notices of the Royal Astronomical Society

DOI: 10.1093/mnras/stag1563