Empirical calibration for helium abundance determinations in Active Galactic Nuclei

TL;DR

This paper introduces an empirical calibration linking the HeI $876$/H$eta$ emission line ratio to helium abundance in Seyfert 2 AGN, enabling easier helium measurements from strong lines with high accuracy.

Contribution

It presents the first calibration method for helium abundance in Seyfert 2 nuclei based on emission line ratios, validated with observational data and direct abundance estimates.

Findings

Calibration estimates helium abundance with 0.02 dex uncertainty.

Application yields helium distribution similar to $T_e$-method results.

Cautions for high electron temperature or ionization parameter regimes.

Abstract

For the first time, a calibration between the HeI $\lambda5876$/H$\beta$ emission line ratio and the helium abundance $y$=12+log(He/H) for Narrow line regions (NLRs) of Seyfert~2 Active Galactic Nuclei (AGN) is proposed. In this context, observational data (taken from the SDSS-DR15 and from the literature) and direct abundance estimates (via the $T_{\rm e}$-method) for a sample of 65 local ($z \: < \: 0.2$) Seyfert~2 nuclei are considered. The resulting calibration estimates the $y$ abundance with an average uncertainty of 0.02 dex. Applying our calibration to spectroscopic data containing only strong emission lines, it yields a helium abundance distribution similar to that obtained via the $T_{\rm e}$-method. Some cautions must be considered to apply our calibration for Seyfert~2 nuclei with high values of electron temperature ($\gtrsim\: 20\,000$ K) or ionization parameter ($\log U > -2.0$).