Empirical calibration for helium abundance determinations in Active Galactic Nuclei

For the first time, a calibration between the HeI λ5876/Hβ 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 Tₑ-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 Tₑ-method. Some cautions must be considered to apply our calibration for Seyfert~2 nuclei with high values of electron temperature (\: 20\,000 K) or ionization parameter (log U > -2.0).