Cosmic evolution of the helium and oxygen abundances in type 2 Active Galactic Nuclei: Helium-loud AGNs

Abstract

We derive the helium and oxygen abundances in the Narrow Line Regions (NLRs) of 84 Active Galactic Nuclei (AGNs) spanning two redshift ranges: 65 objects at z < 0.2 and 19 objects at 2.8 ≲ z ≲ 6.8. Spectroscopic data in the optical rest-frame range [3000 < λ(Å) < 7000] from the JADES survey and from the literature were used to estimate He/H and O/H via the Te-method (7 objects) and strong-emission line calibrations (19 objects). Our results indicate that the ionization degree in AGNs increases toward higher redshifts, exhibiting a trend similar to that observed in star-forming galaxies. We find that the majority of our sample of AGNs at z > 2.8 show oversolar helium abundances and subsolar oxygen abundances. We identified, via the Te-method, an object (goods-s-mediumhst-58850; z = 6.2615) with the highest helium abundance estimated to date, i.e. 12 + log (He/H) = 11.64, corresponding to $\rm (He/He_{\odot }) \sim 4.4$. This result remains robust even when we adopt larger electron densities (varying Ne from 500 to 10 000 $\rm cm^{-3}$) in abundance estimates using the Te-method and empirical strong-line methods. We found marginal evidence for a decline in the He/H abundance (~0.04 dex per redshift unit) toward lower redshifts. In contrast, we found evidence for an increase in O/H toward the local universe at a rate of approximately 0.06 dex per redshift unit.