Cosmic evolution of the helium and oxygen abundances in type 2 active galactic nuclei: helium-loud AGNs

We derive the helium and oxygen abundances in the narrow-line regions of 84 active galactic nuclei (AGNs) spanning two redshift ranges: 65 objects at (Formula presented) and 19 objects at (Formula presented). Spectroscopic data in the optical rest-frame range (Formula presented) from the JWST (James Webb Space Telescope) Advanced Deep Extragalactic Survey and from the literature were used to estimate He/H and O/H via the (Formula presented) -method (7 objects) and strong-emission line calibrations (19 objects). Our results indicate that the ionization degree in AGNs increases towards higher redshifts, exhibiting a trend similar to that observed in star-forming galaxies. We find that the majority of our sample of AGNs at (Formula presented) show oversolar helium abundances and subsolar oxygen abundances. We identified, via the (Formula presented) -method, an object (goods-s-mediumhst-58850; (Formula presented) ) with the highest helium abundance estimated to date, i.e. (Formula presented), corresponding to (Formula presented). This result remains robust even when we adopt larger electron densities (varying (Formula presented) from 500 to (Formula presented)  (Formula presented) ) in abundance estimates using the (Formula presented) -method and empirical strong-line methods. We found marginal evidence for a decline in the He/H abundance ((Formula presented) dex per redshift unit) towards lower redshifts. In contrast, we found evidence for an increase in O/H towards the local universe at a rate of approximately 0.06 dex per redshift unit.