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 $2.8\lesssim z\lesssim 6.8$. Spectroscopic data in the optical rest-frame range [3000 $\lambda (\text{A˚})$ 7000] from the JADES survey and from the literature were used to estimate $\text{He/H}$ and $\text{O/H}$ via the $T_{\mathrm{e}}$-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 $T_{\mathrm{e}}$-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 $(He/H{\mathrm{e}}_{\odot })\sim 4.4$. This result remains robust even when we adopt larger electron densities (varying $N_{\mathrm{e}}$ from $500$ to $10000$ $\mathrm{c}{\mathrm{m}}^{-3}$) in abundance estimates using the $T_{\mathrm{e}}$-method and empirical strong-line methods. We found marginal evidence for a decline in the He/H abundance ($\sim 0.04$ dex per redshift unit) toward lower redshifts. In contrast, we found evidence for an increase in $\text{O/H}$ toward the local universe at a rate of approximately 0.06 dex per redshift unit.

17 pages, 12 figures, accepted for publication in MNRAS