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arxiv: 2606.25890 · v1 · pith:P7ZO25LXnew · submitted 2026-06-24 · 🌌 astro-ph.GA

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

O. L. Dors , M. Armah , G. F. H\"agele , M. V. Cardaci , P. C. Santos , R. A. Riffel , G. Cresci , R. Riffel
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L. N. Marinho X. Ji Y. Isobe R. Maiolino A. Marconi A. Feltre R. L. Sanders I. N. Morais G. C. Almeida
This is my paper

Pith reviewed 2026-06-25 20:03 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords active galactic nucleihelium abundanceoxygen abundancenarrow line regionscosmic evolutionhigh redshiftTe-method
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The pith

Most type 2 AGNs at z greater than 2.8 show oversolar helium and subsolar oxygen abundances in their narrow line regions.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper measures helium and oxygen abundances in the narrow line regions of 84 type 2 active galactic nuclei, split between 65 nearby objects and 19 at redshifts from 2.8 to 6.8. It uses the electron temperature method on seven objects and strong-emission-line calibrations on the rest, drawing spectra from the JADES survey and published data. The central result is that high-redshift AGNs mostly have more helium than the Sun but less oxygen, with one source at redshift 6.26 reaching the highest helium value recorded so far. This pattern, together with rising ionization at earlier times, indicates that the chemical makeup of AGN gas evolves differently than expected from local samples alone.

Core claim

The majority of AGNs at z > 2.8 show oversolar helium abundances and subsolar oxygen abundances. One object at z=6.2615 reaches 12+log(He/H)=11.64 or (He/He⊙)∼4.4, the highest helium abundance estimated to date. The ionization degree rises with redshift, He/H declines marginally by ~0.04 dex per unit redshift toward lower z, and O/H rises by ~0.06 dex per unit redshift toward the present day.

What carries the argument

The Te-method and strong-emission line calibrations applied to rest-frame optical spectra to derive absolute He/H and O/H values.

If this is right

  • Ionization degree in AGNs increases toward higher redshifts in a manner similar to star-forming galaxies.
  • Helium abundance shows a marginal decline of ~0.04 dex per redshift unit toward the local universe.
  • Oxygen abundance increases toward the present day at ~0.06 dex per redshift unit.
  • The high helium values remain stable even when electron density is raised from 500 to 10 000 cm^{-3}.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Early AGN hosts may experience distinct helium enrichment pathways compared with local counterparts.
  • Models of galactic chemical evolution at high redshift may need to incorporate helium-rich gas in AGN narrow-line regions.
  • Future high-resolution spectra of the same objects could test whether the extreme helium values persist under different analysis assumptions.

Load-bearing premise

The strong-emission-line calibrations and Te-method return accurate absolute abundances without large systematic offsets caused by differing ionization parameters, density distributions, or dust depletion relative to the local calibrating samples.

What would settle it

A direct temperature measurement or independent abundance method on goods-s-mediumhst-58850 that returns 12+log(He/H) below 11.0 would falsify the record helium claim.

Figures

Figures reproduced from arXiv: 2606.25890 by A. Feltre, A. Marconi, G. C. Almeida, G. Cresci, G. F. H\"agele, I. N. Morais, L. N. Marinho, M. Armah, M. V. Cardaci, O. L. Dors, P. C. Santos, R. A. Riffel, R. L. Sanders, R. Maiolino, R. Riffel, X. Ji, Y. Isobe.

Figure 1
Figure 1. Figure 1: Bottom panel: logarithm of [O III]𝜆5007/H𝛽 versus [N II]𝜆6584/H𝛼. Red points represent 19 type 2 AGNs from the JADES survey (see Sect. 2.1.2) located in the redshift range 2.8 ≤ 𝑧 ≤ 6.8. Black points represent 65 local type 2 AGNs (𝑧 < 0.2) taken from Dors et al. (2022). Solid and dashed curves represent the criteria proposed by Kewley et al. (2001) and Kauffmann et al. (2003), respectively, to separate AG… view at source ↗
Figure 2
Figure 2. Figure 2: Diagnostic diagrams comparing the observational data of our sample with photoionization model predictions. Black points represent observational data of local AGNs and red points the JADES sample (see Sect. 2.1). In the left and right panels results of photoionization models (see Sect. 2.2.3) simulating high-𝑧 star-forming regions and AGNs, respectively, are shown. Solid colored lines connect models with sa… view at source ↗
Figure 3
Figure 3. Figure 3: As [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Logarithm of the luminosity (in units of erg s−1 ) of H𝛽 for the JADES sample (red points) and for the local AGNs (back points) versus the redshift. of stellar evolutionary synthesis as input parameters photoionization model grids. These authors found that photoionization models as￾suming distinct SEDs predict strong emission-line ratios (as those used in diagnostic diagrams) differing on average of ∼ 0.1 … view at source ↗
Figure 5
Figure 5. Figure 5: Left bottom panel: Comparison between He/H abundances estimated via the empirical calibration proposed by Dors et al. (2024b) versus those via 𝑇e method (see Sect. 2.3.1). Red and black points represent the JADES and local objects, respectively. Solid line represent the equality between the estimates. Left top panel: Difference between the estimates (D=x-y) versus those via the 𝑇e-method. Line represents t… view at source ↗
Figure 6
Figure 6. Figure 6: Helium [in units of 12+log(He/H)] versus oxygen [in units of 12+log(O/H)] abundances for our JADES (see Sect. 2.1.2) and local (see Sect. 2.1.1) samples. Points represent type 2 AGNs classified using diagnos￾tic diagrams (see [PITH_FULL_IMAGE:figures/full_fig_p011_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Helium and oxygen abundances versus redshift for the JADES (red points) and local (black points) AGNs. In the left panels, the blue lines represent fits to the data points (Eqs. 15 and 16). The triangle represents the helium estimate [12+log(He/H)=10.92+0.08 −0.06] for the quasar HS,1700+6416 (𝑧 = 1.724) by Cooke & Fumagalli (2018), derived via the He I [500 < 𝜆(Å) < 540] absorption lines (see Sect. 4.3). … view at source ↗
Figure 9
Figure 9. Figure 9: Logarithm of the [O III]𝜆5007/[O II]𝜆3727 line ratio versus the redshift. Red and black points represent our JADES and local AGNs (see Sect. 2.1), respectively. Blue line represents a linear fitting to the points given by Eq. 19. The Spearman (𝜌) coefficient and the 𝑝-value are indicated. higher by a factor of ∼ 5 than that derived for NLRs (see Nagao et al. 2006b; Matsuoka et al. 2009, 2018; Dors et al. 2… view at source ↗
Figure 10
Figure 10. Figure 10: Relation between the oxygen abundance [in units of 12+log(O/H)] versus the redshift. Red and green areas represent results obtained in the present work (Eqs. 17 and 18). Gray area represents estimates for type 2 AGNs from the semi-empirical calibration between UV emission lines and 𝑍 proposed by Dors et al. (2025b). Blue area represents estimates for NLRs (Eq. 21) based on comparisons between observations… view at source ↗
Figure 11
Figure 11. Figure 11: Relative ionic abundance fraction for distinct ions (X i /X) versus the nebular radius (normalized by the outermost radius) predicted by pho￾toionization models simulating NLRs of AGNs. The nebular parameters as￾sumed in the models are spherical geometry, solar metallicity, 𝛼𝑜𝑥 = −1.1, electron density profile defined by Eq. 22, and three values for log𝑈 = −1.0, −2.5 and −3.5. stricted to the high-metalli… view at source ↗
read the original 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 ${\lambda}(\text{\AA})$ 7000] from the JADES survey and from the literature were used to estimate $\text{He/H}$ and $\text{O/H}$ via the $T_{\rm e}$-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 $T_{\rm e}$-method, an object (goods-s-mediumhst-58850; $z=6.2615$) with the highest helium abundance estimated to date, i.e. $12+\log({\rm He/H})=11.64$, corresponding to $\rm (He/He_{\odot}) \sim 4.4$. This result remains robust even when we adopt larger electron densities (varying $N_{\rm e}$ from $500$ to $10\:000$ $\rm cm^{-3}$) in abundance estimates using the $T_{\rm e}$-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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 1 minor

Summary. The manuscript derives helium and oxygen abundances in the narrow-line regions of 84 AGNs (65 at z < 0.2 and 19 at 2.8 ≲ z ≲ 6.8) from JADES and literature spectra in the rest-frame optical. Abundances are obtained via the Te-method (7 objects) and strong-emission-line calibrations (19 objects). The paper reports that ionization degree increases with redshift, that the majority of z > 2.8 AGNs show oversolar He and subsolar O, and identifies one object at z = 6.2615 with 12 + log(He/H) = 11.64 ((He/He⊙) ∼ 4.4). Marginal evidence is presented for a decline in He/H (∼0.04 dex per unit redshift) and an increase in O/H (∼0.06 dex per unit redshift) toward lower redshift. The result is stated to remain robust when Ne is varied from 500 to 10 000 cm^{-3}.

Significance. If the absolute abundances are reliable, the work would constrain chemical evolution and enrichment in AGN host galaxies at cosmic dawn, highlighting possible helium over-enrichment at early times. The combination of Te-method and strong-line methods on a high-redshift sample from JADES is a positive feature, as is the explicit check on electron density.

major comments (2)
  1. [Abstract] Abstract: The statement that results 'remain robust' when Ne is varied from 500 to 10 000 cm^{-3} addresses only one parameter; it does not test the impact of the reported increase in ionization degree with redshift on the ionization corrections of the Te-method or on the strong-line calibrations. Because the headline claims (oversolar He, subsolar O at z > 2.8, and the 0.04–0.06 dex per unit redshift slopes) rest directly on these absolute abundances, the lack of validation against higher U or altered temperature structure is load-bearing.
  2. [Abstract] Abstract (methods paragraph): No comparison or cross-check is described between the local Te-method and strong-line calibrations and the high-redshift spectra, despite the explicit finding that ionization increases with redshift. Systematic offsets of several tenths of a dex in He/H and O/H are possible if the NLR conditions at z ∼ 6 differ from the local calibrating samples in ionization parameter or density distribution; this directly affects the central evolutionary claims.
minor comments (1)
  1. [Abstract] Clarify why abundance estimates are reported for only 26 of the 84 objects; state the selection criteria applied to the full sample.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful review and constructive comments on our manuscript. We address the two major comments point by point below.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The statement that results 'remain robust' when Ne is varied from 500 to 10 000 cm^{-3} addresses only one parameter; it does not test the impact of the reported increase in ionization degree with redshift on the ionization corrections of the Te-method or on the strong-line calibrations. Because the headline claims (oversolar He, subsolar O at z > 2.8, and the 0.04–0.06 dex per unit redshift slopes) rest directly on these absolute abundances, the lack of validation against higher U or altered temperature structure is load-bearing.

    Authors: We agree that varying only electron density does not fully address potential systematic effects from the observed increase in ionization parameter with redshift. The direct Te-method applied to the seven objects uses measured temperatures and is less dependent on ionization corrections, but the strong-line calibrations used for the remaining objects could be affected. In the revised version we will add an explicit discussion of ionization parameter effects, including estimates of U from the available line ratios where possible, and test the sensitivity of the reported He/H and O/H trends to plausible variations in U. revision: yes

  2. Referee: [Abstract] Abstract (methods paragraph): No comparison or cross-check is described between the local Te-method and strong-line calibrations and the high-redshift spectra, despite the explicit finding that ionization increases with redshift. Systematic offsets of several tenths of a dex in He/H and O/H are possible if the NLR conditions at z ∼ 6 differ from the local calibrating samples in ionization parameter or density distribution; this directly affects the central evolutionary claims.

    Authors: The same calibrations were applied uniformly, yet we acknowledge that no dedicated cross-check for systematic offsets arising from differing NLR conditions is presented. We will add a new subsection that compares Te-method and strong-line results on the local subsample, quantifies any method-dependent offsets, and discusses how the higher ionization at z > 2.8 might propagate into the high-redshift abundances and the reported evolutionary slopes. revision: yes

Circularity Check

0 steps flagged

No circularity: abundances derived via external standard methods applied to new spectra

full rationale

The paper applies the Te-method (7 objects) and strong-line calibrations (19 objects) to JADES and literature spectra to obtain He/H and O/H values. These are standard techniques calibrated on independent local samples; the paper reports the resulting abundances and redshift trends as direct outputs. No equations, fitted relations, or self-citations within the paper reduce the central claims to inputs by construction. The noted increase in ionization with redshift is an observational finding, not a definitional loop. The Ne robustness test is a sensitivity check on external assumptions, not an internal fit. This is a self-contained application of established methods to new data.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract supplies no explicit free parameters, axioms, or invented entities; abundance derivations implicitly rest on standard strong-line calibrations whose functional forms and zero-points are taken from prior literature.

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