X-rays Mark the Spot: The Effects of Reduced Metallicity on X-ray AGN Obscuration at High Redshift

Yash A. Gursahani, Christopher S. Reynolds

First listed 2026-06-25 | Last updated 2026-06-23

Abstract

The James Webb Space Telescope has pushed the frontier of high-redshift galaxy and active galactic nucleus (AGN) observations firmly past $z=10$. Corresponding to the first 500 Myr after the Big Bang, this coincides with the epoch of supermassive black hole seeding and their early growth, much of which is likely to occur in highly obscured environments. In this work, we investigate the expected X-ray properties of these obscured AGNs focusing on the impact of the significantly lower iron abundance predicted at such early times. We use Monte Carlo methods to model the radiative transfer of X-rays from a central AGN through a surrounding torus of cold gas, characterizing the emergent X-ray spectrum as a function of the metallicity, opening angle of the torus, and column density. Motivated by expectations of high-$z$ systems, we focus on Compton-thick obscurers with columns $N_H=10^{24}-10^{25}\,{\rm cm}^{-2}$. We find that decreased metallicity can significantly increase the fraction of X-ray photons that escape the torus, improving the prospects of detecting these very high-$z$ AGNs. The covering fraction of the obscurer (i.e. torus opening angle) plays a complex role, with repeated scatterings across the interior of the torus (isotropizing the emission) competing with escape through the opening, producing geometric beaming. Additionally, we explore non-solar abundance ratios that mimic the delay-time distribution of Type Ia supernovae. We use our models to address the detectability of highly obscured $z=10$ AGNs in next-generation, high-angular resolution X-ray surveys.

Short digest

This paper uses 3D Monte Carlo radiative-transfer calculations to predict how X-rays from heavily obscured high-redshift AGN propagate through cold toroidal gas when the metallicity, torus opening angle, and column density are varied across the Compton-thick regime of N_H=10^24-10^25 cm^-2. The main result is that lowering metallicity, especially the iron abundance, can substantially reduce photoelectric losses and let many more X-ray photons escape, making deeply buried z~10 AGN more detectable in X-rays than solar-metallicity intuition would suggest. The geometric effect is not monotonic, because covering fraction and inclination set up a competition between repeated internal scatterings that isotropize the emission and escape through the opening that produces beaming. By also testing non-solar abundance patterns motivated by delayed Type Ia enrichment, the paper connects early chemical evolution directly to detectability forecasts for next-generation high-angular-resolution X-ray surveys.

Key figures to inspect

• Figure 6. This is one of the clearest summary figures for the paper’s central claim: it shows how the transmitted 0.2-10 keV flux changes with redshift, opening angle, and inclination for Compton-thick obscuration at N_H=10^24 cm^-2, making the metallicity-driven transmission boost immediately visible in the observational band of interest.
• Figure 8. Use this as the extreme-obscuration counterpart to Figure 6. It tests whether the low-metallicity transmission gain survives at N_H=10^25 cm^-2, which is essential for judging the detectability of the most deeply buried early AGN that motivate the paper.
• Figure 10. This figure isolates the non-solar abundance-ratio experiment motivated by delayed Type Ia enrichment. It is important because it shows that iron-poor but alpha-element-rich mixtures can alter transmitted flux in ways that are not captured by a simple uniform metallicity rescaling.
• Figure 11. This is the key line-diagnostic figure. It shows how Fe K equivalent width varies with line-of-sight column density and metallicity, while the shaded noise region makes clear where weak apparent line measurements should be treated cautiously.
• Figure 13. This figure translates the radiative-transfer results into observability by comparing simulated AXIS and Chandra spectra for edge-on Compton-thick sources. It is the most direct bridge from the model grids to the paper’s bottom-line claim about detecting obscured z~10 AGN in future high-angular-resolution X-ray surveys.