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

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.