AGN ruled out as the dominant source of cosmic reionization

Abstract

Cosmic reionization represents the latest phase transition in the Universe, when the Lyman continuum (LyC) photons turned the intergalactic medium (IGM) from neutral to highly ionized. It has long been debated whether galaxies or active galactic nuclei (AGNs) are the major source of LyC photons responsible for reionization. Previous observations slightly favored galaxies as the major ionizing source. However, the James Webb Space Telescope (JWST) recently discovered an unexpectedly high density of AGN candidates at high redshift, which has largely enhanced the influence of AGNs. Here we derive a definitive upper bound on the AGN contribution to reionization using the latest JWST data, and conclusively rule out AGNs as the dominant ionizing source during the peak epoch of reionization (EoR). We build a sample of galaxies and AGNs in a specific redshift range $7.15\le z\le 7.75$ with a high completeness. Each object is then decomposed into a point-source component and an extended component in their rest-frame far-UV JWST images. We assume all point-source components are AGNs. Our fiducial AGN sample reaches an unprecedentedly low luminosity of $M_{\mathrm{UV}}\approx -15$ mag. Based on this sample, AGNs can contribute at most one third of the LyC photons budget required at $z\sim 7.5$. Our result implies that galaxies dominate the ionizing source during the EoR.

Published in Nature Astronomy on Oct 7, 2025

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