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Arxiv: Ruling out AGNs as the dominant source of cosmic reionization with JWST Published: 2/6/2025 2:12:00 AM Updated: 2/6/2025 2:12:00 AM

Paper abstract: Cosmic reionization represents the latest phase transition of theintergalactic medium (IGM) in the Universe. It has long been debated whethergalaxies or active galactic nuclei (AGNs) are the major source of Lymancontinuum (LyC) photons responsible for reionization. Previous observationsslightly favored galaxies as the major ionizing source. However, the James WebbSpace Telescope (JWST) recently discovered an unexpectedly high density of AGNcandidates at high redshift, which has largely enhanced the influence of AGNs.Here we derive a definitive upper bound on the AGN contribution to reionizationusing the latest JWST data, and conclusively rule out AGNs as the dominantionizing source during the epoch of reionization (EoR). We build a sample ofobjects (including galaxies and AGNs) in a specific redshift range between 7.15and 7.75 that has a high completeness. Each object is then decomposed into apoint-source component and an extended component in their rest-frame far-UVJWST images. Assuming all point-source components are AGNs, we obtain anabsolute upper limit for the density of the AGN population. This fiducial AGNsample reaches an unprecedentedly low luminosity of M_{\rm UV} ~ -15mag. Based on this sample, we find that AGNs can contribute at most one thirdof the LyC photons required to ionize the Universe in this redshift range. Ourresult implies that galaxies dominate the ionizing source during the EoR.