Kavli Affiliate: Linhua Jiang
| First 5 Authors: Danyang Jiang, Linhua Jiang, Shengxiu Sun, Weiyang Liu, Shuqi Fu
| Summary:
Cosmic reionization represents the latest phase transition of the
intergalactic medium (IGM) in the Universe. It has long been debated whether
galaxies or active galactic nuclei (AGNs) are the major source of Lyman
continuum (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 epoch of reionization (EoR). We build a sample of
objects (including galaxies and AGNs) in a specific redshift range between 7.15
and 7.75 that has 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. Assuming all point-source components are AGNs, we obtain an
absolute upper limit for the density of the AGN population. This fiducial AGN
sample reaches an unprecedentedly low luminosity of $M_{rm UV} approx -15$
mag. Based on this sample, we find that AGNs can contribute at most one third
of the LyC photons required to ionize the Universe in this redshift range. Our
result implies that galaxies dominate the ionizing source during the EoR.
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