AGN ruled out as the dominant source of cosmic reionization

Danyang Jiang, Linhua Jiang, Shengxiu Sun, Weiyang Liu, Shuqi Fu

First listed 2025-02-06 | Last updated 2025-10-08

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 $7.15 \leq z \leq 7.75$ 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 $M_{\rm UV} \approx -15$ mag. Based on this sample, AGNs can contribute at most one third of the LyC photons budget required at $z\sim7.5$. Our result implies that galaxies dominate the ionizing source during the EoR.

Short digest

JWST GOODS-S/N imaging is used to build a highly complete z=7.15–7.75 sample and decompose each source into a point-source (AGN) and extended component in F115W and F150W, yielding 93 faint AGNs down to M_UV ≈ −15. From the PSF-only UV magnitudes they measure the faint-end AGN UV luminosity function and integrate a 1–4 Ryd SED with f_esc=1 to set an absolute upper bound on AGN ionizing emissivity. The cumulative result is that AGNs provide at most one third of the LyC photon budget at z≈7.5, with the contribution dominated by the lowest-luminosity bins. This effectively rules out AGNs as the dominant drivers of reionization, implying galaxies supply most ionizing photons during the EoR.

Key figures to inspect

• Figure 1: Inspect the F090W–F115W color track versus redshift to see how the strong Lyα break isolates 7.15≤z≤7.75 and minimizes low‑z contamination in the GOODS fields.
• Figure 2: Examine the PSF+Sérsic decompositions in F115W/F150W to verify how AGN fractions are measured and how host light is removed to obtain PSF‑only M_UV.
• Figure 3: Look at the AGN‑fraction histograms and the ≥20% cut that defines the final AGN sample; note how many sources are excluded as galaxy‑dominated.
• Figure 4a: Compare the derived faint‑end AGN UV LF with galaxy LFs, bright‑quasar QLFs, and LRD LFs to see where the new points sit and how the DPL fit behaves without a strong knee.
• Figure 4b: Trace the cumulative emissivity fraction versus luminosity to confirm AGNs reach only ≲1/3 of the required LyC budget and that faint AGNs dominate the total.