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.

arXiv commentPublished in Nature Astronomy on Oct 7, 2025

References in corpus (37)

• emcee: The MCMC Hammer
• An 800-million-solar-mass black hole in a significantly neutral Universe at redshift 7.5
• Radiative Transfer in a Clumpy Universe: III. The Nature of Cosmological Ionizing Sources
• The Evolution of the Stellar Mass Function of Galaxies from z=4.0 and the First Comprehensive Analysis of its Uncertainties: Evidence for Mass-dependent Evolution
• Reionization of the Inhomogeneous Universe
• The optical morphologies of galaxies in the IllustrisTNG simulation: a comparison to Pan-STARRS observations
• UV-Continuum Slopes of >4000 z~4-8 Galaxies from the HUDF/XDF, HUDF09, ERS, CANDELS-South, and CANDELS-North Fields
• The Final SDSS High-Redshift Quasar Sample of 52 Quasars at z>5.7
• CEERS Epoch 1 NIRCam Imaging: Reduction Methods and Simulations Enabling Early JWST Science Results
• The z=5 Quasar Luminosity Function from SDSS Stripe 82
• Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). V. Quasar Luminosity Function and Contribution to Cosmic Reionization at z = 6
• Testing the Unification Model for AGN in the Infrared: are the obscuring tori of Type 1 and 2 Seyferts different?
• A Significantly Neutral Intergalactic Medium Around the Luminous z=7 Quasar J0252-0503
• On Lyman-limit Systems and the Evolution of the Intergalactic Ionizing Background
• Gas Clumping in Self-Consistent Reionisation Models
• A Candidate of a Least-Massive Black Hole at the First 1.1 Billion Years of the Universe
• Early and Extended Helium Reionization Over More Than 600 Million Years of Cosmic Time
• The Lyman Continuum escape fraction of faint galaxies at z~3.3 in the CANDELS/GOODS-North, EGS, and COSMOS fields with LBC
• Relative Role of Stars and Quasars in Cosmic Reionization
• The contribution of faint AGNs to the ionizing background at z~4
• The Spectral Slope and Escape Fraction of Bright Quasars at $z\sim 3.8$: the Contribution to the Cosmic UV Background
• Minor Contribution of Quasars to Ionizing Photon Budget at z~6: Update on Quasar Luminosity Function at the Faint-end with Subaru/Suprime-Cam
• Evolutionary Paths of Active Galactic Nuclei and Their Host Galaxies
• Quasar Luminosity Function at z = 7
• Little Red Dots: Rapidly Growing Black Holes Reddened by Extended Dusty Flows
• Definitive upper bound on the negligible contribution of quasars to cosmic reionization
• Galapagos-2/Galfitm/GAMA -- multi-wavelength measurement of galaxy structure: separating the properties of spheroid and disk components in modern surveys
• Lyman continuum leaking AGN in the SSA22 field
• Constraints on QSO emissivity using H I and He II Lyman alpha forest
• The Pan-STARRS1 $\mathbf{z}>5.6$ Quasar Survey: III. The $\mathbf{z}\approx 6$ Quasar Luminosity Function
• UNCOVERing the contribution of black holes to reionization in the JWST era
• The JADES Origins Field: A New JWST Deep Field in the JADES Second NIRCam Data Release
• Ionizing radiation from AGNs at z>3.3 with the Subaru Hyper Suprime-Cam Survey and the CFHT Large Area U-band Deep Survey (CLAUDS)
• Constraining the AGN luminosity function from JWST with the X-ray Background
• An unambiguous AGN and a Balmer break in an Ultraluminous Little Red Dot at z=4.47 from Ultradeep UNCOVER and All the Little Things Spectroscopy
• Probing the roles of orientation and multi-scale gas distributions in shaping the obscuration of Active Galactic Nuclei through cosmic time
• The $z\gtrsim 9$ galaxy UV luminosity function from the JWST Advanced Deep Extragalactic Survey: insights into early galaxy evolution and reionization

Cited by in corpus (1)

• Probing Reionization-Era Galaxies with JWST UV Luminosity Functions and Large-Scale Clustering