The early Universe underwent a profound transformation during the epoch known as cosmic reionization, a period when the pervasive intergalactic medium transitioned from being mostly neutral to highly ionized. This transition was driven by the emergence of high-energy ultraviolet photons, specifically Lyman-continuum photons, which have enough energy to ionize hydrogen atoms. Understanding the primary sources of these ionizing photons has remained a pivotal question in cosmology for decades, as it directly informs our grasp of how the first luminous structures formed and evolved.
Traditionally, the debate surrounding the dominant agents of reionization has centered on two primary contenders: star-forming galaxies and active galactic nuclei (AGNs). Galaxies, brimming with young, hot stars, were considered the main architects of reionization because their massive stars emit significant ultraviolet radiation capable of ionizing the surrounding hydrogen gas. Conversely, AGNs—supermassive black holes at the centers of galaxies, blazing with intense radiation as they actively accrete matter—have also been proposed as potentially critical contributors due to their prodigious high-energy photon output.
Previous observational evidence has generally lent slight preference toward galaxies as the prime sources of ionizing photons in the early Universe. Measurements of the ultraviolet luminosity function of galaxies at redshifts around 6 to 8 indicated a substantial population of faint galaxies, which collectively might have provided enough ionizing photons to reionize the Universe. Meanwhile, observational surveys of AGNs at similar epochs revealed too few objects bright enough to account for the necessary ionizing budget, leading to a consensus that galaxies played the dominant role.
However, this picture was recently challenged by findings from the revolutionary James Webb Space Telescope (JWST). Thanks to its unparalleled sensitivity and spatial resolution in the infrared domain, JWST has unveiled a surprisingly high density of candidate AGNs at very high redshifts—significantly more than previously estimated with other telescopes. This discovery suggested that AGNs might contribute a larger fraction of the ionizing photons required for reionization than was conventionally thought, renewing the debate about their role during this epoch.
In this groundbreaking study, researchers utilized the latest JWST data to rigorously quantify the maximum possible contribution of AGNs to the ionizing photon budget during the peak epoch of cosmic reionization, specifically focusing on the redshift interval from 7.15 to 7.75. This narrow but crucial redshift window corresponds to roughly 700 million years after the Big Bang, a time when reionization was likely underway. Crucially, the team compiled a sample of galaxies and AGNs with unprecedented completeness, benefiting from JWST’s extraordinary imaging capabilities.
To differentiate AGNs from galaxies within this sample, the team meticulously analyzed the rest-frame far-ultraviolet images produced by JWST. They decomposed each observed source into two spatial components: a point-like source indicative of a compact AGN, and an extended component representing stellar emission from the host galaxy. By assuming that all detected point-source components are AGNs, the researchers effectively isolated the fraction of the ultraviolet luminosity function attributable to active nuclei.
The resulting sample reaches remarkably low ultraviolet luminosities, roughly corresponding to absolute magnitudes of about −15 in the UV band, pushing five magnitudes deeper than typical earlier surveys. This faint end is critical because it encompasses the vast majority of low-luminosity sources that could have contributed significantly to ionizing photon production. By integrating over the luminosity function, the team derived a robust upper limit on the AGN population’s total ultraviolet output during this epoch.
Their findings decisively show that AGNs could contribute at most one third of the total Lyman-continuum photon budget required to maintain reionization at redshift 7.5, effectively ruling out AGNs as the dominant ionizing agents during this critical phase of cosmic history. This upper limit holds even when considering the smallest AGNs detectable thanks to JWST’s capabilities, implying that a dominant role for AGNs in reionization is highly unlikely.
In stark contrast, this strongly reinforces the scenario that star-forming galaxies are predominantly responsible for generating the ionizing photons that reionized the intergalactic medium. Even if some fraction of ionizing photons escape from AGNs, their overall numbers and integrated luminosities are insufficient to drive the transition from a neutral to an ionized Universe on their own. Hence, the galactic population, especially the abundant faint galaxies harboring massive young stars, remains the main architect of cosmic reionization.
This result carries significant implications for theoretical models of galaxy formation and black hole growth in the early Universe. It constrains the space density and evolution of early AGNs, suggesting that the buildup of supermassive black holes capable of producing detectable AGN activity proceeds more slowly than some predictions. It also emphasizes the importance of faint galaxies—which are often below previous telescopes’ detection thresholds—in shaping the thermal and ionization state of the intergalactic medium.
The methodological approach of decomposing sources into point-like and extended components using JWST’s first direct observations in the far-ultraviolet demonstrates an innovative technique to distinguish AGNs from galaxies at such high redshifts. This technique provides a blueprint for future studies aiming to characterize the faintest and earliest populations in the Universe, advancing the frontier of observational cosmology.
The ongoing analysis of JWST data promises to refine these conclusions further. As more deep-field surveys are conducted with JWST’s instruments, the census of faint galaxies and low-luminosity AGNs will continue to improve, reducing uncertainties on the total ionizing budget. These efforts will progressively illuminate the complex interplay between star formation, black hole accretion, and the intergalactic medium during one of the Universe’s most transformative epochs.
Importantly, this work reinstates cosmic reionization as a predominantly galactic phenomenon while acknowledging that AGNs, though less dominant, still contributed a non-negligible portion of high-energy photons. These photons could have influenced localized regions, possibly affecting early galaxy environments and the timing of reionization in different patches of the cosmos.
Moreover, understanding the contributions to reionization is vital for interpreting observations of the cosmic microwave background and large-scale structure, as these early processes leave subtle imprints on the Universe’s fundamental parameters and subsequent evolution. The constraints provided by this study thus ripple across multiple areas of astrophysics and cosmology.
In conclusion, the latest research driven by JWST’s unprecedented data quality has definitively ruled out AGNs as the main sources of cosmic reionization. The comprehensive analysis in the redshift range 7.15 to 7.75, using sophisticated spatial decomposition techniques to disentangle AGNs from their host galaxies, reveals that AGNs account for no more than a third of the required Lyman-continuum photons at this epoch. This landmark result strengthens the paradigm that early star-forming galaxies were the central actors heralding the Universe’s transformation from darkness to light.
As JWST continues to probe the distant Universe, future discoveries will undoubtedly refine our understanding further, but for now, the cosmic story of reionization appears to be written predominantly by the first generations of galaxies, whose intense star formation nestled within nascent dark matter halos provided the ultraviolet photons to ignite the cosmic dawn.
Subject of Research: Cosmic reionization and the sources of Lyman-continuum photons during the early Universe.
Article Title: AGNs ruled out as the dominant source of cosmic reionization.
Article References:
Jiang, D., Jiang, L., Sun, S. et al. AGNs ruled out as the dominant source of cosmic reionization. Nat Astron (2025). https://doi.org/10.1038/s41550-025-02676-7
Image Credits: AI Generated
