A new study analyzing the James Webb Space Telescope (JWST) spectra of more than 1,400 galaxies suggests a surprisingly small group of “leaky” galaxies was responsible for cosmic reionization. The paper outlining this work was posted to the arXiv preprint server on July 24.
Cosmic reionization happened when ionizing ultraviolet radiation from early stars and galaxies heated the universe’s neutral hydrogen gas and converted it into the ionized state seen today. Based on cosmic microwave background and quasar data, this epoch is estimated to have occurred around redshift 6–8, roughly 600 million to 1 billion years after the Big Bang. The main driver is thought to be star-forming galaxies. Therefore, the epoch of reionization is considered the last major phase transition of the universe.
The ultraviolet radiation responsible for cosmic reionization is mainly composed of Lyman continuum (LyC) photons with wavelengths less than 912 Å. Their escape fraction from galaxies, along with the rate of their production, determines whether they reach and ionize the intergalactic medium because most LyC photons are absorbed internally by gas and dust.
The ionizing budget
In this new study, the team of astronomers led by Emma Giovinazzo of the University of Geneva determined how the rate at which galaxies produce ionizing radiation changes over cosmic history, along with the total rate at which ionizing photons are produced and contributed by the galaxy population as a whole. From this, they traced how the neutral hydrogen fraction in the intergalactic medium evolved across cosmic time and assessed how much “strong leaker” galaxies (those with an escape fraction above 10%) contribute to this process.
The goal of the work was also to determine which type of galaxy drives cosmic reionization: a small number of galaxies that leak ionizing photons very efficiently or a large number of galaxies that each leak only a small fraction of their photons.
The team compiled spectral information from 1,428 galaxies observed by JWST/NIRSpec, spanning redshift 5–10, which translates to roughly 500 million to 1 billion years after the Big Bang. For each galaxy, they calculated how many ionizing photons are produced and how many of those photons escape into the intergalactic medium.
Driven by a few
The analysis showed that the galaxies split into two categories: 20% are strong leakers of ionizing radiation, while the rest are weak leakers. Even though strong leakers are the minority, they produce around 87% of the ionizing photons that reach the intergalactic medium, capable of driving nearly all reionization by themselves. “This indicates that it is necessary to identify highly leaking galaxies to find the drivers of reionization,” the team writes in the paper.
The researchers also found that galaxies dominate shifts with cosmic time: At lower redshift (z = 5–6), bright galaxies contribute the most ionizing photons, while at higher redshift (z>6), faint galaxies contribute roughly equally alongside brighter ones. Combining their results, the team estimates that reionization was complete by around z ~ 5.8, consistent with independent measurements.
The researchers are transparent that their precise numbers depend on modeling choices and one survey that covers a small patch of sky. Their extrapolation to faint galaxies is unverified, so they call it their “biggest caveat.”
The team concludes that resolving some of these uncertainties “would require many more extremely deep field observations with JWST or the upcoming Extremely Large Telescope (ELT).”
Written for you by our author Shreejaya Karantha, edited by Lisa Lock, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive.
If this reporting matters to you, please consider a donation (especially monthly). You’ll get an ad-free account as a thank-you.
Publication details
Emma Giovinazzo et al, Reionization driven by the few: the ionizing budget of galaxies at z=5-10 from JWST/NIRSpec, arXiv (2026). DOI: 10.48550/arxiv.2607.22834
Journal information:
arXiv
Key concepts
© 2026 Science X Network
Citation:
JWST study suggests a handful of ‘leaky’ galaxies reionized the early universe (2026, August 6)
retrieved 7 August 2026
from https://phys.org/news/2026-08-jwst-leaky-galaxies-reionized-early.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no
part may be reproduced without the written permission. The content is provided for information purposes only.