JWST reveals a distant galaxy that may let most of its ionizing radiation escape

October 30, 2026 report JWST reveals a distant galaxy that may let most of its ionizing radiation escape Shreejaya Karantha Author Gaby Clark Scientific Editor Robert Egan Senior Editor Astronomers using the James Webb Space Telescope (JWST) have found evidence that a distant, exceptionally blue galaxy may allow most of its ionizing radiation to escape into space. Known as EBG-1, the gravitationally lensed galaxy is seen at a redshift of 9.244, about 530 million years after the Big Bang. The paper was posted to the arXiv preprint server on Oct. 16. Leaky helpers Scientists estimate that cosmic reionization happened roughly between 600 million and a billion years (around redshifts 6 to 8) after the Big Bang and was complete by redshift 5.5. During this epoch, ionizing ultraviolet radiation from early stars and galaxies converted the universe's neutral hydrogen gas into the ionized state we see today. The ultraviolet radiation driving this transition is mainly made up of Lyman continuum (LyC) photons with wavelengths less than 912 Å. Much of this light is absorbed by gas and dust inside a galaxy, so the fraction that escapes into intergalactic space, called the escape fraction, is key to estimating how much a galaxy contributed. "Which galaxies supplied most of the ionizing photons remains debated," the researchers write in their paper. Some models suggest that dwarf galaxies are the main drivers, while others point to more massive galaxies with efficient star formation rates. As a result, studying quasars and galaxies from this period and beyond can not only constrain the redshift at which reionization occurred but also provide clues about the main drivers. One of the bluest ones! In this work, astronomers led by Yunjing Wu of the University of Tokyo have presented the spectroscopic analysis of one of the bluest galaxies observed when the universe was only 530 million years old. At a redshift of 9.244, Extreme Blue Galaxy 1 (EBG-1) is lensed by a foreground galaxy cluster called MACS0647. This means the galaxy cluster's gravity bends and magnifies EBG-1's light, allowing astronomers to see it in detail. The extreme blueness refers to the ultraviolet (UV) light emission of the galaxy. "[The galaxy] stands out for having one of the bluest UV slopes yet measured at this redshift," researchers say. The UV slope describes how steeply the galaxy's brightness changes with wavelength in the ultraviolet. A very blue slope suggests very little dust, very young stars and a metal-poor composition. Extremely blue slopes can also hint that ionizing photons are escaping rather than being absorbed by surrounding gas. But there are other markers of a genuine "leaky" galaxy. Weak nebular emission lines can also signal that ionizing photons are escaping rather than being absorbed and re-emitted by surrounding gas. To test this, the team obtained a deep 14-hour spectroscopic observation of EBG-1 with JWST's NIRSpec instrument and searched for lines from carbon, hydrogen and oxygen. Spectral clues The spectrum showed weak emission lines overall. The oxygen line was prominent, whereas the hydrogen line was weak and carbon was not detected at all. Using the oxygen-to-hydrogen line ratio, the team estimated that the galaxy's oxygen abundance is only about 3% of the sun's. This is consistent with a young galaxy early in the universe, because heavy elements are made by stars over time. By comparing the observations with synthetic galaxy spectra, the team found that models with an ionizing-photon escape fraction of around 85% best fit the data for EBG-1. The models assumed constant star formation and no dust. "EBG-1 shows spectroscopic signatures of a high escape fraction…," researchers conclude. If this interpretation is correct, EBG-1 could be an example of a highly efficient leaky source that helped reionize the early universe. However, this is a single object, and its environment has not yet been studied. So, astronomers do not know how it contributes locally. "A census of neighboring galaxies would help assess whether the local ionizing budget could be dominated by EBG-1 or by its surroundings," the team writes. Written for you by our author Shreejaya Karantha, edited by Gaby Clark, 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 Yunjing Wu et al., Can Blue UV-Bright Galaxies Drive Reionization at Cosmic Dawn? An MUV = −19.6 Galaxy with fesc ≈ 0.85 at z = 9.244, arXiv (2026). DOI: 10.48550/arXiv.2610.24092 Journal information: arXiv © 2026 Science X Network