Astronomers have identified one of the most distant candidate galaxies known to leak ionizing radiation—the same kind of radiation thought to have transformed the early universe during the epoch of reionization.
Leaky galaxies
Cosmic reionization happened when ionizing UV radiation from early stars and galaxies converted the universe’s neutral hydrogen gas into the ionized state seen today.
Based on cosmic microwave background and quasar data, astronomers estimate this happened around redshift z ~ 6–8, roughly 600 million to 1 billion years after the Big Bang, with star-forming galaxies thought to be the main driver. Studying this epoch is therefore a way to study the first galaxies and stars themselves.
In this context, Lyman continuum (LyC) photons with wavelengths less than 912 Å make up the ionizing UV radiation responsible for cosmic reionization. Their escape fraction from galaxies—not just their production—determines whether they actually reach and ionize the intergalactic medium because most LyC photons are absorbed internally by gas and dust.
Rare catch
In this new study led by Shuairu Zhu of Shanghai Astronomical Observatory, researchers report a potential “leaky” galaxy known as LCEz4-M1 at z = 4.444. That means this galaxy was seen just 1.4 billion years after the Big Bang, making it one of the highest-redshift LyC leaker candidates known. It appears somewhat after the reionization epoch itself, but it is still close enough to offer insight into how such escape happens.
The study was published in The Astrophysical Journal Letters on June 22.
To confirm the detection, the researchers combined data from three observatories: the Very Large Telescope (VLT), the Hubble Space Telescope and the James Webb Space Telescope. They used the galaxy’s hydrogen emission line, which showed a distinctive lopsided shape, to pin down its distance. Then they checked the surrounding images carefully to make sure the light wasn’t coming from an unrelated object sitting in the same patch of sky.
They detected the escaping UV light signal independently in two separate data sets, once in Hubble imaging and once in the VLT spectroscopy.
Slipping through
Most local galaxies known to leak this kind of light tend to be small and undergoing furious bursts of star formation. Interestingly, the galaxy did not appear to be intensely forming stars. While LCEz4-M1 appears relatively compact, its star-formation surface density is lower than that of local leakers.
Researchers explain in their paper that this may “indicate a postburst or geometry-dominated LyC leaking phase rather than a classical extreme emission-line compact starburst.” In other words, this calm phase suggests the galaxy may be past its star-forming peak, caught in a short-lived postburst phase—meaning gas channels opened by that earlier burst’s stellar winds and supernovae may still be allowing light to escape.
Separately, a faint nearby companion was also found, hinting at a possible minor interaction. The main galaxy itself looks compact and undisturbed by any merger, but the surrounding region is unusually crowded with some 15 other galaxies.
“However, given the depth of the available data and the lack of a completeness analysis, the current evidence is not sufficient to establish that the source resides in a protocluster,” the team notes. If real, interactions in this dense environment could also stir up gas and help carve out escape channels.
Overall, this galaxy is a rare, valuable high-redshift example, useful for testing how LyC escapes under conditions closer to the actual reionization era.
“LCEz4-M1 is among the few LyC detections reported at z > 4, providing a valuable laboratory to investigate the physical conditions that allow LyC escape,” they conclude. They call for deeper spectroscopy to study this galaxy and its environment to figure out why it leaks so much ionizing light.
Written for you by our author Shreejaya Karantha, edited by Sadie Harley, 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.
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Publication details
Shuairu Zhu et al, LCEz4-M1: A Lyman Continuum Emitter Candidate at z = 4.444 in the MUSE Hubble Ultra Deep Field, The Astrophysical Journal Letters (2026). DOI: 10.3847/2041-8213/ae75e1
Key concepts
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One of the most distant ‘leaky’ galaxies ever found may reveal how the universe reionized (2026, June 30)
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