A dense, star-packed galaxy from just 1.4 billion years after the Big Bang has shown Hubble how the young universe shed its hydrogen fog - providing the first direct view of cosmic reionization from so close to its conclusion.
NASA’s Hubble Space Telescope has detected ionizing ultraviolet light escaping from galaxy MXDFz4.4, a tiny, dense star-forming system that existed just 1.4 billion years after the Big Bang. The finding - published June 23, 2026 in The Astrophysical Journal - provides the clearest evidence yet for how compact, high-star-formation galaxies drove the Era of Reionization, the period when the young universe shed its primordial hydrogen fog and became the transparent cosmos we observe today.
Galaxy MXDFz4.4 sits in the constellation Fornax, identified within the MUSE eXtremely Deep Field survey. It existed approximately 12.37 billion years ago, placing it around 250 million years after the formal close of the Epoch of Reionization - closer to that cosmic turning point than any previously confirmed source of escaping ionising radiation.
Lead researcher Ilias Goovaerts, a postdoctoral fellow at the Space Telescope Science Institute (STScI) in Baltimore, and colleagues used Hubble’s ultraviolet sensitivity to detect what astronomers call Lyman continuum (LyC) emission - the ionizing radiation capable of stripping electrons from neutral hydrogen atoms and rendering surrounding gas transparent.
By galactic standards, MXDFz4.4 is remarkably small: roughly 100 times smaller in area than the Milky Way. Yet it forms stars approximately ten times faster than our galaxy does today. According to the NASA Science press release, that combination - compact geometry and intense star production - is central to why ionizing light can escape: the radiation punches through a galaxy with comparatively little absorbing gas in its path.
“The tightly clustered young stars produce ionizing light capable of transforming the opaque, neutral gas within and immediately around the galaxy, clearing our view,” NASA states.
The Era of Reionization, which cosmologists place roughly between 150 million and 1 billion years after the Big Bang, represents one of the central transitions in cosmic history. After the Big Bang, the universe cooled from a hot plasma into a sea of neutral hydrogen - a fog so opaque that light could not travel freely. What ended that fog is a foundational question in cosmology.
The leading hypothesis is that ionizing radiation from the first generations of stars and galaxies broke apart the surrounding neutral hydrogen, progressively clearing the universe to its present transparent state. But directly observing this process has been difficult because the same neutral gas that was being ionized also absorbed much of the relevant ultraviolet light.
MXDFz4.4 offers a rare direct window. Hubble resolved multiple distinct bursts of younger stars within the galaxy, indicating successive episodes of star formation rather than a single starburst event. According to the ESA/Hubble announcement, those repeated stellar generations collectively generated and sustained a radiation flux high enough to ionize and clear the surrounding gas, with effects observable at cosmic distances.
Before this discovery, the closest confirmed LyC emitter to the reionization epoch existed when the universe was approximately 1.6 billion years old. MXDFz4.4 pushes that boundary 200 million years nearer to the cosmic dawn.
Beyond the discovery itself, Goovaerts’s team used MXDFz4.4 as the first high-redshift test of whether the spatial distribution of Lyman-alpha emission - a different ultraviolet line - can predict where LyC light escapes from a galaxy. If the morphology of Lyman-alpha emission reliably traces LyC escape, it could give future surveys a practical shortcut for identifying reionization-era sources among large samples of galaxies, without requiring the direct ultraviolet detection that Hubble makes possible.
The paper describes the method’s results as promising but preliminary. A larger statistical sample will be required before it can be used as a reliable diagnostic.
The researchers are explicit that MXDFz4.4 is one example, not a confirmed population-level conclusion. Whether small, compact, high-star-formation galaxies were numerous enough in the early universe to drive all of reionization - rather than brighter, rarer quasars - is still under active investigation.
NASA’s James Webb Space Telescope has also imaged MXDFz4.4 alongside Hubble. Webb’s infrared sensitivity is expected to yield further data on the galaxy’s star-formation history and gas conditions, complementing the ultraviolet portrait that Hubble has provided.
The finding joins a growing body of Hubble and Webb observations that point toward faint, compact galaxies - not rare luminous quasars - as the primary engines of cosmic reionization. The picture that has emerged from deep-field imaging is one in which the transformation of the early universe was driven by the accumulated radiation of countless small beacons rather than a handful of bright ones.
For now, MXDFz4.4 is the earliest and closest confirmed member of that class, and the most direct look yet at a galaxy in the act of helping clear the cosmic fog.
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