Hubble Detects Rare Ionizing Light from a Distant Early-Universe Galaxy

Astronomers have identified an ancient galaxy emitting light through the cosmic fog of the early universe, providing a detailed observation previously considered impossible.

By utilizing NASA’s Hubble Space Telescope in conjunction with data from the James Webb Space Telescope (JWST) and the European Southern Observatory’s Very Large Telescope (VLT), researchers detected “ionizing” ultraviolet photons from a galaxy designated as MXDFz4.4. These high-energy photons are capable of stripping electrons from hydrogen atoms. This represents the earliest such detection on record, occurring approximately 250 million years after the conclusion of the Epoch of Reionization, according to a study published June 23 in The Astrophysical Journal.

In the hundreds of millions of years following the Big Bang, the intergalactic space was filled with neutral hydrogen gas that obscured this specific type of light. Over time, radiation from the first stars and galaxies ionized this gas, effectively clearing the fog and allowing light to travel freely—a cosmic transition that astronomers are still striving to fully understand.

“This was thought to be impossible,” Ilias Goovaerts, a postdoctoral fellow at the Space Telescope Science Institute (STScI) and the study’s lead author, told Live Science. “What’s really special about this galaxy is that it’s getting through so much of the intergalactic medium. Being the furthest away, it has the most intergalactic medium to penetrate.”

MXDFz4.4 is unique due to the contrast between its size and its star-formation rate. While it is roughly 100 times smaller in area than the Milky Way, it forms stars approximately 10 times faster. This high density of massive young stars creates a crowding effect that allows the galaxy to punch clear channels through surrounding gas, enabling ionizing light to escape both the galaxy and the murky intergalactic medium. Researchers estimate that between 50% and 100% of the galaxy’s ionizing light is escaping.

The discovery occurred somewhat serendipitously in October. While preparing a funding proposal, Goovaerts reviewed a deep Hubble image to determine if similar signals had been sought in that region. He discovered a promising signal within a few hours. “It was very quick from having the idea to realizing there was something exciting here,” Goovaerts noted. “While we were excited from day one, it took several months to mature the data and extract the galaxy’s properties.”

The findings were supported by a comprehensive data set: a deep Hubble image resulting from 40 hours of observation; JWST multi-wavelength imaging used to analyze star-formation history; and an exceptionally deep spectrum gathered over six days using the VLT’s Multi-Unit Spectroscopic Explorer. This spectrum confirmed the distance of the galaxy via its Lyman-alpha emission line, a “hydrogen fingerprint” used by astronomers to measure cosmic time and distance.

Marc Rafelski, deputy mission head for the Hubble Space Telescope at STScI and study co-author, noted that no other galaxy from this early epoch had previously shown detectable ionizing light, making MXDFz4.4 a unique specimen.

Researchers believe that vigorous bursts of star formation, such as those seen in MXDFz4.4, may have played a critical role in clearing the early universe’s hydrogen fog, suggesting that more galaxies of this nature likely exist and await discovery.

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