Using the Hubble Space Telescope, astronomers have detected, for the first time, ionizing ultraviolet radiation from a galaxy that existed just 1.4 billion years after the Big Bang. The discovery supports the hypothesis that these galaxies made the early universe transparent to ultraviolet light.
The Era of Reionization
In astrophysics, the first billion years after the Big Bang excluding the initial 200–400 million years are referred to as the Reionization Era. The neutral hydrogen that filled the space effectively absorbed ultraviolet radiation with energy sufficient to ionize it. Gradually, ultraviolet radiation ionized the hydrogen atoms, and the universe became transparent to it.
Until now, scientists had only observed the galaxies of that era themselves, but not the ionizing radiation emanating from them. Now, thanks to Hubble, they have managed to detect it from the galaxy MXDFz4.4.
A dense cluster of young stars
The MXDFz4.4 galaxy is one hundred times smaller in area than our own. However, its star formation rate is ten times higher, and its young, massive stars are concentrated in an extremely compact cluster. It is this cluster that is the source of the powerful ultraviolet radiation capable of ionizing the surrounding gas.
Researchers led by Ilias Goovaerts of the Space Telescope Science Institute (STScI) in Baltimore have determined that the stars in MXDFz4.4 formed in a series of star-forming bursts. Each of these bursts generated a new wave of ionizing radiation, and at the end of their short lives, massive stars exploded as supernovae, carving out increasingly larger holes in the gas through which ultraviolet radiation could escape.
At the time of observation, the galaxy had already completed the reionization of the surrounding gas. According to calculations, this occurred approximately 250 million years before the observation.
Hubble and James Webb
The Hubble Space Telescope is one of those who have access to observing early galaxies in the ultraviolet range. Over billions of years of cosmic expansion, this radiation has been redshifted into the visible and near-infrared parts of the spectrum. Thanks to the extremely long cumulative exposures obtained during many years of sky surveys, Hubble has been able to record such objects even at very large cosmological distances.
Data from the James Webb Space Telescope helped determine the galaxy’s mass and analyze older, cooler stars that were not involved in the reionization of the surrounding gas. The spectral data were obtained using the MUSE (Multi-Unit Spectroscopic Explorer) instrument on the European Southern Observatory’s Very Large Telescope in Chile.
Based on this data, the galaxy’s redshift was determined to be z=4.4, corresponding to a distance of 12.37 billion light-years. Marc Rafelski, deputy mission manager for Hubble, noted that without the James Webb Space Telescope, the team would not have been able to reach these conclusions.
What’s next?
Prior to the discovery of MXDFz4.4, the closest galaxy from which ionizing radiation had been detected existed in a universe that was 1.6 billion years old. MXDFz4.4 pushes this boundary 200 million years closer to the peak of the Reionization Era.
Ilias Goovaerts and Marc Rafelski note that similar galaxies are likely still waiting to be discovered. A larger sample of such objects from a slightly later cosmic epoch will allow for more precise measurements and a more complete picture of what cleared the universe during the final stage of reionization. The results were published on June 23, 2026, in the peer-reviewed journal The Astrophysical Journal.
According to science.nasa.gov
