Just 1.4 billion years after the Big Bang, the galaxy MXDFz4.4, which is 100 times smaller than the Milky Way, erupted with ionizing light, penetrating the neutral hydrogen that once enveloped it. This pivotal moment offers astronomers an unprecedented glimpse into the reionization of the universe.
This diagram illustrates the galaxy MXDFz4.4 as it appeared 1.4 billion years after the Big Bang. Image credit: NASA/ESA/Leah Hustak, STScI.
MXDFz4.4 thrived during the Reionization Era, a crucial period of cosmic change.
Initially, for roughly the first billion years following the Big Bang, the interstellar gas remained opaque to high-energy ultraviolet light.
Eventually, the gas transitioned to a transparent or ionized state, a change that took hundreds of millions of years rather than occurring instantaneously.
“Initially, it was believed that observing galaxies like MXDFz4.4 would be impossible,” states Dr. Ilias Guvarts, a postdoctoral researcher at the Space Telescope Science Institute. “The dense ‘fog’ of neutral hydrogen filling the early universe was thought to obscure ionizing light.”
“However, the Hubble Space Telescope not only detected that light but also unveiled intriguing characteristics of the galaxy,” Dr. Guvarts adds.
Hubble’s detailed visible-light images show how several bursts of young stars cleared MXDFz4.4 and its vicinity. Image credits: NASA / ESA / CSA / STScI / Ilias Goovaerts, STScI / Marc Rafelski, STScI & JHU / Anton Koekemoer, STScI / Alyssa Pagan, STScI.
“While astronomers have identified numerous galaxies from this epoch, none have shown ionizing photons until now, making MXDFz4.4 exceptional,” notes Dr. Mark Rafelski from the Space Telescope Science Institute.
Long exposures from Hubble’s various surveys revealed that a young, massive star in MXDFz4.4 was the source of the ultraviolet light that permeated the surrounding universe.
These stars likely formed within the last few million years, densely clustered together.
Notably, MXDFz4.4, despite being about 100 times smaller than the Milky Way, is forming stars at a rate ten times faster.
“A concentration of young, hot, massive stars in a confined space effectively pierces through the opaque gas,” states Dr. Guberts.
For a deeper insight, refer to the team’s publication dated June 23, 2026, in the Astrophysical Journal.
_____
Elias Guvaarts et al. 2026. MXDFz4.4: First test of LyC emitters 250 million years post-reionization and the Lyα form as a tracer of LyC escape at high redshifts. APJ 1005, 34; doi: 10.3847/1538-4357/ae75b0
Source: www.sci.news