The Hubble Space Telescope has detected ultraviolet light escaping from a galaxy that existed just 1.4 billion years after the Big Bang, a feat astronomers previously considered impossible. The discovery provides direct evidence that compact clusters of hot, massive stars, not just black holes, were responsible for clearing the neutral hydrogen fog that once filled the early universe.
The galaxy, designated MXDFz4.4, lies 12.37 billion light-years away and was identified by the MUSE eXtremely Deep Field survey using the Very Large Telescope in Chile. It is roughly 100 times smaller than the Milky Way but forms stars 10 times faster, cramming young, hot, massive stars into an extraordinarily compact space.
“Observing a galaxy like this was thought to be impossible,” said Ilias Goovaerts, a postdoctoral fellow at the Space Telescope Science Institute and lead author of the study published June 23 in The Astrophysical Journal (DOI: 10.3847/1538-4357/ae75b0). “Researchers expected the fog of neutral hydrogen that filled the early universe would be too thick and obscure our view of its ionizing light.”
A cosmic fog lifting
In the first billion years after the Big Bang, the universe was filled with neutral hydrogen gas, opaque to ultraviolet light. During the so-called Epoch of Reionization, this gas was gradually ionized, making the universe transparent to UV radiation. The big question has always been what drove this transformation: radiation from active supermassive black holes, or the first generations of hot, massive stars.
MXDFz4.4 settles the question in favor of stars. Hubble detected Lyman continuum photons, ionizing UV light, streaming out of the galaxy. The galaxy’s stars form in episodic bursts, each producing fresh quantities of UV radiation. The young, massive stars live only a few million years before exploding as supernovae, whose blast waves carve bubbles hundreds of light-years across, clearing pathways for the UV light to escape.
The observations show that 50 to 100 percent of the young stars’ ionizing light escapes the galaxy. “A lot of young, hot, massive stars in a small space do a better job of blasting through opaque gas,” Goovaerts said.
A multi-telescope effort
The detection was possible only through the combined power of three major observatories. The Very Large Telescope’s MUSE instrument identified the galaxy and measured its redshift. Hubble, the only telescope capable of capturing these specific UV wavelengths from the early universe, directly detected the escaping ionizing light. The James Webb Space Telescope contributed by measuring the galaxy’s mass and reconstructing its star formation history through its older, cooler stellar populations.
The result pushes the frontier for detected ionizing-photon emitters 200 million years closer to the start of the Epoch of Reionization. While JWST had previously shown galaxies heating and reionizing gas around them at even earlier times (around 900 million years after the Big Bang), those observations did not directly detect the escape of ionizing light from within the galaxy itself.
“Astronomers have found many galaxies that existed at this point in the history of the universe, but we haven’t detected ionizing photons from any of them, making MXDFz4.4 one of a kind,” said Marc Rafelski, Hubble Deputy Mission Head at STScI and a co-author of the study.
What comes next
The discovery establishes a new methodology for finding similar galaxies. The researchers plan to identify more galaxies at slightly later cosmic times, where larger samples are within reach, to more precisely measure how reionization proceeded as the epoch wound down.
“These insights into MXDFz4.4 were possible thanks to the powerful combination of Hubble, Webb and the VLT,” said co-author Alexander Beckett of the Laboratoire d’Astrophysique de Marseille. “Even then, only using state-of-the-art analysis software, that was primarily developed in Marseille, were we able to measure the properties of this remarkable galaxy.”
The findings demonstrate that small but prolific galaxies, rather than rare supermassive black holes, played the dominant role in making the universe transparent to light, a process that ultimately enabled the formation of galaxies like our own Milky Way.