Hubble & JWST Reveal Earliest Galaxy Breaking Through Cosmic Fog

Astronomers detected ionizing ultraviolet photons from the ancient galaxy MXDFz4.4, marking the earliest such detection on record. According to a study published June 23 in The Astrophysical Journal, researchers used NASA’s Hubble and James Webb telescopes along with the European Southern Observatory’s Very Large Telescope to identify light that was previously thought to be blocked by cosmic fog.

The detection is significant because the early universe was filled with a fog of neutral hydrogen gas that blocked energetic ultraviolet light for hundreds of millions of years after the Big Bang. This period, known as the Epoch of Reionization, ended as radiation from the first stars and galaxies ionized the gas, allowing light to travel freely across space.

MXDFz4.4 appears roughly 1.4 billion years after the Big Bang, arriving approximately 250 million years after the end of the Epoch of Reionization. This allows the galaxy to provide a detailed view of the early universe that researchers previously believed was impossible to obtain.

Why was this light previously thought to be invisible?

For a vast period following the Big Bang, the intergalactic medium consisted of neutral hydrogen gas. This gas acted as a barrier, absorbing ionizing ultraviolet photons—energetic light capable of stripping electrons from hydrogen atoms—before they could reach distant observers.

Ilias Goovaerts, a postdoctoral fellow at the Space Telescope Science Institute (STScI) in Baltimore and the study’s first author, told Live Science that the galaxy is special because it is getting through a significant amount of the intergalactic medium. Because it is the furthest away, it has the most intergalactic medium to get through.

Ilias Goovaerts, a postdoctoral fellow at the Space Telescope Science Institute (STScI) in Baltimore and first author of the new study, told Live Science that this was thought to be impossible.

How does MXDFz4.4 differ from the Milky Way?

The galaxy possesses a rare combination of compact size and intense activity. Researchers found that MXDFz4.4 is roughly 100 times smaller in area than the Milky Way. Despite its size, it forms stars approximately 10 times faster.

According to Goovaerts, this high density of massive young stars creates a crowding effect. This intensity helps the galaxy punch clear channels through its own surrounding gas, allowing ionizing light to escape the galaxy and travel through the murky intergalactic space. The research team estimates that between half and all of the galaxy’s ionizing light is escaping.

What tools were used to find the galaxy?

The discovery relied on three different major astronomical instruments to verify the signal. The team used an extremely deep Hubble image based on 40 hours of observations to identify the initial signal.

They further characterized the galaxy’s star-formation history and stellar population using multi-wavelength imaging from the James Webb Space Telescope (JWST). To confirm the distance, the team spent roughly six days of observing time with the European Southern Observatory’s Very Large Telescope (VLT) using the Multi-Unit Spectroscopic Explorer instrument.

The VLT spectrum identified the Lyman-alpha emission line. This line acts as a hydrogen fingerprint, allowing astronomers to measure cosmic distance and time. Marc Rafelski, deputy mission head for the Hubble Space Telescope at STScI and study co-author, noted that no other galaxy from this early period had previously shown detectable ionizing light.

How was the discovery made?

The discovery occurred in October by chance. Goovaerts was preparing a funding proposal and examined an existing Hubble image to see if anyone had previously searched for this specific signal. He found a promising signal within a few hours.

Goovaerts told Live Science that while the initial discovery was quick, it took several months of analysis to extract the galaxy’s properties and mature the findings.

Researchers believe that bursts of vigorous star formation, similar to those seen in MXDFz4.4, were instrumental in clearing the early universe’s hydrogen fog. The team suggests more galaxies with these characteristics likely exist and are waiting to be discovered.