Webb Sees the Galaxies that Cleared Out the Cosmic Fog

By Mark Thompson - June 14, 2025 at 10:46 PM UTC | Cosmology
White diamonds show the locations of 20 of the 83 young, low-mass, starburst galaxies found in infrared images of the giant galaxy cluster Abell 2744. This composite incorporates images taken through three NIRCam filters (F200W as blue, F410M as green, and F444W as red). The F410M filter is highly sensitive to light emitted by doubly ionized oxygen — oxygen atoms that have been stripped of two electrons — at a time when reionization was well underway. Emitted as green light, the glow was stretched into the infrared as it traversed the expanding universe over billions of years. The cluster’s mass acts as a natural magnifying glass, allowing astronomers to see these tiny galaxies as they were when the universe was about 800 million years old. NASA/ESA/CSA/Bezanson et al. 2024 and Wold et al. 2025
White diamonds show the locations of 20 of the 83 young, low-mass, starburst galaxies found in infrared images of the giant galaxy cluster Abell 2744. This composite incorporates images taken through three NIRCam filters (F200W as blue, F410M as green, and F444W as red). The F410M filter is highly sensitive to light emitted by doubly ionized oxygen — oxygen atoms that have been stripped of two electrons — at a time when reionization was well underway. Emitted as green light, the glow was stretched into the infrared as it traversed the expanding universe over billions of years. The cluster’s mass acts as a natural magnifying glass, allowing astronomers to see these tiny galaxies as they were when the universe was about 800 million years old. NASA/ESA/CSA/Bezanson et al. 2024 and Wold et al. 2025

Scientists using NASA's James Webb Space Telescope (JWST) have made an exciting discovery about the early universe. They found dozens of small galaxies that played a huge role in transforming our cosmos from a dark, foggy place into the bright, clear universe we see today.

Imagine the early universe as a room filled with thick fog. For its first billion years, space was clouded by neutral hydrogen gas that blocked most light from traveling far. Today, this same gas is "ionized," meaning it has been stripped of electrons and can let light pass through freely. This transformation, called reionization, essentially cleared the cosmic fog and allowed galaxies to shine brightly across vast distances.

Artist impression of the James Webb Space Telescope (Credit : NASA) Artist impression of the James Webb Space Telescope (Credit : NASA)

The newly discovered galaxies are surprisingly tiny compared to our Milky Way. You would need between 2,000 to 200,000 of these small galaxies to equal the mass of our own Galaxy. Despite their size, these miniature powerhouses were incredibly effective at producing ultraviolet light.

"When it comes to producing ultraviolet light, these small galaxies punch well above their weight," - Isak Wold from the Catholic University of America

These tiny galaxies had two key advantages that made them perfect fog-clearers. First, their small size meant they couldn't gather as much hydrogen gas around them, making it easier for their powerful ultraviolet light to escape into space. Second, they experienced intense periods of star formation called "starbursts" that not only created lots of ultraviolet light but also carved pathways through their own material, helping that light break free.

The research team used JWST's incredible infrared vision to peer back in time to when the universe was only 800 million years old, just 6% of its current age of 13.8 billion years. They focused on a galaxy cluster called Abell 2744, nicknamed "Pandora's cluster," which acts like a natural magnifying glass, making distant objects appear larger and brighter.

Abell 2744, nicknamed Pandora's Cluster. The galaxies in the cluster make up less than five percent of its mass. (Credit : NASA, ESA) Abell 2744, nicknamed Pandora's Cluster. The galaxies in the cluster make up less than five percent of its mass. (Credit : NASA, ESA)

The scientists looked for a specific green light signature from oxygen atoms that had lost electrons, a telltale sign of the high energy processes happening in these ancient galaxies. This green light, originally visible to the naked eye billions of years ago, had been stretched into infrared light by the time it reached Webb's sensors.

The team discovered 83 of these small starburst galaxies and studied 20 in detail. Their analysis suggests that if these ancient galaxies released about 25% of their ultraviolet light into surrounding space, similar to comparable galaxies today, they could account for all the energy needed to clear the fog that pervades the universe.

This discovery helps solve a long standing mystery about which types of objects were responsible for reionization. While astronomers previously debated whether large galaxies, small galaxies, or supermassive black holes drove this cosmic renovation, JWST's observations strongly support the small galaxy theory.

These findings remind us that sometimes the smallest players can have the biggest impact, fundamentally changing our understanding of how the universe evolved from darkness into light.

Mark Thompson

Mark Thompson

Science broadcaster and author. Mark is known for his tireless enthusiasm for making science accessible, through numerous tv, radio, podcast and theatre appearances, and books. He was a part of the aware-nominated BBC Stargazing LIVE TV Show in the UK and his Spectacular Science theatre show has received 5 star reviews across UK theatres. In 2025 he is launching his new pocast Cosmic Commerce and is working on a new book 101 Facts You Didn't Know About Deep Space In 2018, Mark received an Honorary Doctorate from the University of East Anglia.

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