Faint, small galaxies ionized the opaque fog that obscured the early universe.

Pandora's Cluster of galaxies
Behind Pandora's Cluster (Abell 2744), imaged here by the James Webb Space Telescope, are eight background galaxies hailing from the early universe.

An international team of astronomers has used the James Webb Space Telescope to make the first spectroscopic observations of the faintest galaxies present during the universe’s first billion years. The results offer vital clues toward solving a mystery in the early years of the universe.

For the first few hundred million years after the Big Bang, the universe was a thick soup of hydrogen fog with no stars to illuminate it. This period is known as the cosmic dark ages. The fog is thought to have eventually cleared when radiation ripped hydrogen atoms apart, a process known as reionization. But debate remained as to whether this reionizing radiation came solely from the first stars that lit up the earliest galaxies or whether material falling onto supermassive black holes also played a significant role.

A team of astronomers led by Hakim Atek (Sorbonne University, France) used the James Webb Space Telescope to peer at extremely faint dwarf galaxies in the early universe. These distant galaxies wouldn’t normally be visible — even to Webb’s powerful capabilities — but astronomers received a helping hand from the gravity of Pandora’s Cluster, a car crash of galaxies 4 billion light-years away. The cluster’s immense mass magnified the more distant galaxies’ light. The team's findings are published in the February 28th Nature.

Atek’s team was able to study eight early galaxies in total, taking both images and spectroscopic data with Webb's Near-InfraRed Spectrograph (NIRSpec). The stand-out finding is that the galaxies produced four times more ultraviolet radiation than astronomers had derived from previous scenarios.

“Despite their tiny size, these low-mass galaxies are prolific producers of energetic radiation, and their abundance during this period is so substantial that their collective influence can transform the entire state of the Universe,” Atek says.

“They produce ionizing photons that transform neutral hydrogen into ionized plasma during cosmic reionisation,” adds team member Iryna Chemerynska (also at Sorbonne).  

“It’s fantastic to see the pieces fitting together, and pointing squarely at tiny galaxies as the culprits,” says Sean McGee (University of Birmingham, UK), who was not involved in the research. Astronomers have long suspected that such galaxies helped light up the early universe, but McGee adds that “this was becoming more debatable recently as JWST has been finding more AGN than previously expected.” Now, the new JWST data on these tiny galaxies puts them back in the limelight.

“It had been an assumption that . . . these small galaxies would play an important role, but it’s very satisfying to see it directly,” McGee says. “It wouldn't have been possible without JWST.”

The next step is an upcoming Webb observing program named GLIMPSE. Astronomers will target another massive galaxy cluster — Abell S1063 — in order to see even fainter galaxies behind it in the early universe. This will allow them to verify whether the dwarf galaxies in the current study are typical of the large-scale distribution of galaxies.

McGee also notes the importance of the upcoming Square Kilometer Array (SKA). “We will be able to map out exactly where the universe hasn’t been reionized,” he says. If the findings of Atek's teams are right, we should only find neutral hydrogen — which gives out a distinctive radio signal at 21 centimetres — far from dwarf galaxies. “The lack of a radio signal around the tiny galaxies will be like the dog who didn’t bark in the night.”


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