Astronomers using the James Webb Space Telescope (JWST) have discovered a stellar bar in GN20, a massive galaxy seen just 1.5 billion years after the Big Bang. The new paper was submitted to the preprint server arXiv on May 14.
Cosmic funnels
Stellar bars are elongated arrangements of stars that cut across the center of a galaxy, rotating as a single rigid unit. As they spin, they act like a funnel and pull gas inward toward the galaxy’s nucleus, which can trigger intense star formation, feed the central black hole, and build up a dense core. In the nearby universe, bars are common. Even the Milky Way hosts one.
But formation of a stellar bar is thought to be slow, taking place over billions of years. Early galaxies were also significantly gas-rich, and gas was thought to suppress or delay bar formation. Therefore, when JWST discovered stellar bars within the first 2 billion years after the Big Bang, it challenged expectations from the standard model.
It was in this context that, in a new study, astronomers led by Leindert A. Boogaard of Leiden University, examined GN20, a massive, gas-rich galaxy at redshift 4. GN20 is distant, faint, and shrouded in dust. Yet JWST’s Mid-infrared instrument and Near-infrared camera turned the dust transparent and revealed its internal structure.
Against all odds
Measuring how the brightness of the galaxy’s light stretches and rotates from the center outward—also known as isophotal analysis—revealed a clear bar structure spanning seven kiloparsecs from end to end.
A completely separate mathematical analysis of the light pattern confirmed it independently. The structure also aligns with a bar-shaped feature seen in dust mapped by another telescope, the NOrthern Extended Millimeter Array (NOEMA).
This detection is remarkable because, according to the theories, it technically should not exist for three reasons. Bars that typically form are so strong that they collapse under their own weight. Even if one survives, growing it to seven kiloparsecs should take billions of years, and the abundance of gas should have delayed or halted its formation.
“Our new results demonstrate that all three of these obstacles can be overcome by a single ingredient directly implicated by the observations: the presence of highly turbulent gas across the inner disk at high gas fraction,” the team writes in the paper.
The researchers do note some uncertainties: estimating the stellar mass of the bar and core regions is complicated by the galaxy’s extreme dust, and some measurements will require better data to pin down. However, they say that none of this changes the paper’s main conclusion—GN20 is a gas-rich system, and the stellar bar is real.
The triggering agent
The observations also show where the star formation is concentrated. Where the bar meets the outer disk to the south, gas piles up and ignites a hotspot of intense star formation. At the center, the bar is sweeping material inward, fueling a nuclear starburst and possibly feeding a supermassive black hole—likely a major driver of GN20’s extraordinary star formation rate of over 1,000 solar masses per year.
“Part of this high SFR is likely being driven by the bar funneling gas and dust into the center, where it triggers an intense nuclear starburst in the gas-rich disk, and fuels the potential active galactic nucleus,” the team explains.
In that case, GN20-like galaxies may not simply be a phase in a galaxy’s evolution. The bar-driven star formation rate could explain another puzzle of the universe.
Once the star-forming gas is spent, the galaxy stops forming new stars and goes quiet. This could be the missing link that explains how the massive, dead elliptical galaxies seen in today’s universe got that way, and why some of them appear to have quenched so early.
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Publication details
Leindert A. Boogaard et al, A stellar bar hidden in an extreme gas-rich disk galaxy at z=4.055, arXiv (2026). DOI: 10.48550/arxiv.2605.15273
Journal information:
arXiv
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JWST finds a stellar bar in the early universe that breaks all rules (2026, May 31)
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