Galaxy cluster at cosmic noon defies growth models
Key takeaways
- XLSSC 122, located 10.4 billion light-years away, is the most distant galaxy cluster known to exhibit strong gravitational lensing.
- Webb revealed curved arcs of light created by the cluster's gravity bending light from more distant galaxies.
- The cluster's mass is unusually concentrated at its core, a distribution that contradicts standard cosmological model predictions for structures at this early stage.
The galaxy cluster XLSSC 122 sits 10.4 billion light-years away. XLSSC 122 dates to cosmic noon, roughly 10 billion years ago, when the universe experienced its most intense era of star formation. At that time, galaxies were producing stars at rates up to 100 times higher than in the present-day universe, and galaxy clusters were becoming increasingly common as smaller systems combined into larger structures.
Yet XLSSC 122 appears to have developed faster than expected. The cluster already displayed the size and organised structure typically associated with clusters in the modern universe, despite existing when such enormous structures were only beginning to take shape.
Strong lensing discovered
Webb revealed strong gravitational lensing from the cluster, creating curved arcs of light. XLSSC 122 is the most distant galaxy cluster known to exhibit strong gravitational lensing, a phenomenon that occurs when a massive structure bends and distorts light travelling from galaxies even farther away.
Kyle Finner, a staff scientist at IPAC, Caltech's science and data centre, led the first paper examining the cluster. "When we got those first images back from JWST, we said, 'wow, look at this, there's strong lensing coming from this cluster!'" Finner said. "XLSSC 122 has now set the record for the most distant galaxy cluster displaying strong lensing, which is a valuable tool for astronomers."
Unexpected mass concentration
The gravitational lensing has given scientists an exceptionally precise way to measure how mass is distributed throughout the cluster. The measurements reveal that a remarkably large amount of XLSSC 122's mass is packed into its central region. According to Finner, "XLSSC 122 is one of the first clusters we know of that formed in the universe, and it has a mass concentration that doesn't agree with our cosmological model predictions."
Standard cosmological models predict that the growth of massive structures should have been more gradual, making such a concentrated cluster particularly surprising at this early stage of cosmic history. The research was presented at a press conference on 17 June 2026 during the 248th meeting of the American Astronomical Society.
