Summary
JWST observations show that the distant cluster XLSSC 122 strongly lenses light from more distant galaxies, making it the most distant known galaxy cluster with this effect. The team also found an unusually concentrated mass distribution, a result that challenges predictions from conventional cosmological models.
New James Webb Space Telescope (JWST) observations show that the galaxy cluster XLSSC 122 is the most distant known cluster whose gravity strongly lenses light from galaxies behind it. In a set of three studies, researchers led by Caltech’s IPAC used the lensing arcs to measure the cluster’s mass distribution and found it unusually concentrated toward the centre—a feature that challenges predictions from conventional models of cosmic structure formation.
The results were presented on June 17, 2026, at the 248th meeting of the American Astronomical Society. XLSSC 122 lies about 10.4 billion light-years away, in an era when massive galaxy clusters were still taking shape. Its appearance is strikingly developed for that period, according to the IPAC team.
How the cluster’s gravity revealed its mass
The cluster’s gravity bends and magnifies light from more distant galaxies aligned behind it. In JWST images, this strong gravitational lensing appears as arcs around the cluster’s centre. By studying those distorted images, astronomers can infer the mass responsible for bending the light, including matter that does not emit detectable light.
The visible stars and hot gas in XLSSC 122 account for only a small part of the mass driving the lensing, the researchers report. Most of the effect comes from dark matter, which exerts gravity but cannot be observed directly through its own light. The lensing therefore provides a way to map the cluster’s mass, including its dark matter, rather than relying only on its luminous contents.
Earlier Hubble observations helped establish the cluster’s distance and mature appearance, but did not show definitive lensing arcs. JWST’s resolving power and sensitivity brought those features into view, enabling a more detailed assessment of the cluster’s central mass concentration.
That concentration is the main cosmological puzzle. The team reports that XLSSC 122’s mass is packed unusually tightly toward its centre, unlike the slower buildup of massive structures predicted by the models they examined. One cluster is a focused test of those predictions; finding and studying more early clusters will help establish whether this system is an outlier or points to a broader challenge.
Lensing and intracluster light show a cluster still assembling
A second study used weak gravitational lensing, which produces subtler distortions than the prominent arcs of strong lensing. Those small distortions require statistical analysis, but they let researchers examine mass farther from the centre and across more of the cluster. Combined with X-ray and radio observations, the weak-lensing results indicate that XLSSC 122 is merging: its constituent galaxies are still coming together.
A third study detected intracluster light—the glow from stars that have drifted between galaxies within a cluster. IPAC describes this as the earliest known detection of intracluster light. Its broad extent supports the picture of a cluster in the process of merging, with scattered stars not yet settled into the central region. The shape of the light near the centre also aligns with the mass concentrations mapped through strong lensing. If that relationship is seen in other clusters, intracluster light could offer another way to trace where dark matter is concentrated.
The discovery also points to the need for wider surveys. JWST’s field of view is narrow, so the team says that X-ray and radio surveys are important for finding rare, distant cluster candidates before detailed follow-up observations. Researchers hope to study dozens more systems like XLSSC 122; a larger sample could put models of how cosmic structures formed to a broader test.