NASA's James Webb Space Telescope has found that galaxy cluster XLSSC 122, from 10.4 billion years ago, is the most distant known strong gravitational lens - with a dark matter core so concentrated that the standard Lambda CDM cosmological model cannot account for it.
Galaxy cluster XLSSC 122 - located 10.4 billion light-years from Earth - is the most distant galaxy cluster known to act as a strong gravitational lens, according to three studies published in The Astrophysical Journal Letters using data from NASA’s James Webb Space Telescope. The research also found that the cluster’s dark matter core is more concentrated than the standard Lambda CDM cosmological model predicts for objects at this early stage of the universe, establishing XLSSC 122 as a precision test of how cosmic structure assembled in the universe’s first few billion years.
The cluster was first identified in 2014 by the European Space Agency’s XMM-Newton X-ray observatory, but its remote location placed it well beyond the detailed observational reach of earlier instruments. JWST, with its infrared sensitivity and resolution, changed that.
XLSSC 122 belongs to the era astronomers call “cosmic noon” - roughly 10 billion years ago, when the universe was forming stars at approximately 100 times the rate it does today, and when the first large galaxy clusters were still assembling. The standard Lambda CDM framework - the cosmological model describing a universe dominated by cold dark matter and a cosmological constant driving accelerating expansion - predicts that clusters at this epoch should be loosely organised and only beginning to concentrate gravitationally. XLSSC 122 does not match that description.
Kyle Finner, a staff scientist at the Infrared Processing and Analysis Center (IPAC) at Caltech, led the first paper. His team identified XLSSC 122 as the most distant galaxy cluster yet observed to produce strong gravitational lensing - the warping of light from background galaxies into visible arcs and multiple images by the cluster’s mass. “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, according to IPAC.
Strong lensing is useful because it provides a direct measurement of the mass enclosed within the lensing region, including dark matter that cannot be observed directly. “Strong lensing is a way to measure the dark matter without actually seeing the dark matter,” Finner noted. What the lensing revealed was a central dark matter concentration that, in Finner’s words, “has a mass concentration that doesn’t agree with our cosmological model predictions.”
A companion paper led by Zachary Scofield of Yonsei University applied weak gravitational lensing - a statistical analysis of subtle distortions in the shapes of background galaxies - across a wider area of the cluster. That dataset confirmed the cluster is in an active merger state, with X-ray, radio, and Sunyaev-Zeldovich observations corroborating that the constituent galaxies are still coalescing. The cluster is concentrated yet simultaneously still growing - a combination that strains simple evolutionary timelines.
A third paper, led by Hyungjin Joo, also of Yonsei University, reported what the teams describe as the earliest known detection of intracluster light (ICL) at this distance. Intracluster light is the diffuse glow produced by stars that have been stripped from their host galaxies during close galactic encounters. In XLSSC 122, this light extends roughly 100 kiloparsecs from the cluster’s brightest central galaxy. Critically, its spatial distribution closely follows the dark matter distribution mapped by strong lensing. “In this cluster, the intracluster light essentially traces the dark matter,” Finner said. “That light tells us the cluster is in a merging state.”
The three studies jointly confirm: XLSSC 122 bends background light at the greatest confirmed distance for a galaxy cluster lens; its dark matter core is more concentrated than Lambda CDM predicts for this cosmic epoch; it is actively merging; and it shows the earliest known intracluster light at this redshift.
What the data do not resolve is whether XLSSC 122 is a statistical outlier or the leading example of a broader population of early clusters that the standard model inadequately describes. Lambda CDM has accounted for a wide range of observations across cosmic history, and the research teams are careful about the scope of the finding. Meaningfully testing cosmological models at “cosmic noon” requires observations of many clusters, not a single object.
“It’s still early in the JWST era,” Finner said, “and if we can start to get data on tens or hundreds of these types of objects at this stage in the universe, then we can really start putting our cosmological models to the test.”
XLSSC 122 now functions as the reference case for that programme. Before JWST, the cluster’s distance and redshift made this level of detail unachievable. The combination of strong lensing, weak lensing, and intracluster light detection in a single cluster at this distance had not been accomplished before. Whether the standard model requires revision, refinement, or simply more data is a question the next generation of JWST observations is positioned to answer.
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