JWST Discovers Most Distant Lensing Galaxy Cluster
The James Webb Space Telescope has identified XLSSC 122, a galaxy cluster 10.4 billion light-years away, as the most distant known to exhibit strong gravitational lensing. Its dense core and mature structure, observed during cosmic noon, challenge current models of how massive cosmic structures form.
The galaxy cluster XLSSC 122 is the most distant known to exhibit strong gravitational lensing. Located approximately 10.4 billion light-years from Earth, it dates to a period known as cosmic noon, when the universe was about 3.4 billion years old. The discovery, made with NASA's James Webb Space Telescope, was presented on June 17, 2026, at the 248th meeting of the American Astronomical Society. The cluster was originally identified in 2014 through an X-ray survey by the European Space Agency's XMM-Newton spacecraft.
JWST reveals strong lensing and intracluster light
JWST captured distinctive curved arcs of light around the cluster's center, providing the first definitive signs of strong gravitational lensing there. These arcs, a hallmark of the phenomenon, were missed by earlier Hubble Space Telescope observations. The gravitational distortions indicate a significant contribution from dark matter. The telescope also detected the earliest known intracluster light-a faint glow from stars not bound to any single galaxy. This light is believed to originate from stars dispersed throughout the cluster.
Kyle Finner, a staff scientist at IPAC at Caltech and a lead author, described the moment of discovery. "When we got those first images back from JWST, we said, 'wow, look at this, there's strong lensing coming from this cluster!'"
Lensing and light trace dark matter in a merging cluster
The distribution of the faint intracluster light in XLSSC 122 closely matches the distribution of dark matter inferred from the strong gravitational lensing. Strong lensing allows astronomers to measure the mass of otherwise invisible matter. Weak gravitational lensing was used to study the mass distribution in the cluster's outer regions. Combined observations from lensing, X-ray, and radio data indicate that XLSSC 122 is still undergoing a galaxy cluster merger.
"In this cluster, the intracluster light essentially traces the dark matter," said Kyle Finner. That light tells us that the cluster is in a merging state.
Central mass concentration challenges cosmological models
The cluster exhibits a surprisingly dense concentration of matter in its central region. This observation contradicts current cosmological models that predict more gradual growth of massive structures over time. During cosmic noon, the universe experienced a peak in star formation. The research has significant implications for cosmological models, challenging existing theories about the timeline of cosmic structure formation.
Future research on ancient clusters
The research team, led by scientists at IPAC, Caltech's science and data center, aims to identify more ancient galaxy clusters. They want to determine if XLSSC 122's properties are isolated or indicative of a broader trend. The plan is to study dozens of additional ancient clusters using wide-area X-ray and radio surveys and by searching for signatures in the cosmic microwave background via the Sunyaev-Zel'dovich effect. This will further test cosmological models.
