Webb maps an unexpectedly dense core in an ancient galaxy cluster
· BehindLines newsroom
New observations from the James Webb Space Telescope reveal more about the structure of XLSSC 122, a galaxy cluster known since 2014. The findings were presented in June 2026. Its mass, consisting mainly of dark matter, appears more concentrated at its centre than models predict. Seen as it was more than ten billion years ago, the cluster already looks almost “grown-up” at a time when models suggest it should still be taking shape.
A cluster found in 2014, seen anew by Webb
XLSSC 122 was identified in 2014 in an X-ray survey by the European satellite XMM-Newton. Observations with the Hubble telescope then placed it at around 10.4 billion light-years away. Hubble, however, did not show a pronounced gravitational lensing effect there. Webb detected bright arcs around the cluster's centre: light from galaxies behind it is distorted by its mass. XLSSC 122 thus becomes the most distant known cluster to produce strong gravitational lensing.
Mostly dark matter
The cluster's stars and hot gas account for only a small share of this distortion. Most of it comes from invisible dark matter. According to Kyle Finner, an IPAC researcher at Caltech and the lead author, the mass is more concentrated at the centre than current cosmological models predict.
The team also detected a diffuse glow from stars stripped from their galaxies and scattered between them, known as “intracluster light”. This is the earliest detection of that phenomenon. Near the centre, its shape closely follows the distribution of dark matter inferred from lensing. By adding X-ray and radio observations, the researchers conclude that the cluster is still merging with its neighbours.
Testing models with more clusters
In IPAC's statement, Kyle Finner explains that the team wants to study dozens of other very distant clusters. Comparing them would help put cosmological models to the test. One remarkable observation opens a line of research, but is not enough to describe the entire population of clusters from that era.