New observations from the James Webb Space Telescope show that XLSSC 122, a galaxy cluster about 10.4 billion light-years away, was already mature during cosmic noon, when the universe was forming stars at a furious rate. The cluster is now the most distant one known to produce strong gravitational lensing, bending light from even more distant background galaxies into curved arcs.
Using that lensing to map the cluster's mass, researchers found an unusually large fraction of it concentrated in the central region. Standard cosmological models predict that such structures should have grown more gradually, so the dense core is hard to explain. "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," said Kyle Finner of IPAC, lead author of one of three new studies.
Most of the lensing signal comes from dark matter, not the cluster's visible stars and gas. Hubble had not shown clear lensing in XLSSC 122; JWST's sharper images revealed the telltale arcs. The findings, presented at the 248th American Astronomical Society meeting, give astronomers a new way to probe how dark matter was distributed in the early universe.