JWST Spots Ancient Galaxy Cluster at Cosmic Noon That Challenges Cosmology
Featured image: JWST NIRCam image of XLSSC 122 showing gravitationally lensed arcs around the brightest cluster galaxy. [Credit: Finner et al. / NASA / ESA / CSA]
The James Webb Space Telescope has peered into one of the universe’s oldest known galaxy clusters at a time when the cosmos was just beginning its middle age, and what it found has astronomers rethinking how structure forms in the universe. Designated XLSSC 122, the cluster sits at redshift 1.98, meaning its light has traveled roughly 10.4 billion years to reach Earth. The universe was just 3.4 billion years old when the light we see today left this gargantuan assembly of galaxies.
The findings, published in three companion papers in The Astrophysical Journal Letters and presented at the 248th meeting of the American Astronomical Society on June 17, reveal a cluster that is far more mature and centrally concentrated than standard cosmological models predict. The central dark matter concentration measured inside 100 kiloparsecs, determined through strong gravitational lensing of background galaxies, is so high that it deviates from nearly every major mass-concentration relation at a significance of 3 to 8 standard deviations.
“We were stunned when we saw those first images,” said Kyle Finner of IPAC/Caltech, lead author of the strong-lensing paper. “We said, ‘wow, look at this, there’s strong lensing coming from this cluster!’ and it quickly became clear we were looking at something the models don’t easily explain.”
A mature structure in a young universe
XLSSC 122 was first discovered in 2014 by ESA’s XMM-Newton X-ray observatory as part of the XMM-Newton Large-Scale Structure Survey. But it was JWST’s infrared sensitivity and high resolution that revealed its true nature. On August 10, 2024, JWST’s NIRCam instrument observed the cluster across four filters reaching a 5-sigma depth of 29-30 AB magnitude, penetrating through cosmic dust to resolve the cluster’s intricate structure.
What emerged was the most distant galaxy cluster known to exhibit strong gravitational lensing, four giant arc systems wrapped around the brightest cluster galaxy, breaking the previous record held by IDCS J1426.5+3508 at redshift 1.75.
“We already knew XLSSC 122 existed from X-ray data, but we didn’t know it was this massive and this concentrated,” Finner said. “JWST opened a new window. Before Webb, we couldn’t do this level of science in the early, distant universe.”
The NFW concentration parameter of the cluster’s dark matter halo was measured at c = 6.3 ± 0.5 within the inner 100 kiloparsecs. Compared to five standard mass-concentration relations, only one, Prada et al. 2012, predicts a higher value. The others underestimate the concentration by between 2.8 and 8.2 standard deviations.
The dark matter connection
Strong gravitational lensing is among the most powerful tools for measuring dark matter, because the warping of background galaxy light traces the total mass distribution, including the invisible dark matter component.
“Strong lensing is a way to measure the dark matter without actually seeing the dark matter,” Finner explained. “It gives us a sensitive probe of our cosmological models.”
JWST’s observations probed the cluster through three independent methods: strong gravitational lensing of background galaxies to measure the core mass distribution; weak gravitational lensing of more distant galaxies to trace the outer halo; and the detection of intracluster light, free-floating stars not bound to any single galaxy, whose shape traces the dark matter distribution.
The intracluster light detection, reported in a companion paper led by Hyungjin Joo, is the earliest such detection ever made. It reveals that the cluster is actively undergoing a merger, confirmed independently by weak-lensing analysis and X-ray and radio data, adding further complexity to the picture.
Not just a cluster, a test of fundamental physics
The discovery has implications beyond cluster astrophysics. If JWST continues finding similarly mature clusters at early epochs, the standard Lambda-CDM model of hierarchical structure formation, in which massive structures assemble slowly over cosmic time, may need significant revision.
One possibility is that XLSSC 122 formed in an unusually early collapse, consistent with the accelerated size evolution already observed in its population of quiescent galaxies. Another is that our understanding of how dark matter behaves in dense environments is incomplete. A third option, not ruled out by the data, is that non-standard cosmology is at work.
“It’s still early in the JWST era,” Finner said. “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.”
The three companion papers are published in The Astrophysical Journal Letters. The JWST imaging data are publicly available on Zenodo.