New JWST images of abnormally well-developed galaxy cluster open up the “cosmic noon” frontier

A stunningly concentrated and hefty galaxy cluster, from a time in the universe’s history when such massive structures aren’t expected to have fully formed yet, is challenging cosmic evolution theories. Across a series of three recent papers, a team led by researchers from IPAC—a science and data center for astrophysics and planetary science at Caltech—have revealed that the cluster is the most distant example of strong gravitational lensing with a galaxy cluster. The new results, based on observations from NASA’s James Webb Space Telescope (JWST), were presented in a press conference on June 17, 2026 at the 248th meeting of the American Astronomical Society.

When astronomers first laid eyes on this galaxy cluster dubbed XLSSC 122, they knew they had found something special. The cluster appeared highly evolved—meaning big and organized like a galaxy cluster in the nearby, modern universe—despite being more than 10 billion light-years away, back in an era when other galaxy groupings had only just started to come together.

Now thanks to the unmatched resolving and light-gathering power of JWST, researchers have discovered that XLSSC 122 is luckily aligned with one or more even more distant galaxies. This chance alignment is causing the giant cluster’s gravity to warp the far-off galaxy’s light—an ultra-rare phenomenon called strong gravitational lensing—in a way that enables newly precise mass measurements of XLSSC 122.

“When we got those first images back from JWST, we said, ‘wow, look at this, there's strong lensing coming from this cluster!’” said Kyle Finner, a staff scientist at IPAC and lead author of the first paper studying the cluster. “XLSSC 122 has now set the record for the most distant galaxy cluster displaying strong lensing, which is a valuable tool for astronomers.”

The serendipitous lensing has provided the most detailed look yet at mass distribution in an early galaxy cluster from the period known as “cosmic noon,” around 10 billion years ago. In this critically formative era, galaxy clusters began to form in bulk as the universe hit its peak in star formation, cranking out stars at a torrid place as much as 100 times faster than the present-day cosmos.

In keeping with XLSSC 122’s iconoclastic reputation, its mass turned out to be extremely concentrated toward the cluster’s center. Such a feature at so early a time in cosmic history challenges conventional cosmological models that chronicle a far slower buildup of massive structure. 

“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 Finner. 

Following up their strong lensing discovery, Finner and colleagues have recently published two more papers, first authored by Zachary Scofield and Hyungjin Joo at Yonsei University, examining other aspects of XLSSC 122 with JWST. Their slew of new results have positioned XLSSC 122 as a trailblazer of galaxy clusters at cosmic noon. 

 

Seeing in the dark

XLSSC 122 first literally came to light in 2014 during an x-ray survey conducted by the European Space Agency’s XMM-Newton spacecraft. Subsequent observations by the Hubble Space Telescope helped firm up the cluster’s distance, about 10.4 billion light-years away, and its unexpectedly mature features. 

Hubble data, however, showed no definitive signs of strong lensing, which appears as arcs of light around the cluster center in JWST images. Capturing these arcs with JWST thus proved a very welcome surprise. “Before JWST, we couldn’t do this level of science in the early, distant universe,” said Finner.

The directly observable mass of XLSSC 122—its bright stars, its illuminated gases, and so on—actually contributes little to the strong lensing effect, however. Instead, the overwhelming source is dark matter—astronomers’ term for an invisible, little-understood, and yet deeply evidenced substance that exerts gravity but otherwise no detectable signatures. Estimates hold that dark matter outweighs regular matter—the stuff of stars, planets, and us—five times over. 

Dark matter is a cornerstone of cosmology, responsible for holding galaxies together and creating the large-scale structure of the universe, where galaxies glom together in groups that scale up into gargantuan filaments stretching across space. Gauging the dark matter distribution in XLSSC 122 is a robust test of this framework and how well it describes the origins of structure since the big bang 13.8 billion years ago. 

“Strong lensing is a way to measure the dark matter without actually seeing the dark matter,” said Finner. “It gives us a sensitive probe of our cosmological models.”

 

A complete view of the galaxy cluster

For their second paper, Finner and colleagues turned to the subtler version of gravitational lensing, called weak lensing. Unlike strong lensing, which jumps off the page, weak lensing involves slight distortions of galaxies caused again by gravity, but so fine that statistical analyses must be run to tease out weak lensing’s effects. 

Whereas strong lensing let the research team weigh the core of the cluster, weak lensing offered better coverage of the broader cluster, extending to its peripheral galactic members. “Weak gravitational lensing can constrain mass much further out, so you can get a better picture of the surrounding cluster area,” said Finner. 

This overall perspective, combined with X-ray and radio wave data from a suite of other telescopes, showed that XLSSC 122 is in the process of merging, with its constituent galaxies still coming together. The second study also corroborated the first study regarding XLSSC 122’s whopping, centrally concentrated mass.

In the third and most recent paper, Finner and colleagues used JWST to trace XLSSC’s so-called intracluster light, a glow generated by stars that float freely between galaxies in clusters. This detection—the earliest known instance of intracluster light—further tells the story of XLSSC 122. The broad extent of the intracluster light bolsters that the cluster is undergoing a merger, where stars scattered out of the colliding galaxies have not yet settled into the cluster’s gravitationally powerful core.

Intriguingly, the researchers noticed that the shape of the intracluster light, where centrally located, matches up well with the dark matter mass concentrations disclosed by strong gravitational lensing. This result, if found in other galaxy clusters and especially in the early universe, could prove to be another tool for astronomers to suss out where hidden dark matter resides. 

“In this cluster, the intracluster light essentially traces the dark matter,” said Finner. “That light tells us that the cluster is in a merging state.”

 

The more, the merrier

Looking ahead, Finner and colleagues hope to find and study dozens more ultra-distant galaxy clusters.     

Discovery of such rarefied objects does not happen with JWST, which intentionally has a narrow, targeted field of view. Instead, wide-area surveys of the sky conducted in X-rays—like that which turned up XLSSC 122—as well as radio telescope surveys are needed to spot candidates. One particularly promising avenue is the Sunyaev–Zel'dovich effect, where telltale “holes” sometimes show up in observations of the afterglow from the big bang, called the cosmic microwave background. Those holes happen when afterglow light gets scattered by high-energy particles in galaxy clusters, betraying the clusters’ locations.

Should more objects like XLSSC 122 emerge, with skewed dark matter concentrations and otherwise unexplainable precociousness, then cosmologists might have to significantly revamp their foundational frameworks of the universe’s development. 

“It's still early in the JWST era,” said Finner, “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.”
 

More about these results:

IPAC at Caltech is a science and data center for astrophysics and planetary science. IPAC provides science operations, user support, archives and data services, and scientific vision to maximize discovery with observatories both in space and on the ground. JWST is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency). The Space Telescope Science Institute hosts the science and mission operations centers for JWST. 

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