By
Christina Nunez
|
July 6, 2026
An Argonne-led research team has released a catalog of more than 7,000 confirmed galaxy clusters from five years of South Pole Telescope data, opening a powerful new window into the large-scale structure and evolution of the universe.
A cutout of SPT-3G data (left) showing microwave signals, alongside a map (right) with colorful boxes marking confirmed galaxy clusters detected in the same region. These maps represent approximately 0.5% of the main field survey area. (Image by SPT-3G Collaboration/Argonne National Laboratory.)
Researchers working with data from the South Pole Telescope (SPT) have released a major catalog of galaxy clusters, giving scientists a powerful new tool for studying how the universe grew and changed over billions of years.
The catalog is based on five years of observations from the SPT-3G experiment at the U.S. National Science Foundation (NSF) Amundsen-Scott South Pole Station in Antarctica and involves a collaboration of researchers from institutions around the world.
Using exceptionally sensitive measurements of the cosmic microwave background (CMB) — the faint afterglow of the Big Bang — the multi-institutional team identified 8,892 possible galaxy clusters and confirmed 7,190 of them using optical and infrared data.
“Our analysis draws on the SPT-3G’s phenomenally deep CMB data to open a new window onto the ancient universe. It’s a new milestone for cluster cosmology to have this catalog as a resource. It will be the core of many, many studies over the years to come.” — Lindsey Bleem, Argonne physicist and lead author
Of those confirmed clusters, roughly 20% do not appear in any previous catalog, but the findings go even further: For 67% of the sample — some 4,824 systems — this marks the first time the hot gas within these clusters has ever been detected, making the majority of the catalog a genuinely new window into the large-scale structure of the universe.
“Our analysis draws on the SPT-3G’s phenomenally deep CMB data to open a new window onto the ancient universe,” said Lindsey Bleem, a physicist at the U.S. Department of Energy’s (DOE) Argonne National Laboratory and lead author of the study. “It’s a new milestone for cluster cosmology to have this catalog as a resource. It will be the core of many, many studies over the years to come.”
SPT-3G refers to the camera mounted on the telescope, which was upgraded in 2017 with 16,000 detectors built at Argonne. The sample in the newly published study reaches across about 4% of the sky, or 1,600 square degrees, and includes about 1,800 clusters that date back more than 7.8 billion years.
Galaxy clusters are enormous structures containing hundreds to thousands of galaxies, hot gas and large amounts of dark matter, all bound together by gravity. Because they are the largest gravitationally bound systems in the universe, they are useful for probing ideas about dark matter, dark energy and how cosmic structures form over time.
While earlier surveys have scanned larger areas of sky, the new catalog is remarkable for its depth, showing more faint and distant clusters.
The team found the clusters by looking for distortions they cause in the CMB known as the Sunyaev-Zeldovich effect. As the CMB’s light travels through a galaxy cluster, high-energy particles in the cluster alter the light, creating a subtle signal that can be detected in microwave observations. The Sunyaev-Zeldovich effect allows researchers to image the clusters as shadows on the backdrop of the CMB.
This image shows galaxy clusters detected at a high signal-to-noise ratio in the SPT-3G survey, with optical data from the Dark Energy Survey overlaid with Sunyaev-Zeldovich effect detection contours. (Image by SPT-3G Collaboration/Argonne National Laboratory.)
“Building a catalog like this takes a lot of careful checking behind the scenes,” said Kayla Kornoelje, a University of Chicago graduate student working at Argonne, whose earlier research helped validate signals from a portion of the data featured in the catalog. “A big part of our work was making sure the detections are reliable so that this sample can be used with confidence in future cosmological studies.”
The work also showed that the survey is sensitive enough to study not only the clusters themselves, but also how conditions within them changed over time. The researchers found a strong increase in dust-related emission from cluster environments at earlier times in the universe, helping reveal how activity such as star formation evolved in and around these massive systems.
“With the SPT-3G cluster sample, we will probe the evolution of cosmic structure formation over the past 10 billion years,” said Sebastian Bocquet, senior staff scientist at the Ludwig Maximilian University (LMU) Observatory in Munich, Germany — a member of the SPT collaboration.
Future work will focus on refining cluster mass measurements and using the catalog to test models of the universe. Upcoming surveys, including observations from the Legacy Survey of Space and Time camera at the DOE- and NSF-funded Vera C. Rubin Observatory in Chile and the European Space Agency’s Euclid mission, are also expected to provide optical confirmation of even more distant galaxy clusters in the SPT-3G data sample.
“Working on the confirmation and redshift measurements of SPT-3G candidates gave me the opportunity to inspect many of these clusters in optical and infrared imaging data,” said Matthias Klein, a senior staff scientist at the LMU Observatory. Redshift refers to how much of a wavelength of light from a galaxy cluster has been stretched by the expansion of the universe — a measure that tells scientists how far away, and how far back in time, the cluster is. “Seeing the galaxy clusters behind the microwave signals made me excited about the science this sample will enable.”
The SPT is funded primarily by the NSF and the DOE Office of Science, Office of High Energy Physics, and is operated by a collaboration led by the University of Chicago. Optical data used to confirm the clusters came from the DOE/NSF-supported Dark Energy Survey.
The Dark Energy Survey is jointly supported by the DOE Office of Science and the NSF. Learn more about the collaboration and the funding for this project.
Co-authors with Bleem and Kornoelje at Argonne are Amy Bender, Clarence Chang, Giulia Campitiello, Florian Kéruzoré, Wei Quan and John Carlstrom, as well as Zhaodi Pan, a former Maria Goeppert Mayer fellow.
Christina Nunez is a freelance writer and editor who covers science, technology, and innovation at Argonne and other research facilities under the U.S. Department of Energy. Her work also appears at National Geographic and other publications. She has been writing for Argonne since 2018.
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