South Pole Telescope analysis releases new catalog of more than 7,000 galaxy clusters

Researchers working with data from the South Pole Telescope 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 findings are published on the arXiv preprint server.

The catalog is based on five years of observations from the South Pole Telescope at the National Science Foundation Amundsen-Scott South Pole Station in Antarctica and involves a collaboration of researchers from institutions around the world, including the University of Chicago.

Using exceptionally sensitive measurements of the cosmic microwave background—the faint afterglow of the Big Bang—the team identified 8,892 possible galaxy clusters and confirmed 7,190 of them using optical and infrared data.

Of those confirmed clusters, roughly 20% do not appear in any previous catalog. And for 67% of the sample—some 4,824 systems—this marks the first time the hot gas within these clusters has 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 cosmic microwave background data to open a new window onto the ancient universe,” said lead study author Lindsey Bleem, a physicist at Argonne National Laboratory and senior associate at the UChicago Kavli Institute for Cosmological Physics. “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 structure formed 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 cosmic microwave background, known as the Sunyaev-Zeldovich effect. As the cosmic microwave background 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 cosmic microwave background.

“Building a catalog like this takes a lot of careful checking behind the scenes,” said Kayla Kornoelje, a UChicago 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.”

Probing cosmic structure formation

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 Observatory in Munich, Germany—a member of the South Pole Telescope 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 Department of Energy- and National Science Foundation-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.

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