Observed the birth of a remote cluster of galaxies in the early universe

Galaxy clusters are some of the largest known objects in the universe and, as the name suggests, are home to huge numbers of galaxies, sometimes up to thousands. Among them is the so-called intracluster medium (ICM) of gas, which extends beyond the galaxies themselves.

Much of the physics of galaxy clusters is well known; however, observations of the early stages of ICM formation remain sparse. Previously, it had only been studied in fully formed clusters of nearby galaxies.

A large reservoir of hot gas has been detected in the galaxy cluster still forming around the Spider Web galaxy.

The detection of ICM in protoclusters (that is, clusters of galaxies still in formation) located at great distances, would allow the astronomical community to capture these clusters in the early stages of formation. A team led by Luca Di Mascolo, a researcher at the University of Trieste (Italy) and first author of a study published this week in Naturesought to detect this average introcluster in a protocluster from the early stages of the universe.

Galaxy clusters are so massive that they can accumulate gas that heats up as it falls towards the cluster. “Cosmological simulations have been predicting the presence of hot gas in protocumulus for over a decade, but observational confirmations were lacking,” explains Elena Rasia, a researcher at the Italian National Institute of Astrophysics (INAF) in Trieste, Italy, and co-author of the study “A The search for key observational confirmation led us to carefully select one of the most promising candidate protoclusters.”

The study focused on the Spider Web protocluster, located at a time when the universe was just 3 billion years old.

This was the Spider Web protocluster, located at a time when the universe was only 3 billion years old and so named because it surrounded the Spider Web galaxy (formerly known as MRC 1138-262).

Despite being the most studied protocumulus, ICM detection has not been successful. Finding a large reservoir of hot gas in this protocluster would indicate that the system is on its way to becoming a long-lasting, stable cluster of galaxies rather than dispersing.

The Sunyaev-Zeldovich thermal effect

Di Mascolo’s team detected the ICM of the Cobweb protocluster through what is known as the Sunyaev-Zeldovich (SZ) thermal effect. This effect occurs when light from the cosmic microwave background (the radiation left over from the Big Bang) passes through the ICM.

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When that light interacts with fast-moving electrons in the hot gas, it gains a little energy and its color, or wavelength, changes slightly. “At the right wavelengths, the SZ effect appears as a shadow effect of a galaxy cluster on the cosmic microwave background,” explains Di Mascolo.

By measuring these shadows in the cosmic microwave background, the astronomical community can infer the existence of hot gas, estimate its mass and map its shape. “Today, thanks to its unrivaled resolution and sensitivity, ALMA is the only facility capable of making this measurement of the distant progenitors of massive clusters,” says Di Mascolo. The Atacama Large Millimeter/submillimeter Array (ALMA) is operated by the European Southern Observatory (ESO) and other institutions in Chile.

They determined that the Cobweb Protocluster contains a vast reservoir of hot gas at a temperature of a few tens of millions of degrees Celsius. Cold gas had previously been detected in this protocluster, but the mass of hot gas found in this new study exceeds it by thousands of times.

This discovery shows that the Cobweb Protocluster is on track to become a huge cluster of galaxies in about 10 billion years, increasing its mass by at least a factor of ten.

This discovery shows that the Cobweb Protocluster is on track to become a huge cluster of galaxies in about 10 billion years.

Tony Mroczkowski, co-author of the paper and an ESO researcher, explains that “this system presents huge contrasts. The hot thermal component will destroy much of the cold component as the system evolves, and we are witnessing a delicate transition.”

He concludes by stating that it “provides observational confirmation of long-standing theoretical predictions about the formation of the largest gravitationally bound objects in the universe”.

These results help lay the groundwork for synergies between ALMA and the upcoming Extremely Large Telescope (ELT) from ESO, which “will revolutionize the study of structures like this one”, says Mario Nonino, co-author of the study and researcher at the Astronomical Observatory of Trieste.


Luca Di Mascolo et al. “Forming intracluster gas in a protocluster of galaxies with a redshift of 2.16″. Nature, 2023.

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