This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Astronomers have detected an enormous cloud of gas tens of millions of degrees hot surrounding one of the most extreme structures known in the early universe. The study offers one of the clearest views yet of a galaxy cluster in the making.
Using more than 600,000 seconds of observations with NASA's Chandra X-ray Observatory, the team found diffuse, extended X-ray emission around a quasar embedded in a dense concentration of galaxies more than 11 billion light-years away. The paper describing the results was published in Astronomy & Astrophysics on July 24. Thousands of galaxies gravitationally bound together in a crowded region are collectively called a galaxy cluster.
They can form through gravitational collapse and mergers with smaller structures. When gas is accreted into a reservoir of hot gas—the intracluster medium (ICM)—that fills the gaps between galaxies in a cluster, the infalling gas undergoes shock heating, reaching temperatures of tens to hundreds of millions of Kelvin. The hot gas is primarily detected through X-ray emission.
Astronomers understand this hot gas well in mature, nearby clusters. But they want to know when and how this hot gas envelope first started forming, when the universe was young. What they expect to see in the early universe (some 2 to 3 billion years after the Big Bang) is a young halo of gas that might be starting to heat up, called a "proto-ICM"—a hot circumgalactic medium in the nascent stage.
Signals from these young halos are difficult to detect for several reasons: faint signals, instrumental limitations in sensitivity and resolution, and the presence of actively feeding black holes, also known as active galactic nuclei (AGNs)—such as quasars—whose signals may dominate those from hot gas. In this new study, a team of researchers led by Andrea Travascio of the University of Milano-Bicocca investigated a protocluster centered on a bright quasar (nicknamed ID1) at a redshift z = 3.25, when the universe was less than 2 billion years old. The team published a related paper in 2025 that reported an unusually crowded region with six actively feeding black holes.
The number of black holes is far greater than expected in a typical patch of the universe, confirming that this is indeed a protocluster. This second paper examines this young galaxy cluster in depth using 634,000 seconds of Chandra X-ray observations. Because this protocluster is centered on an active galactic nucleus, the aim was to make sure the protocluster was genuine and not contaminated by light from the quasar itself.
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