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.
Nascent stage
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.
634,000 seconds of X-ray data
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. The team simulated exactly what the quasar’s light alone should look like, then compared that simulation with the real data. Beyond a certain distance from the quasar, there was a clear excess of X-ray light that the “quasar alone” model couldn’t explain. The full X-ray emission extended to at least 98,000 light-years from the quasar.
Confirming and ruling out
Next, they divided the area around the quasar into eight pie-shaped “sectors” and checked whether the X-ray light was brighter in some directions than in others. It was statistically even in all directions. This is consistent with genuine, gravitationally settled gas. The researchers also tested and ruled out an alternative explanation involving a jet from the quasar, finding no evidence to support it.
The team compared the shape of the newly detected X-ray hot gas with the previously known cooler gas cloud around the quasar, finding partial overlap but not a perfect match. This suggested the two were related but physically distinct gas components.
They fit the extra X-ray signal not explained by the quasar-alone model with a model representing the hot, thin, ionized gas expected in galaxy clusters. The fit matched, revealing a gas temperature of around 21 million Kelvin.
“This likely represents the first evidence of thermal emission from proto-ICM (or hot CGM) at z > 3,” the team writes in the paper.
It consists of hot gas with a mass of about 2.6 trillion times the sun’s mass and a total mass of 30 trillion suns, including the dark matter halo. They also calculated that this hot gas alone accounts for a large fraction—around 56%—of all the “normal” matter (not dark matter) that should theoretically exist in a halo of this size. That fraction matches what is seen in nearby, mature galaxy clusters.
This is a significant result because it shows this early-universe structure has already reached a share of hot gas comparable to fully mature clusters seen nearby today, even though the universe was less than 2 billion years old at the time.
Researchers caution that the physical quantities reported here should be regarded as rough estimates with large uncertainties, given the limited number of detected thermal photons. More sensitive future observations (with next-generation telescopes) are needed to fully confirm and refine these findings.
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Publication details
A. Travascio et al, X-ray view of a massive node of the cosmic web at z ∼ 3, Astronomy & Astrophysics (2026). DOI: 10.1051/0004-6361/202557020
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Astronomers catch hot birth of galaxy cluster more than 11 billion light-years away (2026, August 19)
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