Just 1.4 billion years after the Big Bang, astronomers have identified a galaxy cluster “atmosphere” that appears far too hot for its agean observation that strains the usual sequence by which cosmic structures are expected to assemble and heat.
The system is SPT2349-56, a dense knot of vigorous galaxies seen at a time when large, gravitationally settled clusters were not expected to carry a mature reservoir of superheated gas. In the standard picture, the gas between cluster galaxies known as the intracluster medium warms gradually as gravity pulls matter inward over billions of years. Yet here the thermal energy arrives early, implying that the pace and power sources of cluster “birth” can differ sharply from the canonical script.
“We didn’t expect to see such a hot cluster atmosphere so early in cosmic history,” said lead author Dazhi Zhou. “In fact, at first I was skeptical about the signal as it was too strong to be real,” he added. “But after months of verification, we’ve confirmed this gas is at least five times hotter than predicted, and even hotter and more energetic than what we find in many present-day clusters.”
SPT2349-56 is compact but extreme: its core spans about 500,000 light-years, comparable to the Milky Way’s outer halo, while hosting more than 30 active galaxies. The same confined region drives a combined star-formation output estimated at over 5,000× the Milky Way’s pace, concentrating dust, gas, and radiation in a way that resembles a laboratory for rapid galaxy build-up rather than a quiet proto-cluster in formation.
That heat was not inferred from a thermometer-like spectrum of the gas. Instead, the team used ALMA and the thermal Sunyaev–Zeldovich effect, a technique that reads how energetic electrons in hot gas subtly reshape the cosmic microwave background. Because this imprint depends on the integrated pressure of electrons along the line of sight, it can reveal cluster-scale heating even when the system is too distant for conventional X-ray mapping to be straightforward. The result places SPT2349-56 in an unusual category: not merely a crowded proto-cluster, but a young structure already exhibiting a high-energy intracluster medium.
The most compelling clue to the energy source sits at the centre. “This tells us that something in the early universe, likely three recently discovered supermassive black holes in the cluster, were already pumping huge amounts of energy into the surroundings and shaping the young cluster,” said coauthor Scott Chapman, “much earlier and more strongly than we thought.”
Independent work has shown that SPT2349-56 hosts heavily obscured active galactic nuclei among its dusty star-forming galaxies, including an object consistent with a Compton-thick AGN with NH ≈ 2 × 10^24 cm−2 and intrinsic X-ray luminosity in the 10^45 erg s−1 regime. Such buried growth phases are difficult to diagnose in optical light, but they are precisely the kind of engines capable of injecting heat mechanically and radiatively into surrounding gas. In SPT2349-56, the engineering problem is astrophysical: how to channel black-hole feedback, merger-driven turbulence, and starburst activity into an intracluster medium quickly enough to reproduce a signal that appears “overbuilt” for its cosmic age.
Earlier studies of the same structure emphasized a remarkable concentration of dusty, rapidly star-forming members, consistent with a colossal merger in progress. The new temperature result raises the stakes, because it suggests that assembly and heating can be tightly coupled: the processes that pack galaxies together may also accelerate the creation of the hot gas envelope that later governs how the biggest cluster galaxies mature.
SPT2349-56 now serves less as a curiosity and more as a stress test. If a “baby” cluster can carry a mature, overheated atmosphere, the early universe may have produced some large structures through shorter, more explosive pathways than previously assumed.
