Astronomers sifting through signals from the primordial universe have encountered something unexpected: a nascent galaxy cluster already igniting at temperatures that existing models assert it should not achieve for several billion more years.
The entity, designated as SPT2349-56, resides 12 billion years in the past, having formed merely 1.4 billion years post-Big Bang. At that cosmic epoch, galaxy clusters are predicted to be cool, chaotic collections still coming together. This one opted to leap forward. The gas interlacing its galaxies is at least five times hotter than theoretical expectations, with total thermal energy approximately ten times what gravity alone could provide.
Dazhi Zhou, a doctoral candidate at the University of British Columbia who spearheaded the study published in Nature, confesses he was initially doubtful about the measurements.
“In fact, initially I was skeptical of the signal as it seemed too robust to be genuine,” Zhou clarifies. “However, after months of validation, we’ve verified this gas is at least five times hotter than predicted, and even more intense and energetic than what we observe in many contemporary clusters.”
The team assessed the heat utilizing the Atacama Large Millimeter/submillimeter Array, detecting a phenomenon known as the Sunyaev-Zeldovich effect. Simply put, the cluster’s electrons possess such high energy that they provide a measurable energy boost to the passing cosmic microwave background radiation. The signal was unmistakable, unveiling an atmosphere that peers decades into its own future.
Dense, Turbulent, and Rapidly Forming Stars
SPT2349-56 is not merely hot; it’s extraordinarily packed. Over 30 galaxies are crammed into a core spanning nearly 500,000 light years, about the size of the Milky Way’s halo. Collectively, they’re generating stars at a pace 5,000 times quicker than our galaxy can manage. Among this turmoil are at least three supermassive black holes actively consuming surrounding matter.
In mature clusters today, most ordinary matter doesn’t reside within stars. It drifts between galaxies as diffuse plasma known as the intracluster medium, usually heated to tens of millions of degrees by the gradual pressure of gravity as the cluster stabilizes. Young clusters should still be in the process of consolidating, their gas relatively cool and sparse. SPT2349-56 appears to have bypassed that entire phase.
The black holes seem to be crucial. As they consume matter, supermassive black holes can emit jets and winds that inject vast amounts of energy into their vicinity. In the dense, high-pressure conditions of the early universe, this energy had no way of escaping. It remained contained, superheating the gas and creating what could be termed a cosmic pressure cooker.
Implications for Cluster Formation
This finding throws into question a fundamental assumption embedded in cosmological simulations: that intracluster gas heating follows a consistent, gradual timeline related to the cluster’s development. If black holes can predominantly influence heating this early, the sequence becomes disordered. Clusters might undergo brief, intense instances of energy injection that current models fail to consider.
Scott Chapman, a co-author from Dalhousie University, identifies the black holes as likely instigators. The young cluster, he suggests, was already being sculpted by factors beyond mere gravity, factors that left a thermal imprint detectable even 12 billion years later.
Whether SPT2349-56 signifies a rare anomaly or a widespread yet overlooked phase is still uncertain. The researchers are persistently examining the cluster, endeavoring to comprehend how its extreme star formation and black hole activity interconnect. For now, the discovery presents a divergent vision of the nascent universe: one where some of its largest structures evolved quickly, chaotically, and far hotter than previously anticipated.
Nature: 10.1038/s41586-025-09901-3
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