Invisible Storm Lights Up Galaxy Cluster With Record-Breaking Radio Glow

Astronomers have stumbled upon the largest-ever cloud of energetic particles surrounding a galaxy cluster, stretching nearly 20 million light-years, rewriting what we thought we knew about how particles stay energized.

Rather than being powered by galaxies, this colossal structure appears to be driven by turbulence and massive shockwaves moving through intergalactic gas. A surprise discovery of a gigantic radio halo, previously invisible at higher frequencies, challenges current models and hints at unknown forces re-accelerating cosmic rays.

Enormous Particle Cloud Defies Expectations

Astronomers have uncovered something extraordinary: the biggest known cloud of energetic particles ever found around a galaxy cluster. This massive cloud stretches across nearly 20 million light-years — about 200 times the size of our Milky Way.

What makes this discovery even more exciting is how it challenges what scientists thought they knew. Until now, experts believed that such clouds were energized by nearby galaxies. But this one appears to be powered by something else entirely — gigantic shockwaves and turbulent flows of hot gas moving between galaxies.

The discovery was presented by researchers from the Center for Astrophysics | Harvard & Smithsonian at a recent meeting of the American Astronomical Society. It opens up new questions about how energy moves across the largest structures in the universe.

Radio Glow Signals Hidden Forces

Located about five billion light-years from Earth, the galaxy cluster known as PLCK G287.0+32.9 has intrigued scientists since it was first spotted in 2011. Earlier observations had revealed two bright features on its edges, thought to be shockwaves lighting up the outer rim. But what researchers missed was even more surprising — a faint but enormous radio glow stretching between and beyond those edges.

Now, with sharper radio images, astronomers have found that the entire cluster is glowing. This glow is a sign of highly energetic particles and magnetic fields spread throughout the region, hinting at powerful processes shaping the space between galaxies.

“We expected a bright pair of relics at the cluster’s edges, which would have matched prior observations, but instead we found the whole cluster glowing in radio light,” said lead author Dr. Kamlesh Rajpurohit, a Smithsonian astronomer at the CfA. “A cloud of energetic particles this large has never been observed in this galaxy cluster or any other.” The prior record holder, Abell 2255, spans roughly 16.3 million light-years.

Record-Breaking Radio Halo Found

Deep in the cluster’s central region, the team detected a radio halo approximately 11.4 million light-years across, the first of its size seen at 2.4 GHz, a radio frequency where halos this large are usually not visible. The findings raise questions for the team because they provide strong evidence for the presence of cosmic ray electrons and magnetic fields stretched out to the periphery of clusters. However, it remains unclear how these electrons accelerated over such large distances.

“Very extended radio halos are mostly only visible at lower frequencies because the electrons that produce them have lost energy — they’re old and have cooled over time,” said Rajpurohit. “With the discovery of this enormous size halo we are now seeing radio emission extending all the way between the giant shocks and beyond, filling the entire cluster. That suggests something is actively accelerating, or re-accelerating the electrons, but none of the usual suspects apply. We think that giant shockwaves or turbulence could be responsible, but we need more theoretical models to find a definitive answer.” The discovery provides researchers a new way to study cosmic magnetic fields— one of the major unanswered questions in astrophysics— that could help scientists understand how magnetic fields shape the Universe on the largest scales.

New Frontiers in Cosmic Magnetism

“We’re starting to see the Universe in ways we never could before,” said Rajpurohit. “And that means rethinking how energy and matter move through its largest structures.”

Observations with NASA’s Chandra X-ray Observatory, operated by the Smithsonian Astrophysical Observatory, reveal a box-shaped structure, a comet-like tail, and several other distinct features in the cluster’s hot gas, suggesting that the cluster is highly disturbed. Some of these X-ray features coincide with radio-detected structures, suggesting giant shocks and turbulence driven by mergers accelerating or re-accelerating electrons. In the center of the cluster, some of these features may be caused by a merger of two smaller galaxy clusters, or from outbursts produced by a supermassive black hole, or both.

Reference: “Radial Profiles of Radio Halos in Massive Galaxy Clusters: Diffuse Giants Over 2 Mpc” by K. Rajpurohit, A. Botteon, E. O’Sullivan, W. Forman, M. Balboni, L. Bruno, R. J. van Weeren, M. Hoeft, G. Brunetti, C. Jones, A. S. Rajpurohit, S. P. Sikhosana, Submitted, The Astrophysical Journal.
arXiv:2505.05415

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