9 Jun 2025

Astronomers have discovered the largest known cloud of energetic particles surrounding a galaxy cluster— spanning nearly 20 million light-years. The finding challenges long-standing theories about how particles stay energized over time. Instead of being powered by nearby galaxies, this vast region seems to be energized by giant shockwaves and turbulence moving through the hot gas between galaxies.

The results of the new study, led by scientists at theCenter for Astrophysics | Harvard & Smithsonian (CfA), were presented today in a press conference at the 246th meeting of the American Astronomical Society (AAS).

Located five billion light-years from Earth, PLCK G287.0+32.9 is a massive galaxy cluster that has piqued the interest of astronomers since it was first detected in 2011. Earlier studies spotted two bright relics— giant shockwaves that lit up the cluster's edges. But they missed the vast, faint radio emission that fills the space between them. New radio images reveal that the entire cluster is wrapped in a faint radio glow, nearly 20 times the diameter of the Milky Way, suggesting that something much larger and more powerful is at work.

"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.

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.

"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.

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This new composite image made with X-rays from NASA’s Chandra X-ray Observatory (blue and purple), radio data from the MeerKAT radio telescope (orange and yellow), and an optical image from PanSTARRS (red, green, and blue) shows PLCK G287.0+32.9. This massive galaxy cluster, located about 5 billion light-years from Earth, was first detected by astronomers in 2011.