Largest cosmic particle cloud ever found has stunned astronomers

A vast, ghostly “cloud” of high-energy particles – stretching nearly 20 million light-years from end to end – has been discovered enveloping the distant galaxy cluster PLCK G287.0+32.9.

The discovery reveals the largest structure of its kind ever seen and overturns long-held ideas about where cosmic particles get their power.

Instead of being fed by jets from supermassive black holes inside individual galaxies, the electrons lighting up this cloud appear to be jolted repeatedly. Colossal shockwaves and turbulent eddies rumble through the thin gas filling the space between galaxies, re-energizing them.

Biggest particle cloud ever discovered

Astronomers have watched PLCK G287.0+32.9 since 2011, drawn by earlier radio images that revealed a striking pair of “relics.” Bright arcs on opposite rims mark shockwaves from past collisions with smaller clusters.

Those relics were already record-breakers, but new observations with South Africa’s MeerKAT radio telescope have revealed something far bigger.

The low-frequency images show a faint, continuous radio haze that fills the entire cluster, located 5 billion light-years away. This haze stretches nearly 20 times wider than our Milky Way, connecting the relics and spilling beyond them.

Dr. Kamlesh Rajpurohit, the study’s lead author, is an astronomer at the Center for Astrophysics | Harvard & Smithsonian.

“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,” he said. “A cloud of energetic particles this large has never been observed in this galaxy cluster or any other.”

The previous record holder, Abell 2255, measures about 16 million light-years across; the new halo beats it by roughly 25 percent.

Faint signals from aging electrons

Radio halos form when fast-moving electrons spiral around intergalactic magnetic fields, emitting faint radio waves.

The catch is that such electrons lose energy quickly – on cosmic timescales, they should fade in a few hundred million years and fall silent long before they can drift across a region this huge. Something, therefore, must be re-energizing them along the way.

In quieter clusters, astronomers usually point to active galaxies whose central black holes pump out relativistic jets.

The jets stir the surrounding plasma and inject fresh cosmic-ray electrons. But in PLCK G287.0+32.9, the newly mapped glow extends far beyond any galaxy’s sphere of influence, leaving no obvious power source.

Clues from colossal cluster collisions

Dr. Rajpurohit’s team suspects that the same merger events that generated the edge relics also sent slower, large-scale shockwaves and turbulence washing through the cluster’s 100-million-degree gas.

Those motions could repeatedly accelerate aging electrons, giving them multiple “booster shots” that keep them radiating across the full 20-million-light-year span.

Confirming that idea will require new computer models; present theories do not predict efficient re-acceleration over such distances.

A history of violent upheaval

Clues come from NASA’s Chandra X-ray Observatory. When the researchers overlaid Chandra’s view onto the MeerKAT radio map, they saw box-shaped edges, comet-like tails, and other scars in the X-ray-bright gas. These features lined up with structures seen in the radio map.

The shapes reveal a history of violent upheaval – sub-clusters crashing into one another and supersonic gas fronts ripping through the surrounding medium.

Outbursts from a central black hole may also be adding local turmoil. Each disturbance is another candidate engine for electron re-acceleration.

Huge halo maps hidden fields

Magnetic fields thread the cosmos from planets to galaxy clusters, yet their origin and evolution remain murky.

By lighting up those fields across unprecedented scales, the PLCK G287.0+32.9 halo provides a new laboratory.

Mapping radio brightness and polarization helps scientists study the strength and shape of magnetism across the galaxy cluster. This could reveal whether the fields formed in the early universe or emerged later from astrophysical activity.

Hidden energy in clusters

If giant shocks and turbulent cascades can keep electrons lively across tens of millions of light-years, similar “re-acceleration zones” may lurk in many massive clusters still awaiting sensitive low-frequency surveys.

Next-generation arrays such as the Square Kilometer Array and upgraded Very Large Array will be able to hunt for them systematically.

Each new detection would force cosmologists to factor in previously hidden reservoirs of relativistic particles and magnetic energy. These particles and energy sources must be considered when modeling how clusters grow, merge, and heat the gas that ultimately forms new galaxies.

Significance of the giant particle cloud

PLCK G287.0+32.9 sits five billion light-years away, so the light reaching Earth today left when our solar system was still forming its first oceans.

Catching the cluster mid-commotion offers a time-machine glimpse into how violently particles and the universe’s largest building blocks evolve.

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

A preprint of the study can be found on arXiv.

Image Credit: X-ray: NASA/CXC/CfA/K. Rajpurohit et al.; Optical: PanSTARRS; Radio: SARAO/MeerKAT; Image processing: NASA/CXC/SAO/N. Wolk

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