When a team of astronomers working with the pan-European Low Frequency Array (LOFAR) telescope sat down to inspect new data, they expected to see the usual point-like radio beacons of distant quasars and star-forming galaxy clusters.

Instead, they stumbled upon something altogether rarer: a sprawling, ghostly glow surrounding the galaxy cluster SpARCS1049.

Because the cluster lies so far away, the light reaching Earth today was emitted 10 billion years ago – when the universe was about 3.8 billion years old. Since then, fully two-thirds of cosmic history has passed.

Distant cluster’s radio halo

LOFAR excels at detecting long-wavelength radio waves, the hallmark of clouds of ultra-relativistic electrons spiraling around magnetic-field lines throughout the universe.

In nearby clusters, astronomers occasionally see small, roundish patches of such emission called mini-halos. These hover around the brightest central galaxy and reveal that the otherwise invisible space between galaxies teems with fast particles.

Spotting an analog at twice the previous record distance, however, came as a shock.

“It’s as if we’ve discovered a vast cosmic ocean, where entire galaxy clusters are constantly immersed in high-energy particles,” said co-lead author Julie Hlavacek-Larrondo, an astrophysicist at the University of Montreal.

Her collaborator, Roland Timmerman from Durham University shared the excitement. “It’s astonishing to find such a strong radio signal at this distance.”

“It means these energetic particles and the processes creating them have been shaping galaxy clusters for nearly the entire history of the universe,” said Timmerman.

A million-light-year fog

Follow-up analysis showed that the diffuse emission spans more than a million light-years – roughly ten Milky Ways laid end to end.

That size rules out any explanation based on individual galaxies. Instead, the radiation must arise in the thin, hot plasma that pervades the whole cluster.

Astronomers call such features mini-halos because they are smaller cousins of giant radio halos seen in some of the universe’s more turbulent clusters closer to Earth. But even “mini” is relative: compared with any single galaxy, the newfound halo is huge.

The signal is so faint that astronomers hadn’t detected one this far away until now. Engineers spread LOFAR’s 100,000 simple antennae across eight European countries and linked them using high-speed optical fibers. Together, they form one of the most sensitive low-frequency observatories ever built.

Only with such a powerful array could scientists tease the subtle glow out of a background filled with brighter, compact sources.

Black holes as power sources

What pumps so much energy into the cluster’s intergalactic medium? Two main ideas dominate the discussion.

First, the central galaxies of most clusters host supermassive black holes that intermittently swallow gas and spew out jets of plasma traveling at near-light speeds. Over millions of years, these outflows can inject an enormous amount of energy into their surroundings.

If the jets in SpARCS1049 fired often enough, they might have filled the cluster with fast electrons, which then diffused outward to produce the observed haze.

The challenge for theorists is to explain how those particles retain enough energy while wandering hundreds of thousands of light-years from their birthplace.

Collisions in hot cluster gas

The second possibility invokes cosmic ray hadrons – high-speed protons and heavier nuclei already sloshing around in the searing 100-million-degree gas.

When two such particles collide, the impact can create lighter particles, including electrons, that inherit a share of the kinetic energy. These electrons immediately begin to spiral in the cluster’s magnetic fields, giving off radio light.

This mechanism would operate wherever the gas is dense enough, meaning mini-halos could persist for billions of years without fresh input from black-hole jets.

Determining which process dominates is a hot topic because the answer affects how astronomers model everything from black-hole feedback to the way clusters shine in X-rays.

Radio clues from long ago

Before this discovery, every confirmed mini-halo sat in the comparatively nearby universe, less than five billion light-years away.

Doubling that record distance shows that clusters became “wired” with magnetic fields and relativistic particles remarkably soon after they formed. This newfound halo therefore offers a rare window onto the physical conditions inside young clusters.

Because the LOFAR data include multiple sub-bands, the team could measure how the halo’s brightness changes with frequency. That “radio spectrum” encodes information about the ages and energies of the electrons. It also reveals when and how they were accelerated.

By comparing the spectrum with those of younger halos, researchers can infer whether black-hole outbursts were more violent in the past or whether proton-proton collisions supplied most of the power.

Radio clues from the early universe

Extracting the halo from the LOFAR data required painstaking calibration to remove artifacts and foreground sources. Even so, the scientists suspect that many weaker halos lie just below the telescope’s current sensitivity.

The forthcoming Square Kilometre Array (SKA), slated to become the world’s largest radio observatory, will push that threshold far lower.

SKA will map distant halo shapes and probe their magnetic fields with unmatched resolution and collecting power. It may even capture them flickering as black holes cycle through active and dormant phases.

“We are just scratching the surface of how energetic the early universe really was,” Hlavacek-Larrondo said. “This discovery gives us a new window into how galaxy clusters grow and evolve, driven by both black holes and high-energy particle physics.”

For now, the mini-halo around SpARCS1049 stands as a spectacular signpost. It’s evidence that ten billion years ago, vast clouds of relativistic particles already crackled through the cosmic web, illuminating the universe’s darkness with a quiet radio glow.

A preprint of the study can be found on arXiv.

Image Credit: Chandra X-ray Center (X-ray: NASA/CXC/SAO; Optical: NASA/ESA/STScI; Radio: ASTRON/LOFAR; Image Processing: NASA/CXC/SAO/N. Wolk)

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–