Discovery of a ‘mini halo’ offers new clues about the formation of the early universe

Astronomers have discovered a massive cloud of high-energy particles surrounding one of the most distant galaxy clusters ever observed, offering fresh insights into how galaxy clusters evolved in the early universe.

The study, co-led by Julie Hlavacek-Larrondo from the Université de Montréal, reveals that galaxy clusters—some of the universe’s largest structures—have been enveloped in energetic particles for much of their history.

Located so far away that its light takes 10 billion years to reach Earth, this find marks the most distant "radio mini-halo" ever detected, surpassing the previous record by a wide margin.

These mini-halos are made up of fast-moving charged particles spread through the space between galaxies within a cluster, emitting radio waves that are detectable from Earth.

Soon to be published in Astrophysical Journal Letters, the research—currently available as a preprint—demonstrates that dynamic energy processes were already influencing galaxy clusters in the early universe.

According to the researchers, understanding how energy flows within these clusters is key to unraveling the broader history of cosmic evolution.

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Two likely explanations

There are two main theories regarding how the mini-halo may have formed.

One possibility involves supermassive black holes located at the centers of galaxies within the cluster, which can release jets of high-energy particles into space.

However, astronomers are still investigating how these particles could travel such vast distances from the black hole and still retain a significant amount of their energy to form such an enormous particle cloud.

The second possible explanation involves collisions between cosmic particles.

In this scenario, charged particles within the hot plasma of the galaxy cluster collide at speeds close to that of light, breaking apart and producing the highly energetic particles that make up the mini-halo.

According to the researchers, this discovery provides a rare glimpse into the state of galaxy clusters shortly after their formation. It reveals that these clusters have been permeated by high-energy particles for billions of years longer than previously believed, and it opens new opportunities to explore the origins of these particles.

“It’s remarkable to detect such a powerful radio signal from this distance,” said Roland Timmerman, an astronomer at Durham University (U.K.) who co-led the study with Hlavacek-Larrondo. “It shows that energetic particles and the mechanisms that generate them have been influencing galaxy clusters for almost the entire history of the universe.”