Evrim Yazgin
Cosmos science journalist
Astronomers have found a gigantic cosmic structure which defies understanding.
The object is a cloud of energetic particles – the largest ever found. The cloud is so large that it surrounds a massive galaxy cluster, spanning 20 million light-years. For comparison, our home Milky Way galaxy has a diameter about 200 times smaller at roughly 100,000 light-years.
Long standing theories about how particles stay energised have been challenged by the new discovery.
The results were presented at the 246th meeting of the American Astronomical Society and have been submitted for publication in the Astrophysical Journal. A preprint of the paper is available on the arXiv server.
The giant cloud surrounds the galaxy cluster PLCK G287.0+32.9 which was first detected in 2011. The cluster is 5 billion light-years from Earth.
New radio images of the cluster revealed a faint glow in the space between the 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,” says lead author Kamlesh Rajpurohit from the Harvard-Smithsonian’s Center for Astrophysics. “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 centre of the cluster the team detected a radio halo about 11.4 million light-years across at the frequency 2.4 GHz. Halos this large are normally not seen at this frequency. This suggests the presence of cosmic ray electrons and magnetic fields stretched out to the periphery of clusters. How electrons could be accelerated over such large distances remains a mystery.
“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,” Rajpurohit explains. “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 findings provide more insight into cosmic magnetic fields and how they help shape the largest structures in the universe.
“We’re starting to see the universe in ways we never could before,” says Rajpurohit. “And that means rethinking how energy and matter move through its largest structures.”
