The biggest collisions in the cosmos seem to be acting as giant particle accelerators, generating some of the mysterious “ultra-high energy” cosmic rays that slam into Earth.
Collisions between clusters of galaxies should produce shockwaves whose magnetic fields can boost loose protons and electrons up to very high energies, but until now there was no clear evidence of it happening.
Now, researchers led by Reinout van Weeren of Leiden University in the Netherlands have used radio telescopes in the Netherlands, India and the US to image a huge bright arc at the interface of two colliding galaxy clusters, known collectively as CIZA J2242.8+5301. The energy spectrum of the radio waves changes across the arc in a way that fits models of shock acceleration.
The shockwave stretches for more than 6 million light years, and van Weeren calculates that it could easily accelerate protons to energies of up to 1019 eV. That is millions of times as high as the particles in any atom smasher and in the energy range of mysterious charged particles called ultra-high energy cosmic rays that occasionally hit Earth’s atmosphere.
A few of the protons hitting our atmosphere may even have come all the way from CIZA J2242.8+5301, 3 billion light years away.
At even higher energies, protons can only travel about 160 million light years before being destroyed in collisions with low-energy photons that permeate space, radiation known as the cosmic microwave background.
No known cluster shock is that close to us, so the most extreme cosmic rays – which can reach energies of 1021 eV – must have another source – probably active galaxies whose black holes are violently devouring matter, or gamma-ray bursts, explosions thought to herald the birth of black holes.
“The case for shock acceleration in this object seems compelling,” says Charles Dermer of the Naval Research Laboratory in Maryland, who was not involved in the study. But he believes that active galaxies are a better bet for producing the ultra-high energy cosmic rays that zap Earth, as they can account for a much wider energy range of these particles.