Violent black holes linked to high-energy cosmic rays

Physics

University of Adelaide scientists are among a leading international research group that has made an important discovery about the highest-energy cosmic rays that hit the Earth - and the discovery leads back to supermassive black holes.

The scientists, in the University's School of Chemistry & Physics, are among researchers from 17 countries participating in the Pierre Auger Collaboration, using the largest cosmic ray observatory in the world, the Pierre Auger Observatory in Argentina.

The team has discovered that active galactic nuclei (AGN) - thought to be powered by supermassive black holes that devour large amounts of matter - are the most likely candidate for the source of the highest-energy cosmic rays that hit Earth.

Using the Pierre Auger Observatory, the team found that the sources of the highest-energy particles are not distributed uniformly across the sky. Instead, the Auger results link the origins of these mysterious particles to the locations of nearby galaxies that have active nuclei in their centres.

Active Galactic Nuclei (AGN) have long been considered sites where high-energy particle production might take place. They swallow gas, dust and other matter from their host galaxies and spew out particles and energy.

While most galaxies have black holes at their centre, only a fraction of all galaxies have an AGN. The exact mechanism of how AGNs can accelerate particles to energies 100 million times higher than the most powerful particle accelerator on Earth is still a mystery.

"We have taken a big step forward in solving the mystery of the nature and origin of the highest-energy cosmic rays, first revealed by French physicist Pierre Auger in 1938," said Nobel Prize winner James Cronin, of the University of Chicago, who conceived the Pierre Auger Observatory together with Alan Watson of the University of Leeds.

"In the next few years our data will permit us to identify the exact sources of these cosmic rays and how they accelerate these particles."

Cosmic rays are comprised of protons and atomic nuclei, which travel across the universe at close to the speed of light. When these particles smash into the upper atmosphere of our planet, they create a cascade of secondary particles called an "air shower" that can spread across 40 or more square kilometres as they reach the Earth's surface.

"These enormously energetic particles are very rare, but they pack a real punch," said Associate Professor Bruce Dawson from the University of Adelaide.

"This discovery is a major step towards understanding some of the most extreme processes in the universe.

"The most exciting thing is that the Observatory is only just beginning, so there is huge scope for further discoveries."

The Adelaide research group, led by associate Professor Bruce Dawson and Professor Roger Clay in the University's School of Chemistry & Physics, was a foundation member of the Auger collaboration.

The first design workshop for the Auger Observatory was held at the University of Adelaide in January 1993. Since then, Adelaide scientists have made many key contributions to the experiment during its design and construction stages. With data now being collected, they have assumed leadership roles in data analysis and interpretation for the international group.

The team's latest results were published in a recent issue of the international journal Science.