Violent galaxies ‘spew out’ energetic cosmic rays
09 Nov 07
The stormy centres of galaxies powered by supermassive black holes are most likely to be the source of the highest-energy cosmic rays that hit Earth, according to research reported in this week's Science.
The Auger Collaboration, a team of 370 scientists and engineers from 17 countries, made new observations using the largest cosmic-ray observatory in the world: the Pierre Auger Observatory in Argentina.
Professor Subir Sarkar of the Physics Department at Oxford University, a member of the Auger Collaboration, said: ‘The Auger data indicates that the sources of ultrahigh energy cosmic rays are associated with nearby 'active galaxies' which harbour supermassive black holes that are gobbling up stellar matter and ejecting huge jets of plasma. Our own galaxy too has such a black hole at its centre but, fortunately for us, it is not 'feeding' at the moment!’ Scientists call the centres of such violently active galaxies Active Galactic Nuclei (AGNs).
Cosmic rays are protons and nuclei that whizz across the universe at close to the speed of light. Only the highest-energy particles are undeflected by cosmic magnetic fields and can be linked to their sources with any precision. There have been previous attempts to associate AGNs with high-energy cosmic rays but this new study is the first to demonstrate that such a link is statistically robust and not down to chance.
‘One has to be careful to conduct a 'prescription' search and to not let one's prejudices influence the choice of the objects or cosmic ray events being considered. Only then can we compute reliably the probability of the observed correlations occurring by chance: if this is sufficiently low then there is likely to be a physical association between the cosmic rays and the AGN. After months of careful work we are reasonably confident that this connection is real,’ said Professor Sarkar, a theoretical physicist who was invited to join the international collaboration because of his long-standing interest in cosmic rays. ‘The credit for this breakthrough goes of course to the experimentalists who constructed and operate this superb experiment, including my colleagues at Leeds University.’
However, determining the origin of these high-energy cosmic rays opens up a host of intriguing new questions. ‘A major challenge is to understand how such objects can boost particles to energies 100 million times higher than what we can achieve in the most powerful accelerators on Earth – for example the Large Hadron Collider being constructed at CERN,’ said Professor Sarkar. ‘Also, as is obvious from pictures of active galaxies such as Centaurus A, it is difficult to see into the maelstrom around the actual black hole, which is shrouded by in-falling matter. Soon, however, another experiment I am involved with – a cubic-kilometre detector called IceCube, which is being assembled 3 km under the South Pole – might be able to detect neutrinos from AGNs. This will let us peer right into the heart of the regions where the cosmic rays are probably accelerated. Together, such observations will open up a new astronomy and reveal the physics of the most energetic phenomena in the universe.’
A report of the research, entitled ‘Correlation of the Highest-Energy Cosmic Rays with Nearby Extragalactic Objects’ is published in the journal Science on 9 November 2007.