The Pierre Auger Collaboration has discovered that active galactic nuclei are the most likely candidate for the source of the ultra-high-energy (UHE) cosmic rays arriving on Earth. Using the world's largest cosmic-ray observatory, the Pierre Auger Observatory (PAO) in Argentina, the team of 370 scientists from 17 countries has found that the sources of the highest-energy particles are not distributed uniformly across the sky. Instead, the results link the origins of these mysterious particles to the locations of nearby galaxies that have active nuclei at their centres (Pierre Auger Collaboration 2007).

Low-energy charged cosmic rays (by far the majority) lose their initial direction when travelling through galactic or intergalactic magnetic fields, and therefore cannot reveal their point of origin when detected on Earth. UHE particles, by contrast, with energies of more than 40 EeV (4 × 1019 eV) are only slightly deflected, so they come almost straight from their sources. These are the particles that the Auger Observatory was built to detect.

When UHE cosmic rays hit nuclei in the upper atmosphere, they create cascades of secondary particles that can spread across an area of around 30 km2 as they arrive at the Earth's surface. The PAO records these extensive air showers using an array of 1600 particle detectors placed 1.5 km apart in a grid spread across 3000 km2. A group of 24 specially designed telescopes record the emission of fluorescence light from excitation of the atmospheric nitrogen by the air shower, and water tanks record shower particles arriving at the Earth's surface by detecting Cherenkov radiation. The combination of particle detectors and fluorescence telescopes provides an exceptionally powerful instrument for determining the energy and direction of the primary UHE cosmic ray (CERN Courier July/August 2006 p12).

While the observatory has recorded almost a million cosmic-ray showers, the Auger team can link only the rare, highest-energy cosmic rays to their sources with sufficient precision. The observatory has so far recorded 81 cosmic rays with energy of more than 40 EeV – the largest number of cosmic rays at these energies ever recorded. At these ultra-high energies, there is only a degree or so uncertainity in the direction from which the cosmic ray arrived, allowing the team to determine the location of the particle's source.

The Auger Collaboration discovered that the 27 highest-energy events, with energy of more than 57 EeV, do not come from all directions equally. Comparing the clustering of these events with the known locations of 381 active galactic nuclei (AGNs), the collaboration found that most of these events correlated well with the locations of AGNs in some nearby galaxies, such as Centaurus A. Astrophysicists believe that AGNs are powered by supermassive black holes that are devouring large amounts of matter. They have long been considered sites where high-energy particle production might take place, but the exact mechanism of how AGNs can accelerate particles to such high energies is still a mystery.

These UHE events are rare and, even with its large size, the PAO can record only about 30 of them each year. The collaboration is already developing plans for the construction of a second, larger installation in Colorado. This will extend coverage to the entire sky while substantially increasing the number of high-energy events recorded; there are, it turns out, even more nearby AGNs in the northern sky than in the southern sky visible from Argentina.