Research Highlight
February 28, 2008
A Century-Old Mystery Unveiled: The Most Energetic Particles to Reach Earth Come From Outside the Milky Way
by The Auger Group at KICP
Cosmic rays are energetic subatomic particles from space that can have energies 10 million times greater than those created by man-made particle accelerators. Their origin has been a mystery for almost a century, ever since their discovery in 1912. The Pierre Auger Observatory in Argentina, an international effort involving 17 countries and hundreds of scientists, including an active group at KICP led by Auger Spokesperson Emeritus James Cronin and faculty members Angela Olinto and Paolo Privitera, has recently measured the arrival directions of the highest energy cosmic rays. The pattern in the sky created by these particles reveals that the most energetic particles do not come from every direction in space. Instead, their arrival directions trace the sky distribution of the most energetic galaxies in our neighborhood of the Universe, the suburbs around our Milky Way Galaxy - galaxies that are suspected of harboring active supermassive black holes at their centers.

Simulation of Auger detection of ultrahigh energy proton, created by the Space Visualization Laboratory at the Adler Planetarium and KICP scientists.
The origin of cosmic rays has been a mystery for decades due to the difficulty of correlating their arrival direction with their source direction - cosmic rays are charged particles (such as protons and atomic nuclei), and so are deflected by magnetic fields as they travel to and through the Milky Way. The magnitude of this deflection depends on the energy of the cosmic tray - at lower energies, the particles are wildly diverted from their original path by encounters with magnetic fields; those with higher energies are deflected by a smaller amount. For the very highest energy particles the magnetic deflection is weak enough that a possible correlation between source position and arrival direction can be determined.

The arrival directions of the 27 highest energy cosmic rays detected by Auger projected onto the celestial sphere (black circles of radius 3.1o). The positions of 472 AGN within 75 Megaparsecs are shown as red stars.
The detection of the highest energy cosmic rays also provides another clue to their origin. Interactions between these ultra-high energy particles and the radiation left over from the Big Bang tend to decrease the energy of the cosmic rays - and these interactions become increasingly more likely the farther the cosmic rays travel through space. This limits the distance ultra-high energy cosmic rays can have traveled before arriving at Earth, implying that they must have been produced relatively close-by (cosmologically speaking) - in our local neighborhood of the Universe.

Pierre Auger Collaboration members.
The highest energy particles are very rare: over an area of one square kilometer, only one particle per century reaches the Earth. To meet this challenge, the Pierre Auger Observatory will cover an area of 3000 km2 once its construction is completed in early 2008. Since it began operation in January 2004, the Auger Observatory has collected 27 particles with energies above 5.7 x 1019 eV. These particles are the first to show a positive correlation between nearby active galaxies (AGNs) and the highest energy cosmic rays. Their arrival directions correlate well with the distribution of active galaxies located less than 180 million light years (or 1.7 x 1021 km) from Earth. These findings are summarized in recent publications by the Pierre Auger Collaboration in Science and Astroparticle Physics (in press).

Auger surface detector with the Andes in the background.
The Auger group at KICP and the University of Chicago, including faculty Jim Cronin, Angela Olinto and Paolo Privitera, visiting faculty Joao de Mello, postdoctoral fellows Maximo Ave, Lorenzo Cazon, and Vasiliki Pavlidou, graduate students Florin Ionita, Fabian Schmidt and Tonia Venters, and visiting graduate student Beatriz Siffert, have been working actively on the anisotropy properties of the arrival directions of these highest energy particles in the Universe. Their contributions include assessing the significance of the anisotropy signal with different techniques; evaluating which of the different candidate populations appears to be the most likely sources of the highest-energy cosmic rays; developing tools that will help identify individual point sources once more data have been collected; studying the physical properties of nearby likely ultra-high--energy cosmic-ray sources; and studying the high energy interaction of these particles in the atmosphere.