Tiny Particles from Remote Galaxies


Aachen researchers involved in solving decades-old riddle about cosmic radiation

  Structure with an antenne in the desert Copyright: © Pierre Auger Collaboration

Ever since the existence of cosmic rays with individual energies of several Joules was established in the 1960s, speculation has raged as to whether cosmic particles of mean energy of 2 Joules are created in our Milky Way or in distant extragalactic objects. Germany is represented in the collaboration by RWTH Aachen together with KIT, Universität Hamburg, and the Universities of Siegen and Wuppertal. The Pierre Auger Collaboration has reported evidence about the cosmic rays in a paper published in Science.

Distant Galaxies as a Source

Cosmic rays are the nuclei of of known elements. Above 2 Joules the rate of their arrival at the top of the atmosphere is only about 1 per sq km per year. Such rare particles are detectable because they create showers of electrons, photons and muons through successive interactions with the nuclei in the atmosphere. These showers spread out, sweeping through the atmosphere at the speed of light in a disc-like structure, similar to a dinner-plate, several kilometres in diameter. They contain over ten billion particles and, at the Auger Observatory, are detected through the Cherenkov light they produce.


The detectors are spread over 3000 square kilometers of Western Argentina. The times of arrival of the particles at the detectors, measured with GPS receivers, are used to find the arrival directions of events. By studying the distribution of the arrival directions of more than 30,000 cosmic particles the Auger Collaboration has discovered an anisotropy in a direction where the distribution of galaxies is relatively high.

Professor Thomas Hebbeker from the Institute of Physics III at RWTH Aachen has conducted research for many years on how an increasing number of measured cosmic particles demonstrated a growing number of particles originating from a certain cardinal direction and, comparably, a fewer number originating from other directions. "The great statistic significance of this signal is convincing and demonstrates the central importance of this result for our understanding of high-energy cosmic radiation."

Search for Actual Source

Although this discovery clearly indicates an extragalactic origin for the particles, the actual sources have yet to be pinned down. The direction of the excess points to a broad area of sky rather than to specific sources as even particles as energetic as these are deflected by a few degrees in the magnetic field of our galaxy. The direction, however, cannot be associated with putative sources in the plane or centre of our Galaxy for any realistic configuration of the Galactic magnetic field. Even higher energy particles are to be used in the future to find the sources themselves. Professor Martin Erdmann from the Institute of Physics III at RWTH investigates deviations of cosmic particles in magnetic fields. "The measured asymmetry in the arrival directions of the cosmic radiation is hardly changed by the galactic magnetic field, thus increasing the chances of identifying distant galaxies as the sources of the radiation," says Erdmann.

With funding from the Federal Ministry of Education and Research, the institutes are already working together with RWTH professor Thomas Bretz to expand the Pierre Auger Observatory. The scientists expect exciting research even in the coming years with the large experiment: "With our more than 100 additional detectors, we are contributing to once again significantly improve the Observatory's sensitivity to cosmic atomic nuclei."

The articles can be found in Science.

Source: Press and Public Relations