Galaxy Clusters
Galaxy Clusters and Cosmic Rays
Clusters of galaxies are the largest gravitationally-bound objects in
the Universe. They are storehouses of cosmic rays, since all the cosmic
rays produced in the galaxies of the cluster since the beginning of the
Universe will be confined to the cluster. Probing the density of cosmic
rays in clusters via their gamma-ray emission thus provides a
measurement of the total integrated non-thermal energy output of
galaxies.
The observation of mainly radio (and in some cases X-ray) emission
proves the existence of non-thermal phenomena in galaxy clusters, but
gamma-rays have not yet been detected. Gamma-ray emission from galaxy
clusters is predicted at levels just below the sensitivity of current
instruments, so should be easily observed with CTA.
Dark Matter
A possible additional source of non-thermal radiation from galaxy
clusters is the annihilation of dark matter. The increased sensitivity
of CTA will help to establish a dark matter signal if it exists,
possibly making it the first instrument to map dark matter on the scale
of galaxy clusters.
Further Reading
The MAGIC Collaboration, MAGIC Gamma-ray Telescope Observation of the
Perseus Cluster of Galaxies: Implications for Cosmic Rays, Dark Matter,
and NGC 1275, The Astrophysical Journal (2010), 710, 1, p. 634-647; http://arxiv.org/abs/0909.3267