|Chandra X-ray image of the Perseus galaxy cluster, revealing the cloud of hot gas in which the cluster is immersed. (Image: NASA/CXC/IoA/Fabian et al., http://xxx.soton.ac.uk/abs/astro-ph/0510476)|
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.
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.
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