Department of Astronomy Department of Astronomy

CTAO Science

Galaxy Clusters

Particles are trapped in the magnetic fields of galaxy clusters and accumulate for the entire age of the universe. Hence, clusters keep the memory of the entire non-thermal activity of their components. CTAO will be sensitive enough to detect clusters of galaxies at high energies and to measure their CR content. What is the CR yield from galaxies and AGNs over the age of the Universe? Hierarchical structure formation generates shocks, do they accelerate particles? Are particles accelerated in cluster collisions/mergers? What about injection by large-scale radio lobes and jets?

Galaxy clusters are very large structures in the Universe grouping thousands of galaxies and CR storehouses since CRs produced in its member galaxies remain confined into them for a long time. Probing the density of CRs in clusters via their gamma-ray emission provides a calorimetric measure of the total integrated non-thermal energy output of galaxies. Accretion or merger shocks in clusters of galaxies provide an additional source of high-energy particles. The emission from galaxy clusters is predicted at levels just below the sensitivity of current instruments. Scientists at the Astronomy Department/UniGe detected the Perseus cluster up to 0.1 MeV using INTEGRAL and measured upper limits on the GeV gamma-ray emission of the cluster of galaxies. Observations by the ETHZ group with MAGIC provided upper limits in the energy range 100 GeV to TeVs that indicate an energy density of CRs of less than 5% of that of the thermal gas. CTAO will provide more information about the energy of higher-energy galactic CRs.
Jets and radio lobes generated by AGNs have a dramatic impact on their surrounding environment and on galaxy clusters. X-ray emission from regions affected by jet lobes can be the result of inverse Compton scattering of accelerated electrons, indicating that particle acceleration can have an important role. CTAO will detect many more of these regions to better understand such injection processes from lobes and jets.

CTA-Perseus.png
Shock waves in the Perseus cluster from radio and X-ray observations. They might have been created by the collision of a small cluster (including only 10’000 billions stars) with the main cluster [arXiv:1705.00011].