Nonthermal Emission from Clusters of Galaxies

Some clusters of galaxies have diffuse nonthermal synchrotron radio halos, which extend in a megaparsec scale. This indicates that there exists a relativistic electron population with energy of a few GeV in intracluster space in addition to the thermal intracluster medium (ICM). Furthermore, it is well known that such clusters of galaxies show evidence of recent major mergers in X-ray observations. In such clusters of galaxies with radio halos, nonthermal X-ray radiation due to inverse Compton (IC) scattering of cosmic microwave background (CMB) photons by the same electron population is expected. Indeed, nonthermal X-ray radiation was recently detected in a few rich clusters and several galaxy groups, although their origins are still controversial.

The origin of such relativistic electrons is still unclear. From N-body + hydrodynamical simulations, however, it is expected that shock waves and strong bulk-flow motion exist in ICM during a merger. This suggests that relativistic electrons are produced around the shock fronts through first-order Fermi acceleration and that propagation of the shock waves and bulk flow of ICM are responsible for extension of radio halos.

We introduce some results of evolution of nonthermal emission from clusters of galaxies in a merger shock acceleration model by one of our group (MT). To get more detailed information, please see Takizawa & Naito 2000 (ApJ vol.535, p.586).