Nonthermal Emission from Accreting and Merging Clusters of Galaxies

We compare the nonthermal emission from clusters of galaxies undergoing minor mergers (accreting'' clusters) and major mergers (merging’’ clusters). For accreting clusters, the radial distribution of the nonthermal emission in the clusters is also calculated. The relativistic electrons, which are the origin of the nonthermal radiation through inverse Compton (IC) and synchrotron mission, are assumed to be accelerated at shocks produced by accretion or mergers. We estimate the typical accretion rate and merger probability according to a hierarchical clustering model. We show that the total luminosity of IC emission from accreting and merging clusters are similar. On the other hand, the luminosity of synchrotron radio emission of the former is much smaller than that of the latter. We show that about 10% of clusters at z~0 should have hard X-ray and radio nonthermal emissions due to their last major merger, which are comparable to or dominate those due to ongoing accretion. Moreover, 20-40% of clusters should have significant EUV emission due to their last merger. We also investigate the case where the criterion of mergers is relaxed. If we extend the definition of a merger to an increase in the mass of the larger subcluster by at least 10% of its initial mass, about 20-30% of clusters at z~0 should have hard X-ray and radio nonthermal emissions due to the merger even in a low density universe. We compare the results with observations. We find that the observed EUV emission from clusters is not attributed to accretion. If the diffuse radio emission observed in clusters is synchrotron emission from electrons accelerated via accretion or merging, the magnetic fields of clusters are generally as small as ~0.1 mu G.