Nonthermal Emission from Accreting and Merging Clusters of Galaxies

Abstract

We compare the nonthermal emission from clusters of galaxies undergoingminor mergers (``accreting'' clusters) and major mergers (``merging''clusters). We define major mergers as mergers that change the inner darkmatter structure of clusters; minor mergers are all others. Foraccreting clusters, the radial distribution of the nonthermal emissionin the clusters is also calculated. The relativistic electrons, whichare the origin of the nonthermal radiation through inverse Compton (IC)and synchrotron emission, are assumed to be accelerated at shocksproduced by accretion or mergers. We estimate the typical accretion rateand merger probability according to a hierarchical clustering model. Wepredict that in the inner region of accreting clusters the nonthermalemission has a flat spatial distribution at all frequencies. Forsynchrotron and hard X-ray emissions, we predict an increase in theemissions at the cluster edge due to accretion. We show that the totalluminosities of IC emission from accreting and merging clusters aresimilar. On the other hand, the luminosity of synchrotron radio emissionof the former is much smaller than that of the latter. We show thatabout 10% of clusters at z~0 should have hard X-ray and radio nonthermalemissions due to their last major merger that are comparable to ordominate those due to ongoing accretion. Moreover, 20%-40% of clustersshould have significant EUV emission due to their last merger. We alsoinvestigate the case where the criterion of mergers is relaxed. If weextend the definition of a merger to an increase in the mass of thelarger subcluster by at least 10% of its initial mass, about 20%-30% ofclusters at z~0 should have hard X-ray and radio nonthermal emissionsdue to the merger, even in a low-density universe. We compare theresults with observations. We find that the observed EUV emission fromclusters is not attributed to accretion. If the diffuse radio emissionobserved in clusters is synchrotron emission from electrons acceleratedvia accretion or merging, the magnetic fields of clusters are generallyas small as ~0.1 {μ}G. One concern is that this is a significantlyweaker field than that implied by Faraday rotation measurements.