Implications of a Nonthermal Origin of the Excess Extreme-Ultraviolet Emission from the Coma Cluster of Galaxies

The inverse Compton (IC) interpretation of the excess EUV emission that was recently reported from several clusters of galaxies suggests that the amount of relativistic electrons in the intracluster medium is highly significant, W_e>10⁶¹ ergs. Considering Coma as the prototype galaxy cluster of nonthermal radiation, with synchrotron and IC fluxes measured in the radio and EUV regions, and possibly also in the hard X-ray region, we discuss implications of the IC origin of the EUV fluxes in the case of low intracluster magnetic fields of order 0.1 μG as required for the IC interpretation of the observed excess hard X-ray flux, and in the case of high fields of order 1 μG, as suggested by Faraday rotation measurements. Although for such high intracluster fields the excess hard X-ray fluxes will require an explanation other than by the IC effect, we show that the excess EUV flux can be explained by the IC emission of a ``relic'' population of electrons driven into the incipient intracluster medium at the epoch of starburst activity by galactic winds and later on reenergized by adiabatic compression and/or large-scale shocks transmitted through the cluster as the consequence of more recent merger events. Radiative cooling will naturally produce a sharp cutoff in the spectrum of this relic electron population, which is required, in the case of microgauss fields, in order to avoid a contradiction with the observed radio fluxes. For high magnetic fields, B>=1 μG, the interpretation of the radio fluxes of Coma requires a second population of electrons injected recently. They can be explained as secondaries produced by a population of relativistic protons. We calculate the fluxes of γ-rays to be expected in both the low and high magnetic field scenarios and discuss possibilities to distinguish between these two principal options by future γ-ray observations.