The EUV Emission in the Coma Cluster of Galaxies and the Underlying Source of this Radiation

摘要

Observations with the Extreme Ultraviolet Explorer (EUVE) have shown the ComaCluster to be a source of EUV emission in excess of that produced by X-ray gasin the cluster. We have re-examined the EUVE data on this cluster in an attemptto obtain clues as to the origin of this emission. We find two important newresults. First, the ratio between the azimuthally averaged EUV excess emissionand the ROSAT hard X-ray flux is constant as a function of distance from thecluster center outward. Second, a correlation analysis between the EUV excessemission and the X-ray emission shows that on a detailed level the EUV excessis spatially closely related to the X-ray emission. These findings contradictprevious suggestions as to the underlying source of the diffuse EUV emission inComa and provide important information in regards to the true source of thisemission. We propose a new explanation for the source of this emission: inverseCompton scattering of microwave background photons by secondary electrons andpositrons. We explore this possibility in some detail and show that it isconsistent with all of the available observational evidence. The parent cosmicray protons may have been produced by any of a number of sources, includingsupernovae, active galaxies, galactic winds, and cluster formation shocks, butwe believe that the most likely source is cluster formation shocks. If the EUVemission in the Coma Cluster is, in fact, the result of secondary electrons,this may be the only direct evidence for secondary electrons in theintracluster medium of a cluster of galaxies, since recent work suggests thatsecondary electrons may not be the cause of radio halos.