Diffuse EUV Emission from Clusters of Galaxies

Luminous extreme ultraviolet (EUV) emission was detected with EUVE from a number of nearby clusters of galaxies. I will review the observations of EUV and very soft X-ray emission from clusters with EUVE, ROSAT, and XMM/Newton. I will discuss the two primary competing theories for this emission. First, it may be thermal emission from a large amount of relatively cool intracluster gas. Second, it may be due to inverse Compton (IC) scattering of Cosmic Microwave Background (CMB) radiation by low energy cosmic ray electrons in the intracluster medium. The cosmic ray electrons would have Lorentz factors of γ ≈ 300 and energies of E ≈ 150 MeV. They would lose energy primarily by emitting EUV radiation. These particles have rather long lifetimes of ⪆ 10⁹ years. The required relativistic electrons might be accelerated in merger shocks in clusters. Support for this work was provided by the National Aeronautics and Space Administration through Chandra Award Numbers GO1-2123X and GO1-2133X, issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-39073, and by NASA XMM/Newton Grant NAG5-10075.