EUV and soft X-ray emission from clusters of galaxies

Clusters of galaxies are found to possess a diffuse EUV emitting component unrelated to the hot intra-cluster medium (ICM) at X-ray temperatures. The EUV emission does not resemble the X-ray morphology of clusters: at the cluster core the EUV contours are organised, but at larger radii they become anisotropic, and therefore unrelated to the hot ICM. Joint spectral analysis of EUVE/DS and ROSAT/PSPC data showed clear evidence for a soft X-ray emission in excess of that expected from the hot ICM: the 'cluster soft-excess' (CSE) phenomenon has been clearly detected for the Virgo, Coma, A2199 and A1795 clusters. Thermal and non-thermal emission models are employed to explain the CSE. Surprising conclusions, such as pressure equipartition between cosmic rays and the hot ICM gas, are unavoidable consequence of non-thermal models. Aging effect due to radiative and collisional losses are taken into account in the spectral modelling. Thermal interpretation of the phenomenon typically implies, on the other hand, large masses of warm gas. Cosmological implications of the CSE are also discussed.