Simulating the soft X-ray excess in clusters of galaxies

The detection of an excess of soft X-ray or Extreme Ultraviolet (EUV) radiation, above the thermal contribution from the hot intracluster medium (ICM), has been a controversial subject ever since the initial discovery of this phenomenon. We use a large-scale hydrodynamical simulation of a concordance \LambdaCDM model, to investigate the possible thermal origin of such an excess in a set of 20 simulated clusters having temperatures in the range 1-7 keV. Simulated clusters are analysed by mimicking the observational procedure applied to ROSAT-PSPC data, which for the first time showed evidence for the soft X-ray excess: we compare the low-energy (e.g. [0.2-1] keV) part of the spectrum of each cluster with that predicted for a plasma having temperature and metallicity as computed after weighting by the emissivity in a harder band (e.g., [1-2] keV). For distances to the cluster center 0.4< R/R<SUB>vir</SUB>< 0.7 we detect a significant excess in most of the simulated clusters, whose relative amount changes from cluster to cluster and, for the same cluster, by changing the projection direction. In about 30 per cent of the cases the soft X-ray flux is measured to be at least 50 per cent larger than predicted by the one-temperature plasma model. We find that this excess is generated in most cases within the virialized regions of the cluster. It mainly comes from low-entropy and high-density gas associated with merging sub-halos, rather than from diffuse warm gas. Only in a few cases does the excess arise from fore/background groups observed in projection, while no evidence is found for a significant contribution from gas lying within large-scale filaments. We compute the distribution of the relative soft excess, as a function of the distance to the cluster center , and compare it with the observational result by Bonamente et al. (\cite{Bonamente03}) for the Coma cluster. As in the observations, we find that the relative excess increases with the distance from the cluster center, with no significant excess detected for R<0.4R<SUB>vir</SUB>. However, an excess as large as that reported for the Coma cluster at scales 0.4⪉ R/R<SUB>vir</SUB>⪉ 0.7 is found to be rather unusual in our set of simulated clusters.