Can radiative cooling and nongravitational heating explain simultaneously the global X-ray properties of clusters and the unresolved cosmic X-ray background ?

摘要

Using a simple analytic approach we address the question of whether radiativecooling, nongravitational heating and cooling plus heating models cansimultaneously explain the observed global X-ray properties (entropy and X-rayluminosity distributions) of groups and clusters and the residual soft X-raybackground (XRB) after discrete sources are removed. Within the framework oftypical cold dark matter structure formation characterized by an amplitude ofmatter power spectrum sigma_8=0.9, it is argued that while radiative coolingalone is able to marginally reproduce the entropy floor detected in the centralregions of groups and clusters, it is insufficient to account for thesteepening of the X-ray luminosity - temperature relation for groups and theunresolved soft XRB. A phenomenological preheating model, in which either anextra specific energy budget or an entropy floor is added to the hot gas ingroups and clusters, fails in the recovery of at least one of the X-rayobserved features. Finally, the soft XRB predicted by our combined model ofcooling plus heating exceeds the observational upper limits by a factor of ~2,if the model is required to reproduce the observed entropy and X-ray luminosity- temperature relationships of groups and clusters. If the discrepancy is not aresult of the oversimplification of our analytic models, this implies thateither our current understanding of the physical processes of the hot gas isstill incomplete, or the normalization of the present power spectrum has beensystematically overestimated. For the latter, both the X-ray properties ofgroups and clusters and the XRB predicted by preheating model and cooling plusheating model can be reconciled with the X-ray observations if a lower value ofthe normalization parameter sigma_8 pprox 0.7 is assumed.