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

Abstract

Using a simple analytic approach we address the question of whether radiative cooling, nongravitational heating and cooling plus heating models can simultaneously explain the observed global X-ray properties (entropy and X-ray luminosity distributions) of groups and clusters and the residual soft X-ray background (XRB) after discrete sources are removed. Within the framework of typical cold dark matter structure formation characterized by an amplitude of matter power spectrum sigma_8=0.9, it is argued that while radiative cooling alone is able to marginally reproduce the entropy floor detected in the central regions of groups and clusters, it is insufficient to account for the steepening of the X-ray luminosity - temperature relation for groups and the unresolved soft XRB. A phenomenological preheating model, in which either an extra specific energy budget or an entropy floor is added to the hot gas in groups and clusters, fails in the recovery of at least one of the X-ray observed features. Finally, the soft XRB predicted by our combined model of cooling 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 a result of the oversimplification of our analytic models, this implies that either our current understanding of the physical processes of the hot gas is still incomplete, or the normalization of the present power spectrum has been systematically overestimated. For the latter, both the X-ray properties of groups and clusters and the XRB predicted by preheating model and cooling plus heating model can be reconciled with the X-ray observations if a lower value of the normalization parameter sigma_8 \approx 0.7 is assumed.