Detection of the entropy of the intergalactic medium: Accretion shocks in clusters, adiabatic cores in groups

The thermodynamics of the diffuse, X-ray-emitting gas in clusters of galaxies is linked to the entropy level of the intracluster medium. In particular, models that successfully reproduce the properties of local X-ray clusters and groups require the presence of a minimum value for the entropy in the center of X-ray halos. Such a minimum entropy is most likely generated by nongravitational processes in order to produce the observed break in self-similarity of the scaling relations of X-ray halos. Likely candidates are supernova heating, stellar winds, radiative processes, and nuclear activity. At present, there is no consensus on the level, the source, or the time evolution of this excess entropy. In this framework a key question is whether the central entropy is the residual of the entropy originally present in the precollapse or intergalactic medium or whether it is generated within the halos after collapse. Answering this question will allow a more precise determination of the energetic budget required to build an entropy floor and lead toward an understanding of the nature of the sources of heating. In this paper we describe a strategy to investigate the physics of the heating processes acting in groups and clusters. We show that the best way to extract information from the local data is by observation of the entropy pro file at large radii in nearby X-ray halos (z similar or equal to 0.1), at both the upper and lower extremes of the cluster mass scale. The spatially and spectrally resolved observation of such X-ray halos provides information on the mechanism of the heating. We demonstrate how measurements of the size of constant entropy (adiabatic) cores in clusters and groups can directly constrain heating models and the minimum entropy value. We also consider two specific experiments: the detection of the shock fronts expected at the virial boundary of rich clusters and the detection of the isentropic, low surface brightness emission extending to radii larger than the virial ones in low-mass clusters and groups. Both experiments are designed to measure the entropy in the low-density gas far from the core, taking advantage of the large collecting power of the present X-ray telescopes (in this case, XMM). Such observations will be a crucial probe of both the physics of clusters and the relationship of nongravitational processes to the thermodynamics of the intergalactic medium.