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 external intergalactic medium at the epoch of accretion. 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 of the pre-collapse, intergalactic medium. Such an entropy floor can be generated by non-gravitational phenomena, such as SNe heating, stellar winds and radiative processes. However, there is no consensus on the level, the source or the time evolution of this entropy. In this paper we propose that the best way to investigate the entropy distribution of the local universe is via the detection of the shock fronts expected at the virial boundary of rich clusters, and the extended isentropic gas distribution in the center of low mass clusters and groups. In particular we describe a strategy to look for accretion shocks with the next generation of X-ray facilities (in this case XMM). The measurement of the entropy level exterior to the shock, and observations of the transition from the shock (clusters) to the adiabatic (groups) regime, will be crucial probes of both the physics of clusters and the relationship of stellar processes to the state of the intergalactic medium. Subject headings: galaxies: clusters: general – intergalactic medium – X–rays: galaxies – 3 – 1.