Competition between shocks and entropy floor: Unifying groups and clusters of galaxies

¹ Department of Physics & Astronomy, University of Leicester, 1 University Road, Leicester LE1 7RH, UK e-mail: ssa@star.le.ac.uk
² Institut d'Astrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France

Corresponding author: S. Dos Santos, dore@iap.fr

Received: 27 June 2001
Accepted: 6 December 2001

Abstract

Semi-analytic models of X-ray clusters and groups of galaxies, relying on the idea that there was a non-gravitational energy injection in these systems, are able to reproduce many observed correlations, in particular the relation and the “central entropy floor” in groups. Limiting models exist which describe the behaviour of clusters and groups separately, but no analytic modeling has yet been found to unify both mass ranges. It is the aim of this paper to provide such an analytic model. Our description relies on a now standard description of the shock thought to occur in these systems near the virial radius (Cavaliere et al. [CITE]), the isothermality and spherical symmetry of the intracluster medium, as well as the reinterpretation of observed quantities (like the X-ray luminosity, the gas mass M_ICM and the central SZ effect y₀) in terms of the specific entropy. This allows the derivation of analytic expressions for several observed correlations (, , ,...) and their normalisation encompassing both the group and the cluster regimes. The analytic predictions compare very well with observations, as well as with more elaborated semi-analytic schemes. This agreement allows a reinterpretation of the relation (via the quantity ) and the relation (via ) as indirect measures of the non-gravitational entropy content of groups and clusters of galaxies. We emphasize the need for shock heating, even in the group mass range: shocks can not be completely suppressed in groups (and thus groups can not be entirely isentropic) unless an unacceptably high entropy floor is needed in order to break the self-similarity in the relation. Our model shows that the normalisation of the entropy after the shock (which is mass-dependent) is a key ingredient and that this quantity alone can explain the shape of the observed correlations between integrated X-ray and SZ quantities over and below 2 keV.