Structure and evolution of magnetized clusters: entropy profiles, S-T and L_X-T relations

Abstract

We study the impact of an intracluster magnetic field on the main structural properties of clusters and groups of galaxies: the radial density and entropy profiles, the S-T relation and the L_X-T relation for groups and clusters of galaxies. To this aim, we develop a description of the intra-cluster gas based on the Hydrostatic Equilibrium condition and on the Magnetic Virial Theorem in the presence of a radial distribution of the magnetic field B(r)=B_* \times [ρ_g(r)]^alpha, with alpha \sim 0.9, as indicated by observations and numerical simulation. Our analysis shows that such a description is able to provide a possible explanation of three problematic aspects of the cluster structure: i) the flattening of the entropy profile in the cluster center; ii) the flatness of the S-T relation; iii) the increasing steepening of the L_X-T relation from the cluster scale towards the group scales. The available entropy and X-ray luminosity data indicate that an increase of the magnetic field B_* \sim T^0.5 is required to reproduce at the same time both the S-T and the L_X-T relations. It follows that a consistent description of the magnetized ICM can provide a simple explanation of several (or of all) of these still open problems, and thus weakens the need for the inclusion of other non-gravitational effects which have been proposed so far for the explanation of some of these features. This (initial, but not conclusive) analysis can be regarded as a starting point for a more refined analytical exploration of the physics of the magnetized intra-cluster medium, and it provides testable predictions that can be proven or disproven with the next coming sensitive observations of groups and clusters in the X-ray band and in the radio frequency band.