Structure and evolution of magnetized clusters: entropy profiles,

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 entropy – temperature relation and the X-ray luminosity – temperature 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 $B(r) = B_* (\rho_{\rm g}(r))^{\alpha}$, with $\alpha \approx 0.9$, as the one indicated by observations and numerical simulation. Our analysis shows that such a description is able to provide, at once, 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 $L_{\rm X}-T$ 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 $B_* \sim T^{0.5 \pm 0.1}$ is required to reproduce at the same time both the $S - T$ and the $L_{\rm X}-T$ 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