The dynamical state of A548 from XMM-Newton data: X-ray and radio connection

Context. We present a detailed study of the X-ray properties of the galaxy cluster Abell 548b (z = 0.04), using XMM-Newton data, and discuss the connection between the thermal properties and the presence of two extended relic radio sources located at the cluster periphery. Aims. We wish to analyze the dynamical state of the cluster and confirm the presence of a major merger. We will discuss the merger effects on the extended nonthermal emission. Methods. We analyzed the X-ray brightness distribution, the surface brightness profiles in different directions, and the spectral properties in several cluster regions. Moreover, to better understand the dynamical history of this cluster, we performed an optical analysis of the cluster galaxies. Results. From the analysis of the temperature distribution and of the surface brightness profiles, we find evidence of a shock in the northern cluster region, just before the location of the two extended peripheral relics. From the optical analysis, we find that the cluster galaxies show a large sigma V, and two components are needed to fit their velocity distribution. Observational results were compared with a cluster simulation. The maps of gas temperature and density distribution from the simulation agree with the observational data in the case of a cluster merger nearly perpendicular to the plane of the sky and in the state after the maximum core collapse. Conclusions. We conclude that we are observing a galaxy cluster in a major merger phase, just after the maximum core collapse. The mass ratio is about 1:2, and the merger collision is nearly perpendicular to the plane of the sky. A shock is present in the northern cluster region, and it is very likely responsible for the electron reacceleration and the magnetic field amplification that will give cause the cluster relics. The relative position of the shock and the relics is strongly affected by projection effects.