Cosmological MHD simulation of a cooling flow cluster

Context. Various observations of magnetic fields in the intra-cluster medium ( ICM), most of the time restricted to cluster cores, point towards a field strength of a few mu G ( synchrotron radiation from radio relics and radio halos, inverse Compton radiation in X-rays and Faraday rotation measure of polarised background sources). Both the origin and the spatial structure of galaxy cluster magnetic fields are still under debate. In particular, the radial profile of the magnetic field, from the core of clusters to their outskirts, is important for cosmic ray propagation within the cosmic web. Aims. In this letter, we highlight the importance of cooling processes in amplifying the magnetic field in the core of galaxy clusters up to one order of magnitude above the typical amplification obtained for a purely adiabatic evolution. Methods. We performed a zoom cosmological simulation of a 3 keV cluster, including dark matter and gas dynamics, atomic cooling, UV heating, and star formation using the newly developed MHD solver in the AMR code RAMSES. Results. Magnetic field amplification proceeds mainly through gravitational contraction. Shearing motions due to turbulence provide additional amplification in the outskirts of the cluster, while magnetic reconnection during mergers causes magnetic field dissipation in the core. Conclusions. Cooling processes have a strong impact on the magnetic field structure in the cluster. First, due to the sharp rise of the gas density in the centre, gravitational amplification is significantly higher, when compared to the non-radiative run. Second, cooling processes cause shearing motions to be much stronger in the core than in the adiabatic case, leading to additional field amplification and no significant magnetic reconnection. Cooling processes are therefore essential for determining the magnetic field profile in galaxy clusters.