Magnetic fields in the intracluster medium

Abstract

Magnetic fields are ubiquitous in the Universe from small to large scales. While various theories have been proposed for their generation, their origin is still not well understood. In order to best address this fundamental question, we study the largest gravitationally bound systems that exist: galaxy clusters. Galaxy clusters are rich, astrophysical laboratories that help us understand phenomena in our Universe from the largest cosmological scales down to the micro-scales ruled by plasma physics. The space in between galaxies in galaxy clusters is filled with hot plasma called the intracluster medium (ICM). This plasma emits in X-ray and radio wavelengths. The acceleration of cosmic-ray electrons in magnetic fields with strengths of microGauss produces Mpc--sized structures of diffuse radio emission in the ICM typically grouped into two categories: radio haloes and radio relics. The particle acceleration mechanisms leading to this large-scale emission is not fully understood. In this doctoral thesis I have focused on studying the role of magnetic fields in galaxy clusters. My motivation was to answer the following questions: 1) What are the magnetic amplification mechanisms that lead to today’s observables?, 2) What is the role of galaxy cluster mergers in shaping magnetic fields?, 3) What defines the substructure in the synchrotron and polarised emission in radio relics?. To this end, I have used results from the cosmological MHD code ENZO, the MHD code FLASH and the hybrid MHD-Lagrangian PLUTO code. I have studied a primordial scenario along with a small-scale dynamo amplification in simulated galaxy clusters tackling questions 1) and 2). I have found that such scenario can reproduce cluster magnetic fields of the order of microGauss.Major and minor mergers are sources of both compression and turbulence and therefore play a key role in the magnetic amplification. In particular, they introduce multiple turbulence cascades that affect the growth of an existing small-scale dynamo. I found that major mergers can delay the dynamo amplification for a period of 1 Gyr. I have studied the diffusive shock acceleration (DSA) mechanism to simulate the radio and polarised emission observed in radio relics tackling question 3). I have found that turbulence has a significant impact on the morphology of synchrotron and polarised emission. The observed discrepancy between the Mach number of shocks derived from X-ray and from radio spectra is explained in this framework. Some observed radio relics exhibit a gradient of polarisation fraction which is higher at the shock front and lower at the downstream regions contrary to theoretical expectations. I find that turbulence may be able to explain this gradient.