Impact des processus physiques dans et autour des amas de galaxies sur l'estimation de leur frontière et de leur masse : étude de cas de la réplique numérique de l'amas de la Vierge

Galaxy clusters are the most massive gravitationally bound objects of the Universe as they are at the endpoint of the hierarchical structure formation. As their name indicates, they are made of galaxies but that only accounts for a few per cent of the total mass budget. In reality, they are mostly constituted of dark matter (DM), only interacting gravitationally, and also of gas called the Intracluster Medium (ICM). Under the action of gravity, a deep gravitational potential well forms within which baryons are heated, thus increasing the thermal pressure, and become a plasma. As a first approximation, galaxy clusters can be seen as spherical and isolated objects that have reached both dynamical and thermal equilibrium. However, since galaxy clusters are at the nodes of the cosmic web, they continuously accrete matter from their local environment, particularly from filaments connected to them, inducing shocks and heating. Moreover, cluster member galaxies host stars that can explode in supernovae and central supermassive black holes that can emit radiation and jets of matter, both injecting energy back to their surrounding medium. All these processes can also induce turbulence, which adds non-thermal pressure, alike magnetic fields and cosmic rays. These interconnected and complex physical processes at play at different scales in and around galaxy clusters are difficult to probe from observations. Moreover, the simplifying approximations assumed to estimate the mass of galaxy clusters, e.g. hydrostatic equilibrium or virialisation, cannot account for the full complexity of the gas inside clusters, potentially leading to biased estimations of their mass from observations. A deeper understanding of the astrophysics of galaxy clusters has thus become a key step to more robustly estimate their mass, which in turn is essential to constrain cosmology through the cluster number counts. The aim of my PhD thesis was to explore how different physics processes impact the gas properties of the ICM and, as a consequence, the estimation of the boundaries and mass of galaxy clusters. I conducted my studies on one specific hydrodynamical simulation constrained so that it reproduces the Virgo galaxy cluster, our closest neighbour, in its local environment. I first analysed the thermodynamic properties of the ICM and the gas motion in and around the Virgo numerical replica. I showed that accretion and feedback processes perturb the medium by generating shocks, inducing complex turbulence initiated in filaments and driven towards the core of the cluster. Moreover, I showed that important projection effects due to filaments connected to Virgo and nearby structures bias the reconstruction of its properties from projected quantities similar to observations. I then estimated two general properties of the Virgo cluster replica: its radius and mass. More precisely, I estimated the splashback radius in different regions with different tracers, showing that this characteristic radius probes the accretion dynamics of the different constituents of a galaxy cluster in different regimes. Therefore, this radius should be used carefully to define the boundary of a galaxy cluster. Finally, I estimated the hydrostatic mass, and the associated bias, demonstrating that all the processes previously studied indeed contribute to the unrelaxedness of Virgo, making its hydrostatic mass estimation unreliable. I also explored methods for correcting the hydrostatic mass accounting for the non-thermal pressure generated by turbulent gas motion, showing promising results for future galaxy cluster mass calibration.