Magnetised stratified turbulence and dynamo effect in the intracluster medium

Soutenance de thèse de Jean Kempf (Salle Coriolis, OMP)

Résumé de la thèse : Magnetic fields pervade the entire Universe, and its largest scales are no exception. At these scales are also found galaxy clusters, the natural endpoint of hierarchical structure formation, which formed only recently in the history of the Universe. Located at the nodes of the cosmic web, they are amongst the most massive, gravitationally bound, astrophysical systems in the Universe. Dark matter, magnetic fields, and a hot and diffuse plasma, called the intracluster medium (ICM), which shines almost exclusively in radio and X-ray wavelengths, permeate the space in between galaxies in clusters. Many questions related to the astrophysics of galaxy clusters remain unsolved, like the cooling catastrophe, the hydrostatic mass bias, chemical enrichment and mixing, and the origin of cluster magnetic fields, to name just the most outstanding instances. Undoubtedly, a common ground between these diverse problems is the large-scale, fluid, magnetised, internal dynamics of the ICM of galaxy clusters, which remain, however, only poorly constrained because observations of the ICM in the relevant frequency bands are intrinsically, and technically, very challenging.

The main goal of this thesis has been to improve our understanding of a possible, efficient driver of internal, turbulent, magnetised dynamics in the ICM, the magneto-thermal instability (MTI), a fundamental, magnetohydrodynamical (MHD) process likely at play in the outskirts of galaxy clusters. We proceeded in four steps, following a guiding thread of increasing theoretical complexity, or, equivalently, from the most linear, to the most non-linear, physics that we probed. First, we constrained the physics and the energetics of the linear MTI and showed that its eigenmodes develop at cluster scales; consequently, we further elucidated the dynamical features of global, compressible, non-linear, magneto-thermal turbulence and developed a diffusive mixing-length theory to unify two previously divergent theories of its non-linear saturation and transport in the ICM. We then tackled the problem of magnetic-field amplification by magneto-thermal turbulence through a fluid dynamo effect, a specific process that could explain the rather small-scale, isotropic, entangled geometry of magnetic fields inferred in clusters. Finally, armed with a state-of-the-art theoretical understanding of magneto-thermal dynamics in clusters, we assessed its possible observational detectability in the ICM with the future X-ray Integrated Field Unit (X-IFU) instrument onboard ATHENA, and found that it would most likely not be detectable, albeit only marginally so. Altogether, our results show that characterising the specifics of magnetised, stratified dynamics in the ICM is of utmost importance for future observations and to further our understanding of large-scale structures like galaxy clusters.

Composition du jury de thèse :

• Pascale GARAUD, reviewer, University of Santa Cruz
• Christopher REYNOLDS, reviewer, University of Maryland
• Geoffroy LESUR, examiner, IPAG
• Katia FERRIÈRE, examiner, IRAP
• NicolasCLERC, examiner, IRAP
• FrançoisRINCON, PhD supervisor, IRAP