Multi-scale analysis of turbulence evolution in the density stratified intracluster medium

Abstract

The diffuse hot medium inside clusters of galaxies typically exhibits turbulent motions whose amplitude increases with radius, as revealed by cosmological hydrodynamical simulations. However, its physical origin remains unclear. It could either be due to an excess injection of turbulence at large radii, or faster turbulence dissipation at small radii. We investigate this by studying the time evolution of turbulence in the intracluster medium (ICM) after major mergers, using the Omega500 non-radiative hydrodynamical cosmological simulations. By applying a novel wavelet analysis to study the radial dependence of the ICM turbulence spectrum, we discover that faster turbulence dissipation in the inner high density regions leads to the increasing turbulence amplitude with radius. We also find that the ICM turbulence at all radii decays in two phases after a major merger: an early fast decay phase followed by a slow secular decay phase. The buoyancy effects resulting from the ICM density stratification becomes increasingly important during turbulence decay, as revealed by a decreasing turbulence Froude number $Fr \sim \mathcal{O}(1)$. Our results indicate that the stronger density stratification and smaller eddy turn-over time are the likely causes of the faster turbulence dissipation rate in the inner regions of the cluster.