A Multifiltering Study of Turbulence in a Large Sample of Simulated Galaxy Clusters

We present results from a large set of N-body/smoothed particle hydrodynamics (SPH) hydrodynamical cluster simulations aimed at studying the statistical properties of turbulence in the intracluster medium (ICM). The numerical hydrodynamical scheme employs an SPH formulation in which gradient errors are strongly reduced by using an integral approach. We consider both adiabatic and radiative simulations. We construct cluster subsamples according to the cluster dynamical status or gas physical modeling, from which we extract small-scale turbulent velocities obtained by applying different multiscale filtering methods to cluster velocities. The velocity power spectra of nonradiative relaxed clusters are mostly solenoidal and exhibit a peak at wavenumbers set by injection scales ≃r 200 /10; at higher wavenumbers, the spectra are steeper than Kolmogorov. Cooling runs are distinguished by much shallower spectra, a feature which we interpret as the injection of turbulence at small scales due to the interaction of compact cool gas cores with the ICM. Turbulence in galaxy clusters is then characterized by multiple injection scales, with the small-scale driving source acting in addition to the large-scale injection mechanisms. Cooling runs of relaxed clusters exhibit enstrophy profiles with a power-law behavior over more than two decades in radius and a turbulent-to-thermal energy ratio ≲1%. In accordance with Hitomi observations, in the core of a highly relaxed cluster, we find a low level of gas motions. In addition, the estimated cluster radial profile of the sloshing oscillation period is in very good agreement with recent Fornax measurements, with the associated Froude number satisfying Fr ≲ 0.1 within r/r 200 ≲ 0.1. Our findings suggest that in cluster cores, ICM turbulence approaches a stratified anisotropic regime, with weak stirring motions dominated by gravity buoyancy forces and strongly suppressed along the radial direction. We conclude that turbulent heating cannot be considered the main heating source in cluster cores.