The impact of numerical viscosity in SPH simulations of galaxy clusters

Abstract

A SPH code employing a time-dependent artificial viscosity scheme is used to construct a large set of N-body/SPH cluster simulations for studying the impact of artificial viscosity on the thermodynamics of the ICM and its velocity field statistical properties. Spectral properties of the gas velocity field are investigated by measuring for the simulated clusters the velocity power spectrum E(k). The longitudinal component E_c(k) exhibits over a limited range a Kolgomorov-like scaling k^{-5/3}, whilst the solenoidal power spectrum component E_s(k) is strongly influenced by numerical resolution effects. The dependence of the spectra E(k) on dissipative effects is found to be significant at length scales 100-300Kpc, with viscous damping of the velocities being less pronounced in those runs with the lowest artificial viscosity. The turbulent energy density radial profile E_{turb}(r) is strongly affected by the numerical viscosity scheme adopted in the simulations, with the turbulent-to-total energy density ratios being higher in the runs with the lowest artificial viscosity settings and lying in the range between a few percent and ~10%. These values are in accord with the corresponding ratios extracted from previous cluster simulations realized using mesh-based codes. At large cluster radii, the mass correction terms to the hydrostatic equilibrium equation are little affected by the numerical viscosity of the simulations, showing that the X-ray mass bias is already estimated well in standard SPH simulations. Finally, simulations in which the gas can cool radiatively are characterized by the presence in the cluster inner regions of high levels of turbulence, generated by the interaction of the compact cool gas core with the ambient medium.