Properties of Cosmic Shock Waves in Large Scale Structure Formation

Abstract

We have examined the properties of shock waves in simulations of large scale structure formation for two cosmological scenarios (a SCDM and a LCDM with Omega =1). Large-scale shocks result from accretion onto sheets, filaments and Galaxy Clusters (GCs) on a scale of circa 5 Mpc/h in both cases. Energetic motions, both residual of past accretion history and due to current asymmetric inflow along filaments, generate additional, common shocks on a scale of about 1 Mpc/h, which penetrate deep inside GCs. Also collisions between substructures inside GCs form merger shocks. Consequently, the topology of the shocks is very complex and highly connected. During cosmic evolution the comoving shock surface density decreases, reflecting the ongoing structure merger process in both scenarios. Accretion shocks have very high Mach numbers (10-10^3), when photo-heating of the pre-shock gas is not included. The typical shock speed is of order v_{sh}(z) =H(z)lambda_{NL}(z), with lambda_{NL}(z) the wavelength scale of the nonlinear perturbation at the given epoch. However, the Mach number for shocks occuring within clusters is usually smaller (3-10), due to the fact that the intracluster gas is already hot. Statistical fits of shock speed around GCs as a function of GCs temperature give power-law's in accord with 1-D predictions. However, a very different result is obtained for fits of the shock radius, reflecting the very complex shock structures forming in 3-D simulations. The in-flowing kinetic energy across such shocks, giving the power available for cosmic-ray acceleration, is comparable to the cluster X-ray luminosity emitted from a central region of radius 0.5 Mpc/h. Considering their large size and long lifetimes, those shocks are potentially interesting sites for cosmic-ray acceleration, if modest magnetic fields exist within them.