Properties of cosmic shock waves in large-scale structure formation

We have examined the properties of shock waves in simulations of large-scal e structure formation. Two cosmological scenarios have been considered: a s tandard cold dark matter model with Omega (M) = 1 (SCDM), and a cold dark m atter model with cosmological constant and Omega (M) + Omega (A) = 1 (Lambd a CDM) having Omega (Lambda) = 0.55. Large-scale shocks result from accreti on onto sheets, filaments, and knots of mass distribution on a scale of the order of similar to 5 h(-1) Mpc in both scenarios. Energetic motions, part ly residuals of past accretion processes and partly caused by current asymm etric inflow along filaments, end up generating additional shocks. These ex tend on a scale of the order of similar to 1 h(-1) Mpc and envelop and pene trate deep inside the clusters. Collisions between substructures inside clu sters also form merger shocks. Consequently, the topology of the shocks is very complex and highly connected. During cosmic evolution the comoving sho ck surface density decreases, reflecting the ongoing structure merger proce ss in both scenarios. Accretion shocks have very high Mach numbers, typical ly between 10 and a few x 10(3), when photoheating of the preshock gas is n ot included. The characteristic shock velocity is of the order of v(sh)(z) = H(z)lambda (nl)(z), where lambda (nl)(z) is the wavelength scale of the n onlinear perturbation at the given epoch. However, the Mach number for merg er and flow shocks (which occur within clusters) is usually smaller, in the range of similar to 3-10, corresponding to the fact that the intracluster gas is hot (i.e., already shack heated). Statistical fits of shock velociti es around clusters as a function of cluster temperature give power-law func tions in accord with those predicted by one-dimensional solutions. On the o ther hand, a very different result is obtained for the shock radius, reflec ting extremely complex shock structures surrounding clusters of galaxies in three-dimensional simulations. The amount of inflowing kinetic energy acro ss the shocks around clusters, which represents the power available for cos mic-ray acceleration, is comparable to the cluster X-ray luminosity emitted from a central region of radius 0.5 h(-1) Mpc. Considering their large siz e and long lifetimes, those shocks are potentially interesting sites for co smic-ray acceleration, if modest magnetic fields exist within them.