Cosmological shock waves: Clues to the formation history of haloes

Abstract

Shock waves developed during the formation and evolution of cosmic structures are key features encoding crucial information on the hierarchical formation of the Universe. We present the analysis of an Eulerian adaptive mesh refinement hydrodynamical and N-body simulation in a ^ cold dark matter cosmology especially focused on the study of cosmological shock waves. The combination of a shock-capturing algorithm together with the use of a halo finder allows us to study themorphological structures of the shock patterns, the statistical properties of shocked cells and the correlations between the cosmological shockwaves appearing at different scales and the properties of the haloes harbouring them. According to their localization with respect to the population of haloes in the simulation, shocks can be split into two broad classes: internal weak shocks related with evolutionary events within haloes and external strong shocks associated with large-scale events. The shocks' segregation according to their characteristic sizes is also visible in the shock distribution function. This function contains information on the abundances and strength of the different shocks, and it can be fitted by a double power law with a break in the slope around a Mach number of 20. We introduce a generalized scaling relation that correlates the average Mach numbers within the virial radius of haloes and their virial masses. In this plane, Mach number-virial mass, two welldifferentiated regimes appear. Haloes occupy different areas of such plane according to their early evolutionary histories: those haloes with a relatively quiet evolution have an almost constant Mach number independently of their masses, whereas haloes undergoing significant merger events very early in their evolution show a linear dependence on their masses. At high redshift, the distribution of haloes in this plane forms an L-like pattern that evolves with time, bending the vertical branch towards the horizontal one. We prove that this behaviour is produced by haloes reaching low average Mach numbers as they evolve towards a virialized state. The analysis of the propagation speed and size of the shock waves developed around haloes could give some hints on the formation time and main features of the haloes. The possible future detection of all sort of cosmic shocks by the forthcoming telescopes, such as the Square Kilometre Array (SKA), could open a new window to indirectly unravel some of the main features of haloes and, therefore, the formation process of the cosmic structures. © 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.