some shocking Stuff..
(Vazza, Brunetti & Gheller 2009 MNRAS)
Overview

Large Scale Shocks are responsible for
the heating of the ICM and can be
important sources of Cosmic Rays (CR) in the Universe.
However the occurence and properties of these
shocks are still poorly constrained from both a theoretical
and an observational side.

Results

We analysed the properties of Large Scale Shocks
in a (100Mpc/h)^3 cosmological volume simulated
with ENZO. Shocks are identified and charactherised
by means of a novel procedure which studies jumps in the
velocity variables across the cells in the simulations
and this allows us to have a viable description of
shocks also in underdense cosmic regions. The role played by the
modelling of reionization in the simulations
is investigated in detail, and a fitting procedure to
model reionization in a post--processing phase is presented. We
derive the distributions of the number of shocks
and of their energy dissipation as a function
of their Mach number,
and we discuss the evolution of these distributions
with cosmological time and across different cosmic environments
(clusters, outskirts, filaments, voids).

In line with previous numerical studies weak shocks
are found to dominate the energy dissipation process in the simulated
cosmic volume, although our results suggest that these distributions
are steeper that those found in previous numerical works, and that
the bulk of energy dissipation happens at shocks with Mach number \approx 2.
Distributions become steeper with increasing the density of the cosmic
environments, and are in agreement with
semi-analytical results in the case of virialised regions in galaxy
clusters.

We estimate the rate of injection of CR at Large
Scale Shocks
by adopting injection efficiencies taken from previous
numerical calculations. The bulk of the energy dissipation
happens in galaxy clusters and in filaments and the
flux dissipated in the form of CR within the whole
simulated volume is found to be $\approx 0.2$ of the thermal
energy dissipated at shocks.

Finally we discuss in some detail the properties of shocks in the
case of galaxy clusters in relation with their dynamical state.
In these regions the bulk of the energy is dissipated at weak
shocks, with Mach number $\approx 1.6$, although strong
shocks are found in the external regions of merging clusters.
Our results are within present upper limits on the energy density
of CR in clusters obtained from gamma ray observations. Furthermore,
because of the steep spectrum of the CR derived from our simulations,
our results are also consistent with recent upper
limits obtained from radio observations.