Research Highlight
November 23, 2005
Analytical Models of Cosmic Accretion Shocks and the Role of Environment
by Vasiliki Pavlidou
Analytical Models of Cosmic Accretion Shocks and the Role of Environment
After their formation through gravitational collapse, cosmic structures (e.g., clusters of galaxies) continue to accumulate matter from their surroundings. At the locations where the infalling, accelerated gas meets the gaseous content of the largest collapsed structures, cosmic accretion shocks form. Such shocks process the enormous amount of kinetic energy present in the accreting gas, and convert part of it to heat, and part of it to kinetic energy of relativistic particles which are known as 'cosmological cosmic rays'. The properties of each shock, and its potential as a cosmic particle accelerator, depend on the properties of the collapsed accretor, as well as the physical conditions of its immediate environment.
To gain insight in the competing physical processes which determine the properties of the cosmic network of these large-scale accretion shocks, KICP fellow Vasiliki Pavlidou and UIUC Assistant Professor Brian Fields developed an analytical model for the statistical properties of cosmic accretion shock. The model takes into account the mass distribution of accretors and its evolution with time, as well as several environmental effects that modify the properties of the shock hosted by each accretor. Such effects are: the clustering of large-scale structures, the most massive of which tend to be found in denser environments, and therefore accrete high-density gas; adiabatic heating and cooling due to the compression or decompression of gas located in overdense or underdense regions, respectively; and shock-compression and heating of the gas in very-large-scale cosmic filaments and sheets, which pre-process the material accreted by most structures.
The model describes how much energy and mass passes through shock surfaces per unit time; how these quantities depend on the strength of the shocks and on cosmic time; and how the energy processed by accretion shocks compares to the energy released through other processes of cosmological significance, such as star formation and supernova explosions.

One of the most striking findings is that cosmic accretion shocks process more energy than the energy released by supernovae, for all redshifts smaller than 3. In the local universe, the shock-processed energy overtakes the energy output of supernovae by more than an order of magnitude!
The results of this study also demonstrated that most of the energy processing associated with shocks is done by the most massive structures in the universe, despite the fact that these structures are relatively rare compared to the much more numerous smaller structures. The environmental factors were found to be extremely important in determining the properties of the accretion shock population, primarily through increasing the density of the material accreted by the most massive structures. The impact of this density increase is two-fold. On the one hand, it increases the amount of mass and energy processed by the shocks. On the other hand, it tends to make accretion shocks weaker, and hence limit the number of structures which can be viewed as promising sites for the acceleration of very-high-energy particles.
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KICP Members: Vasiliki Pavlidou