A toy model to test the accuracy of cosmological Nbody simulations
Abstract
The evolution of an isolated overdensity represents a useful toy model to test the accuracy of a cosmological Nbody code in the nonlinear regime as it is approximately equivalent to that of a truly isolated cloud of particles, with same density profile and velocity distribution, in a nonexpanding background. This is the case as long as the system size is smaller than the simulation box side, so that its interaction with the infinite copies can be neglected. In such a situation, the overdensity rapidly undergoes to a global collapse forming a quasi stationary state in virial equilibrium. However, by evolving the system with a cosmological code (GADGET) for a sufficiently long time, a clear deviation from such quasiequilibrium configuration is observed. This occurs in a time t_{LI} that depends on the values of the simulation numerical parameters such as the softening length and the timestepping accuracy, i.e. it is a numerical artifact related to the limited spatial and temporal resolutions. The analysis of the LayzerIrvine cosmic energy equation confirms that this deviation corresponds to an unphysical dynamical regime. By varying the numerical parameters of the simulation and the physical parameters of the system we show that the unphysical behaviour originates from badly integrated close scatterings of highvelocity particles. We find that, while the structure may remain virialized in the unphysical regime, its density and velocity profiles are modified with respect to the quasiequilibrium configurations, converging, however, to well defined shapes, the former characterised by a Navarro Frenk Whitetype behaviour.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 April 2013
 DOI:
 10.1051/00046361/201321037
 arXiv:
 arXiv:1303.4531
 Bibcode:
 2013A&A...552A..36S
 Keywords:

 methods: numerical;
 galaxies: halos;
 galaxies: formation;
 largescale structure of Universe;
 dark matter;
 Astrophysics  Cosmology and Extragalactic Astrophysics
 EPrint:
 19 pages, 21 figures. Astronomy and Astrophysics in the press