Gravitational collapse from cold uniform asymmetric initial conditions

Abstract

Using controlled numerical N-body experiments, we show how, in the collapse dynamics of an initially cold and uniform distribution of particles with a generic asymmetric shape, finite N fluctuations and perturbations induced by the anisotropic gravitational field compete to determine the physical properties of the asymptotic quasi-stationary state. When finite N fluctuations dominate the dynamics, the particle energy distribution changes greatly and the final density profile decays outside its core as r−4 with an N-dependent amplitude. On the other hand, in the limit where the anisotropic perturbations dominate, the collapse is softer and the density profile shows a decay as r−3, as is typical of halos in cosmological simulations. However, even in this limit, convergence with N of the macroscopic properties of the virialized system, such as the particle energy distributions, the bound mass, and the density profile, is very slow and not clearly established, including for our largest simulations (with N ∼ 106). Our results illustrate the challenges of accurately simulating the first collapsing structures in standard-type cosmological models.