Collisionless formation of filaments in an expanding universe

A series of numerical simulations of the nonlinear gravitational collapse of collisionless matter in an Omega = 1 expanding universe are reported. The study is focused on the collapse of an isolated and smooth maximum of the density field, and is restricted to the case of one- and two-dimensional perturbations. The influence of dimensionality on the collapse process is investigated. It is found that the filament collapse occurs for an expansion factor 75 percent smaller than in the plane collapse case. Characteristics of the filament density profile and the evolution of the velocity distribution with an expansion factor of 1.6 after the collapse are described. It is determined that, whatever the initial conditions, the collapsed filament follows a self-similar evolution. The measured departure from the expected slope of -1 for the halo power law in the two-dimensional case is attributed to the presence of compensating troughs, which should have important consequences for the three-dimensional case. 37 refs