Growth of perturbations in homogeneous collisionless collapse: discs versus spheres

Abstract

We study the growth of radial perturbations during the collapse of homogeneous spheres and discs. We model the sphere by concentric shells and the disc by concentric rings, initially at rest and allowed to move only radially. Our numerical experiments show only moderate growth in spheres but rapid growth in discs, leading to pronounced ring formation. For a sphere, the perturbations can be treated analytically as in cosmology. The growth rate is that of a power-law in time and independent of scale. For discs, we present an analytic solution for the perturbations using a suitable mode expansion. Shorter wavelength perturbations grow faster, exhibiting exponential growth at early times.