Growth of Velocity Dispersions for Collapsing Spherical Stellar Systems

Abstract

First, we have ensured that spherical nonrotating collisionless systems collapse with almost retaining spherical configurations during initial contraction phases even if they are allowed to collapse three-dimensionally. Next, on the assumption of spherical symmetry, we examine the evolution of velocity dispersions with collapse for the systems which have uniform or power-law density profiles with Maxwellian velocity distributions by integrating the collisionless Boltzmann equation directly. The results show that as far as the initial contraction phases are concerned, the radial velocity dispersion never grows faster than the tangential velocity dispersion except at small radii where the nearly isothermal nature remains, irrespective of the density profiles and virial ratios. This implies that velocity anisotropy as an initial condition should be a poor indicator for the radial orbit instability. The growing behavior of the velocity dispersions is briefly discussed from the viewpoint that phase space density is conserved in collisionless systems.