Collisionless relaxation in gravitacional systems from violent relaxation to gravothermal collapse

Abstract

Theory and simulations are used to study collisionless relaxation of a gravitational N-body system. It is shown that when the initial one-particle distribution function satisfies the virial condition—potential energy is minus twice the kinetic energy—the system quickly relaxes to a metastable state described quantitatively by the Lynden-Bell distribution with a cutoff. If the initial distribution function does not meet the virial requirement, the system undergoes violent oscillations, resulting in a partial evaporation of mass. The leftover particles phase-separate into a core-halo structure. The theory presented allows us to quantitatively predict the amount and the distribution of mass left in the central core, without any adjustable parameters. On a longer time scale TG~N, collisionless relaxation leads to a gravothermal collapse.