Properties of self-gravitating quasi-stationary states

Abstract: Initially far out-of-equilibrium self-gravitating systems form, through a collisionless relaxation dynamics, quasi-stationary states (QSS). These may arise from a bottom-up aggregation of structures or in a top-down frame; their quasi-equilibrium properties are well described by the Jeans equation and are not universal, i.e. they depend on initial conditions. To understand the origin of such dependence, we present results of numerical experiments of initially cold and spherical systems characterized by various choices of the spectrum of initial density fluctuations. The amplitude of such fluctuations determines whether the system relaxes in a top-down or a bottom-up manner. We find that statistical properties of the resulting QSS mainly depend upon the amount of energy exchanged during the formation process. In particular, in the violent top-down collapses the energy exchange is large and the QSS show an inner core with an almost flat density profile and a quasi Maxwell-Boltzmann (isotropic) velocity distribution, while their outer regions display a density profile $\rho(r) \propto r^{-\alpha}$ ($\alpha >0$) with radially elongated orbits. We analytically show that $\alpha=4$ in agreement with numerical experiments. In the less violent bottom-up dynamics, the energy exchange is much smaller, the orbits are less elongated and $0< \alpha(r) \le 4$, with a a density profile well fitted by the Navarro-Frenk-White behavior. Such a dynamical evolution is shown by both non-uniform spherical isolated systems and by halos extracted from cosmological simulations. We consider the relation of these results with the core-cusp problem concluding that this is naturally solved if galaxies form through a monolithic collapse.