Properties of self-gravitating quasi-stationary states

¹ CREF, Centro Ricerche Enrico Fermi, Via Panisperna 89A, 00184 Roma, Italy
² Istituto dei Sistemi Complessi, Consiglio Nazionale delle Ricerche, 00185 Roma, Italy
³ Istituto Nazionale Fisica Nucleare, Dipartimento di Fisica, Università “Sapienza”, 00185 Roma, Italy
e-mail: Francesco.SylosLabini@roma1.infn.it
⁴ Dipartimento di Fisica, Sapienza, Universitá di Roma, Piazzale Aldo Moro 2, 00185 Roma, Italy

Received: 7 September 2020
Accepted: 24 September 2020

Abstract

Initially far out-of-equilibrium, self-gravitating systems form quasi-stationary states (QSS) through a collisionless relaxation dynamics. 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. These QSS depend on initial conditions. To understand the origin of such dependence, we present the 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 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 ρ(r) ∝ r^−α (α >  0) with radially elongated orbits. We show analytically that α = 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 < α(r) ≤ 4, where the density profile is well fitted by the Navarro-Frenk-White behavior. Such a dynamical evolution is shown by both nonuniform spherical isolated systems and by halos extracted from cosmological simulations. We consider the relation of these results with the core-cusp problem and conclude that this can be solved naturally if galaxies form through a monolithic collapse.