Phase-space structure of protohalos: Vlasov versus Particle-Mesh

Abstract

The phase-space structure of primordial dark matter halos is revisited using cosmological simulations with three sine waves and Cold Dark Matter (CDM) initial conditions. The simulations are performed with the tessellation based Vlasov solver ColDICE and a Particle-Mesh (PM) $N$-body code. The analyses include projected density, phase-space diagrams, radial density and pseudo-phase space density. Particular attention is paid to force and mass resolution. Because the phase-space sheet complexity, estimated in terms of total volume and simplices count, increases very quickly, ColDICE can follow only the early violent relaxation phase of halo formation. During the latter, agreement between ColDICE and PM simulations having one particle per cell or more is excellent and halos have a power-law density profile, $\rho(r) \propto r^{-\alpha}$, $\alpha \in [1.5,1.8]$. This slope, measured prior to any merger, is slightly larger than in the literature. The phase-space diagrams evidence complex but coherent patterns with clear signatures of self-similarity in the sine wave simulations, while the CDM halos are somewhat scribbly. After additional mass resolution tests, the PM simulations are used to follow the next stages of evolution. The power-law progressively breaks down with a convergence of the density profile to the well known "NFW"-like universal attractor, irrespectively of initial conditions, that is even in the three-sine wave simulations. This demonstrates again that mergers do not represent a necessary condition for convergence to the dynamical attractor. Not surprisingly, the measured pseudo phase-space density is a power-law $Q(r) \propto r^{-\alpha_Q}$, with $\alpha_{\rm Q}$ close to the prediction of secondary spherical infall model, $\alpha_{\rm Q} \simeq 1.875$. However this property is also verified during the early relaxation phase, which is non trivial.