Dark matter halo dynamics in 2D Vlasov simulations

Understanding dark matter halo dynamics can be pivotal in unraveling the nature of dark matter particles. Analytical treatment of the multistream flows inside the turn-around region of a collapsed CDM (cold dark matter) halo using various self-similar approaches already exist. In this work, we aim to determine the extent of self-similarity in 2D halo dynamics and the factors leading to deviations from it by studying numerical simulations of monolithically growing CDM halos. We have adapted the Fillmore and Goldreich self-similar solutions assuming cylindrical symmetry to data from 2D Vlasov-Poisson (ColDICE package) simulations of CDM halos seeded by sine wave initial conditions. We measured trajectories in position and phase space, mass and density profiles and compared these to predictions from the self-similar model, characterized by two parameters: M 0 , ϵ . The former scales the size of the turn-around region and the latter is inversely related to the mass accretion rate. We find that after turn-around and subsequent shell crossing, particles undergo a period of relaxation, typically about 1−2 oscillations about the center before they start to trace the self-similar fits and continue to do so as long as their orbits are predominantly radial. Overplotting the trajectories from different snapshots in scale-free position-time and phase spaces shows strikingly good superposition, a defining feature of self-similarity. The radial density profiles measured from simulations: ρ ∝ r − α , α = 0.9−1.0 are consistent with Fillmore and Goldreich's prediction α = 1 for 2D halos. The best-fit parameters for each simulation are found to be narrowly distributed, with the spread being entirely systematic. Deviations from self-similarity, on the other hand, are evidently linked to relaxation, inhibited motion due to periodic boundaries, transverse motion in the halo interior, and deficit of infalling mass in limited simulation volume. It could not be conclusively established if the halos tend to grow circular over time. Extension of this work to actual 3D CDM cosmologies necessitates further detailed study of self-similar solutions with ellipsoidal collapse and transverse motion.