Cold dark matter protohalo structure around collapse: Lagrangian cosmological perturbation theory versus Vlasov simulations

TL;DR

This study compares high-order Lagrangian perturbation theory with Vlasov simulations to analyze cold dark matter protohalo structures around shell-crossing, revealing good agreement and insights into caustic and vorticity patterns.

Contribution

It demonstrates that high-order LPT accurately predicts protohalo structures beyond shell-crossing, validated against detailed Vlasov simulations, and explores the effects of symmetry and velocity quantities.

Findings

High-order LPT matches Vlasov simulations well beyond shell-crossing.

Convergence of LPT slows in symmetric configurations with equal wave amplitudes.

LPT predictions align with singularity theory regarding caustic and vorticity patterns.

Abstract

We explore the structure around shell-crossing time of cold dark matter protohaloes seeded by two or three crossed sine waves of various relative initial amplitudes, by comparing Lagrangian perturbation theory (LPT) up to 10th order to high-resolution cosmological simulations performed with the public Vlasov code ColDICE. Accurate analyses of the density, the velocity, and related quantities such as the vorticity are performed by exploiting the fact that ColDICE can follow locally the phase-space sheet at the quadratic level. To test LPT predictions beyond shell-crossing, we employ a ballistic approximation, which assumes that the velocity field is frozen just after shell-crossing. In the generic case, where the amplitudes of the sine waves are all different, high-order LPT predictions match very well the exact solution, even beyond collapse. As expected, convergence slows down when going from quasi-1D dynamics where one wave dominates over the two others, to the axial-symmetric configuration, where all the amplitudes of the waves are equal. It is also noticed that LPT convergence is slower when considering velocity related quantities. Additionally, the structure of the system at and beyond collapse given by LPT and the simulations agrees very well with singularity theory predictions, in particular with respect to the caustic and vorticity patterns that develop beyond collapse. Again, this does not apply to axial-symmetric configurations, that are still correct from the qualitative point of view, but where multiple foldings of the phase-space sheet produce very high density contrasts, hence a strong backreaction of the gravitational force.