Numerical convergence of pre-initial conditions on dark matter halo properties

TL;DR

This study examines how different initial particle arrangements in cosmological simulations affect dark matter halo properties, finding that convergence is generally good but varies with halo merging history and environment, and introducing CCVT as a viable initial load.

Contribution

It is the first to quantify the convergence of CCVT initial loads with other isotropic loads like glass in dark matter halo simulations.

Findings

Median halo properties agree within a few percent across different initial loads.

Poorly converged haloes are often merging or recently merged, with out-of-sync processes.

CCVT loads perform comparably to grid and glass loads in convergence tests.

Abstract

Generating pre-initial conditions (or particle loads) is the very first step to set up a cosmological N-body simulation. In this work, we revisit the numerical convergence of pre-initial conditions on dark matter halo properties using a set of simulations which only differs in initial particle loads, i.e. grid, glass, and the newly introduced capacity constrained Voronoi tessellation (CCVT). We find that the median halo properties agree fairly well (i.e. within a convergence level of a few per cent) among simulations running from different initial loads. We also notice that for some individual haloes cross-matched among different simulations, the relative difference of their properties sometimes can be several tens of per cent. By looking at the evolution history of these poorly converged haloes, we find that they are usually merging haloes or haloes have experienced recent merger events, and their merging processes in different simulations are out-of-sync, making the convergence of halo properties become poor temporarily. We show that, comparing to the simulation starting with an anisotropic grid load, the simulation with an isotropic CCVT load converges slightly better to the simulation with a glass load, which is also isotropic. Among simulations with different pre-initial conditions, haloes in higher density environments tend to have their properties converged slightly better. Our results confirm that CCVT loads behave as well as the widely used grid and glass loads at small scales, and for the first time we quantify the convergence of two independent isotropic particle loads (i.e. glass and CCVT) on halo properties.