Early Growth in a Perturbed Universe: Exploring Dark Matter Halo Populations in 2LPT and ZA Simulations

TL;DR

This study compares the evolution of dark matter halos in cosmological simulations initialized with 2LPT and ZA methods, revealing that 2LPT leads to earlier and more massive halo formation at high redshift, affecting halo properties.

Contribution

It provides a detailed comparison of 2LPT and ZA initialization techniques, highlighting the importance of initial conditions on high-redshift halo evolution and properties.

Findings

2LPT results in earlier collapse and larger high-redshift halos.

Halo concentration differences are significant when viewed as a function of mass.

ZA simulations may underestimate high-z halo populations.

Abstract

We study the structure and evolution of dark matter halos from z = 300 to z = 6 for two cosmological N-body simulation initialization techniques. While the second order Lagrangian perturbation theory (2LPT) and the Zel'dovich approximation (ZA) both produce accurate present day halo mass functions, earlier collapse of dense regions in 2LPT can result in larger mass halos at high redshift. We explore the differences in dark matter halo mass and concentration due to initialization method through three 2LPT and three ZA initialized cosmological simulations. We find that 2LPT induces more rapid halo growth, resulting in more massive halos compared to ZA. This effect is most pronounced for high mass halos and at high redshift. Halo concentration is, on average, largely similar between 2LPT and ZA, but retains differences when viewed as a function of halo mass. For both mass and concentration, the difference between typical individual halos can be very large, highlighting the shortcomings of ZA-initialized simulations for high-z halo population studies.