The Effect of Corner Modes in the Initial Conditions of Cosmological Simulations

TL;DR

This study assesses the impact of including or removing corner modes in initial conditions of cosmological simulations, finding that their effect on small-scale structures and high redshifts is generally minimal, below a few percent.

Contribution

It provides a detailed comparison of simulation outcomes with and without corner modes, quantifying their influence on various cosmological observables.

Findings

Corner modes significantly affect the power spectrum at high redshifts and small scales.

Including corner modes improves low- and high-resolution simulation agreement near the Nyquist frequency.

Effects of corner modes are smaller than particle discreteness effects.

Abstract

In view of future high-precision large-scale structure surveys, it is important to quantify the percent and subpercent level effects in cosmological $N$-body simulations from which theoretical predictions are drawn. One such effect involves deciding whether to zero all modes above the one-dimensional Nyquist frequency, the so-called "corner" modes, in the initial conditions. We investigate this effect by comparing power spectra, density distribution functions, halo mass functions, and halo profiles in simulations with and without these modes. For a simulation with a mass resolution of $m_p \sim 10^{11}\,h^{-1}\,M_{\odot}$, we find that at $z>6$, the difference in the matter power spectrum is large at wavenumbers above $\sim 80$\% of $k_{\rm{Ny}}$, reducing to below 2\% at all scales by $z\sim 3$. Including corner modes results in a better match between low- and high-resolution simulations at wavenumbers around the Nyquist frequency of the low-resolution simulation, but the effect of the corner modes is smaller than the effect of particle discreteness. The differences in mass functions are 3\% for the smallest halos at $z=6$ for the $m_p \sim 10^{11}\,h^{-1}\,M_{\odot}$ simulation, but we find no significant difference in the stacked profiles of well-resolved halos at $z \leq 6$. Thus removing power at $|\mathbf{k}|>k_{\rm{Ny}}$ in the initial conditions of cosmological simulations has a small effect on small scales and high redshifts, typically below a few percent.