Artifacts in Halo Shapes: Imprints of the Initial Condition

TL;DR

This paper reveals that grid-based initial conditions in cosmological simulations cause statistically significant artifacts in halo shapes and orientations, especially at low redshifts, challenging previous assumptions of their negligible impact.

Contribution

It demonstrates the presence of halo shape artifacts due to grid initialization and proposes a mathematical explanation for their redshift-dependent orientation flipping.

Findings

Grid initialization induces significant halo shape artifacts.

Halo orientations show a redshift-dependent alignment pattern.

Artifacts are present despite small amplitude (~1%).

Abstract

Grid type pre-initial conditions are commonly used to initialize particle positions in cosmological simulations. While these conditions are known to produce noticeable numerical artifacts in void regions, their impact on halo properties has generally been assumed to be negligible. In this work, we employ multiple simulations to demonstrate that grid initialization induces statistically significant artifacts in halo shapes, despite the modest absolute amplitude ($\sim 1\%$) making them unimportant for most cosmological studies. We identify a redshift-dependent artificial alignment pattern: at low redshifts ($z<2$), halo shapes preferentially orient away from the simulation box's Cartesian axes, whereas their constituent particles initially exhibit alignment with these axes. We propose a mathematical hypothesis to explain this flipping behavior.