An Excessive Core Collapse in Nbody Cosmological simulations

TL;DR

This paper reveals that the large particle mass in cosmological N-body simulations causes an artificial core collapse in small halos, potentially explaining the small scale crisis in CDM models.

Contribution

It demonstrates that particle mass differences induce excessive core collapse in simulations, providing a physical limit and a method to mitigate this bias.

Findings

Excessive cuspy centers in low mass halos due to simulation particle mass.

A physical limit of about 1000 particles per small halo to avoid bias.

A simple suggestion to reduce core collapse effects in simulations.

Abstract

The particle mass used in cosmology N-body simulations is close to $10^{8}M_{\odot}$, which is about $10^{65}$ times larger than the GeV scale expected in particle physics. However, self-gravity interacting particle systems made up of different particles mass have different statistical and dynamical properties. Here we demonstrate that, due to this particle mass difference, the nowaday cosmology N-body simulations can have introduced an excessive core collapse process, especially for the low mass halos at high redshift. Such dynamical effect introduces an excessive cuspy center for these small halos, and it implies a possible connection to the so called "small scale crisis" for CDM models. Our results show that there exist a physical limit in cosmological simulations, by using about $10^3$ particles to describe smallest halos, and we provide a simple suggestion based on it to relieve those effects from the bias.