The Dynamical State of Dark Matter Haloes in Cosmological Simulations I: Correlations with Mass Assembly History

TL;DR

This study investigates how a dark matter halo's formation history influences its dynamical state, using simulations to identify reliable indicators like the centre of mass offset for assessing relaxation status.

Contribution

It demonstrates that the centre of mass offset is a more robust measure of halo dynamical state than the virial ratio, linking recent accretion history to relaxation indicators.

Findings

Massive, recently formed haloes are less relaxed.

Centre of mass offset correlates strongly with recent accretion.

Delta r < 0.04 effectively identifies relaxed haloes at z=0.

Abstract

Using a statistical sample of dark matter haloes drawn from a suite of cosmological N-body simulations of the Cold Dark Matter (CDM) model, we quantify the impact of a simulated halo's mass accretion and merging history on two commonly used measures of its dynamical state, the virial ratio eta and the centre of mass offset Delta r. Quantifying this relationship is important because the degree to which a halo is dynamically equilibrated will influence the reliability with which we can measure characteristic equilibrium properties of the structure and kinematics of a population of haloes. We begin by verifying that a halo's formation redshift zform correlates with its virial mass Mvir and we show that the fraction of its recently accreted mass and the likelihood of it having experienced a recent major merger increases with increasing Mvir and decreasing zform. We then show that both eta and Delta r increase with increasing Mvir and decreasing zform, which implies that massive recently formed haloes are more likely to be dynamically unrelaxed than their less massive and older counterparts. Our analysis shows that both eta and Delta r are good indicators of a halo's dynamical state, showing strong positive correlations with recent mass accretion and merging activity, but we argue that Delta r provides a more robust and better defined measure of dynamical state for use in cosmological N-body simulations at z~0. We find that Delta r < 0.04 is sufficient to pick out dynamically relaxed haloes at z=0. Finally, we assess our results in the context of previous studies, and consider their observational implications.