Nature and Nurture in Dark Matter Halos

TL;DR

This paper explores the origins and evolution of dark matter halos, contrasting the roles of primordial structure accretion ('Nature') and dynamical relaxation ('Nurture'), and proposes models for central relaxation mechanisms.

Contribution

It introduces a simple self-similar cascade model for halo relaxation and compares it with primordial accretion models, highlighting the presence of weak relaxation in simulations.

Findings

Weak relaxation observed in simulations, often described as radial orbit instability.

Scale-free sub-structure of halos is essential to both formation and relaxation mechanisms.

Models suggest a combination of primordial accretion and dynamical relaxation shapes halo centers.

Abstract

Cosmological simulations consistently predict specific properties of dark matter halos, but these have not yet led to a physical understanding that is generally accepted. This is especially true for the central regions of these structures. Recently two major themes have emerged. In one, the dark matter halo is primarily a result of the sequential accretion of primordial structure (ie `Nature'); while in the other, dynamical relaxation (ie `Nurture') dominates at least in the central regions. Some relaxation is however required in either mechanism. In this paper we accept the recently established scale-free sub-structure of halos as an essential part of both mechanisms. Consequently; a simple model for the central relaxation based on a self-similar cascade of tidal interactions, is contrasted with a model based on the accretion of adiabatically self-similar, primordial structure. We conclude that a weak form of this relaxation is present in the simulations, but that is normally described as the radial orbit instability.