The behaviour of shape and velocity anisotropy in dark matter haloes

TL;DR

This study investigates how the shape and velocity anisotropy of dark matter haloes vary with direction, revealing complex behaviors and emphasizing the importance of cone-based analysis over spherical averages.

Contribution

It demonstrates the directional dependence of velocity anisotropy in dark matter haloes using non-cosmological simulations, highlighting the limitations of spherical averaging.

Findings

Elongated haloes exhibit distinct velocity anisotropies.

Halo behavior can be categorized into three groups based on anisotropy profiles.

Cone-based analysis uncovers hidden information not seen in spherical averages.

Abstract

Dark matter haloes from cosmological N-body simulations typically have triaxial shapes and anisotropic velocity distributions. Recently it has been shown that the velocity anisotropy, beta, of cosmological haloes and major merger remnants depends on direction in such a way that beta is largest along the major axis and smallest along the minor axis. In this work we use a wide range of non-cosmological N-body simulations to examine halo shapes and direction-dependence of velocity anisotropy profiles. For each of our simulated haloes we define 48 cones pointing in different directions, and from the particles inside each cone we compute velocity anisotropy profiles. We find that elongated haloes can have very distinct velocity anisotropies. We group the behaviour of haloes into three different categories, that range from spherically symmetric profiles to a much more complex behaviour, where significant differences are found for beta along the major and minor axes. We encourage future studies of velocity anisotropies in haloes from cosmological simulations to calculate beta-profiles in cones, since it reveals information, which is hidden from a spherically averaged profile. Finally, we show that spherically averaged profiles often obey a linear relation between beta and the logarithmic density slope in the inner parts of haloes, but this relation is not necessarily obeyed, when properties are calculated in cones.