Asymmetric velocity anisotropies in remnants of collisionless mergers

TL;DR

This paper investigates how the velocity anisotropy in dark matter halo remnants varies with direction, revealing a strong dependence linked to merger history, which explains diversity in simulation results.

Contribution

It introduces a directional analysis of velocity anisotropy in merger remnants, highlighting the impact of merger history on anisotropy profiles.

Findings

Velocity anisotropy varies significantly with direction.

Merger history influences anisotropy direction-dependence.

Explains diversity in outer halo velocity profiles.

Abstract

Dark matter haloes in cosmological N-body simulations are affected by processes such as mergers, accretion and the gravitational interaction with baryonic matter. Typically the analysis of dark matter haloes is performed in spherical or elliptical bins and the velocity distributions are often assumed to be constant within those bins. However, the velocity anisotropy, which describes differences between the radial and tangential velocity dispersion, has recently been show to have a strong dependence on direction in the triaxial halos formed in cosmological simulations. In this study we derive properties of particles in cones parallel or perpendicular to the collision axis of merger remnants. We find that the velocity anisotropy has a strong dependence on direction. The finding that the direction-dependence of the velocity anisotropy of a halo depends on the merger history, explains the existence of such trends in cosmological simulations. It also explains why a large diversity is seen in the velocity anisotropy profiles in the outer parts of high-resolution simulations of cosmological haloes.