Radial orbit instability in dwarf dark matter haloes

TL;DR

This study uses N-body simulations to investigate radial orbit instability in dwarf galaxy dark matter haloes, revealing how anisotropy influences shape, angular momentum, and orbital families without significantly altering density profiles.

Contribution

It provides new insights into the effects of anisotropic orbit distributions on the shape and dynamics of dwarf galaxy dark matter haloes through detailed simulation analysis.

Findings

Haloes become axisymmetric and prolate due to instability.

Density profiles remain largely unchanged.

Angular momentum exhibits large oscillations.

Abstract

Using N-body simulations we study the phenomenon of radial orbit instability occurring in dark matter haloes of the size of a dwarf galaxy. We carried out simulations of seven spherical models, with the same standard NFW density profile but different anisotropy profiles of particle orbits. Four of them underwent instability: two with a constant positive anisotropy, one with an anisotropic core and an isotropic envelope and one with a very small isotropic core and an anisotropic envelope. Haloes affected by the instability become approximately axisymmetric and prolate, with the profile of the shortest-to-longest axis ratio increasing with radius. The lower limit for the central value of this axis ratio is 0.3 for an NFW halo. The density profiles of the haloes did not change significantly, whereas the velocity distributions became axisymmetric. The total angular momentum rose due to large-amplitude oscillations of its components perpendicular to the symmetry axis of the halo. We also studied orbits of individual particles assigning them to classical orbit families in triaxial potentials. We find that the membership of a given particle in a family depends on its initial total angular momentum and its components along the principal axes of matter distribution.