A Counterpart to the Radial Orbit Instability in Triaxial Stellar Systems

TL;DR

This paper identifies a new instability in triaxial galaxy models caused by an excess of radial orbits, leading to shape transformation, with implications for dark matter halo structures.

Contribution

It demonstrates the existence of a radial-orbit instability in triaxial models and links it to the proportion of box orbits, providing insights into galaxy stability.

Findings

Instability occurs with high radial orbit fraction.

Evolution leads to more prolate, triaxial shapes.

Reducing radial orbits stabilizes models.

Abstract

Self-consistent solutions for triaxial mass models are highly non-unique. In general, some of these solutions might be dynamically unstable, making them inappropriate as descriptions of steady-state galaxies. Here we demonstrate for the first time the existence in triaxial galaxy models of an instability similar to the radial-orbit instability of spherical models. The instability manifests itself when the number of box orbits, with predominantly radially motions, is sufficiently large. N-body simulations verify that the evolution is due neither to chaotic orbits nor to departures of the model from self-consistency, but rather to a collective mode. The instability transforms the triaxial model into a more prolate, but still triaxial, configuration. Stable triaxial models are obtained when the mass contribution of radial orbits is reduced. The implications of our results for the shapes of dark-matter halos are discussed.