A counterpart to the radial-orbit instability in triaxial stellar systems

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 haloes are discussed.