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 halos
are discussed.
