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 selfconsistency, 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.

Publication Date



This is a pre-copyedited, author-produced version of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review.

F. Antonini, R. Capuzzo-Dolcetta, D. Merritt; A counterpart to the radial-orbit instability in triaxial stellar systems, Monthly Notices of the Royal Astronomical Society, Volume 399, Issue 2, 21 October 2009, Pages 671–682

The version of record is available online at: https://doi.org/10.1111/j.1365-2966.2009.15342.x

Also archived in: arXiv:0901.4485 v1 Jan 28, 2009

Note: imported from RIT’s Digital Media Library running on DSpace to RIT Scholar Works in February 2014.

Document Type


Department, Program, or Center

School of Physics and Astronomy (COS)


RIT – Main Campus