An instability in triaxial stellar systems

The radial-orbit instability is a collective phenomenon that has heretofore only been observed in spherical systems. We find that this instability occurs also in triaxial systems, as we checked by performing extensive N-body simulations whose initial conditions were obtained by sampling a self-consistent triaxial model of a cuspy galaxy composed of luminous and dark matter (Capuzzo-Dolcetta et al. 2007). N-body simulations show a time evolution of the galaxy that is not due to the development of chaotic motions but, rather, to the collective instability induced by an excess of radially biased (box) orbits. The instability quickly transforms such model into a more prolate configuration, with 0.64 < b/a < 0.77 and 0.6 < c/a < 0.7 for the dark halo and 0.64 < b/a < 0.77 and 0.59 < c/a < 0.67 for the luminous matter. Stable triaxial, cuspy galaxies with dark matter halo are obtained when the mass contribution of radially biased orbits to the solution is reduced. These results constitute the first evidence of radial orbit instability in triaxial stellar systems. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim