Dark Halo Shapes and the Fate of Stellar Bars

We investigate the stability of trajectories in barred galaxies with mildlytriaxial halos by means of Liapunov exponents. This method is perfectlysuitable for time-dependent 3D potentials where surfaces of sections and othersimple diagnostics are not applicable. We find that when halos arecentrally-concentrated most trajectories starting near the plane containing thebar become chaotic. Moreover, the shape of many of the remaining regulartrajectories do not match the bar density distribution, being too round.Therefore, time-independent self-consistent solutions are highly unlikely to befound. When the non-rotating non-axisymmetric perturbation in the potentialreaches 10%, almost all trajectories integrated are chaotic and have largeLiapunov exponents. No regular trajectories aligned with the bar have beenfound. Hence, if the evolution of the density figure is directly related to thecharacteristic timescale of orbital instability, bar dissolution would takeplace on a timescale of few dynamical times. The slowly rotatingnon-axisymmetric contribution to the potential required for the onset ofwidespread chaotic behavior is remarkably small. Systems consisting ofcentrally-concentrated axisymmetric halos and stellar bars thus appear to bestructurally unstable, and small (1%) deviations from perfect axisymmetryshould result in a bar dissolution on a timescale significantly smaller thanthe Hubble time. Since halos found in CDM simulations of large scale structureare both centrally-concentrated and triaxial it is unlikely that stellar barsembedded in such halos would form and survive unless the halos are modifiedduring the formation of the baryonic component.Comment: 28 pp., 17 postscript figures. Submitted to Astrophysical Journa