Dark Halo Shapes and the Fate of Stellar Bars

Abstract

We investigate the stability of trajectories in barred galaxies with mildly triaxial halos by means of Liapunov exponents. This method is perfectly suitable for time-dependent 3D potentials where surfaces of sections and other simple diagnostics are not applicable. We find that when halos are centrally-concentrated most trajectories starting near the plane containing the bar become chaotic. Moreover, the shape of many of the remaining regular trajectories do not match the bar density distribution, being too round. Therefore, time-independent self-consistent solutions are highly unlikely to be found. When the non-rotating non-axisymmetric perturbation in the potential reaches 10%, almost all trajectories integrated are chaotic and have large Liapunov exponents. No regular trajectories aligned with the bar have been found. Hence, if the evolution of the density figure is directly related to the characteristic timescale of orbital instability, bar dissolution would take place on a timescale of few dynamical times. The slowly rotating non-axisymmetric contribution to the potential required for the onset of widespread chaotic behavior is remarkably small. Systems consisting of centrally-concentrated axisymmetric halos and stellar bars thus appear to be structurally unstable, and small (1%) deviations from perfect axisymmetry should result in a bar dissolution on a timescale significantly smaller than the Hubble time. Since halos found in CDM simulations of large scale structure are both centrally-concentrated and triaxial it is unlikely that stellar bars embedded in such halos would form and survive unless the halos are modified during the formation of the baryonic component.