Stellar Bar Evolution in Cuspy and Flat-cored Triaxial Cdm Halos

We analyze the formation and evolution of stellar bars in galactic disks embedded in mildly triaxial cold dark matter (CDM) halos that have density distributions ranging from large flat cores to cuspy profiles. We have applied tailored numerical simulations of analytical and live halos which include the feedback from disk/bar system onto the halo in order to test and extend earlier work by El-Zant & Shlosman (2002). The latter employed the method of Liapunov exponents to analyze the fate of bars in analytical asymmetric halos. We find the following: (1) The bar growth is very similar in all rigid axisymmetric and triaxial halos. (2) Bars in live models experience vertical buckling instability and the formation of a pseudo-bulge with a boxy/peanut shape, while bars in rigid halos do not buckle. (3) In live axisymmetric halos, the bar strength shows only marginal secular evolution, whether growth or decay. In such halos, the bar strength is larger for smaller disk-to-halo mass ratios D/H within disk radii, the bar size correlates with the halo core sizes, and the bar pattern speeds correlate with the halo central mass concentration. In contrast, bars embedded in live triaxial halos have a starkly different fate: they dissolve on a timescale of ∼ 1.5− 5 Gyr due to the onset of chaos over continuous zones, sometimes leaving behind a weak oval distortion. The onset of chaos is related to the halo triaxiality, the fast rotating bar and the halo cuspiness. Before the bar dissolves, the region outside it develops strong spiral structures, especially in the live triaxial halos. (4) More angular momentum is absorbed (fractionally) by the triaxial halos as compared to the axisymmetric models. (5) Cuspy halos are more susceptible than flat-core halos to having their prolateness washed out by the action of the bar. The subsequent evolution is then similar to the case of a cuspy axisymmetric halos where the bar survives and its strength shows only marginal secular evolution. We analyze the above results on disk and bar evolution in terms of the stability of trajectories and development of chaos in the system. We set important constraints on the triaxiality of DM halos by comparing our predictions to recent observational results on the properties of bars out to intermediate redshifts z ∼ 1. Subject headings: galaxies: evolution – galaxies: formation – galaxies: halos – galaxies: kinematics and dynamics – galaxies: structure – cosmology: dark matter