Bars and Cold Dark Matter Halos

The central part of a dark matter halo reacts to the presence and evolutionof a bar. Not only does the halo absorb angular momentum from the disk, it canalso be compressed and have its shape modified. We study these issues in aseries of cosmologically motivated, highly resolved N-body simulations ofbarred galaxies run under different initial conditions. In all models we findthat the inner halo's central density increases. We model this density increaseusing the standard adiabatic approximation and the modified formula by Gnedinet al. and find that halo mass profiles are better reproduced by this latter.In models with a strong bar, the dark matter in the central region forms abar-like structure (``dark matter bar''), which rotates together with thenormal bar formed by the stellar component (``stellar bar''). Theminor-to-major axial ratio of a halo bar changes with radius with a typicalvalue 0.7 in the central disk region. DM bar amplitude is mostly a function ofthe stellar bar strength. Models in which the bar amplitude increases or staysroughly constant with time, initially large (40%-60%) misalignment between thehalo and disk bars quickly decreases with time as the bar grows. The halo baris nearly aligned with the stellar bar (~10 degrees lag for the halo) after ~2Gyr. The torque, which the halo bar exerts on the stellar bar, can serve as amechanism to regulate the angular momentum transfer from the disk to the halo.Comment: Modified version after referee's suggestions. 17 pages, 12 figures, accepted by Ap