Bars and Cold Dark Matter Halos

Abstract

The central part of a dark matter halo reacts to the presence and evolution of a bar. Not only does the halo absorb angular momentum from the disk, it can also be compressed and have its shape modified. We study these issues in a series of cosmologically motivated, highly resolved N-body simulations of barred galaxies run under different initial conditions. In all models we find that the inner halo's central density increases. We model this density increase using the standard adiabatic approximation and the modified formula by Gnedin et al., and we find that halo mass profiles are better reproduced by the latter. In models with a strong bar, the dark matter in the central region forms a barlike structure (``dark matter bar''), which rotates together with the normal bar formed by the stellar component (``stellar bar''). The minor-to-major axial ratio of a halo bar changes with radius with a typical value 0.7 in the central disk region. Dark matter bar amplitude is mostly a function of the stellar bar strength. For models in which the bar amplitude increases or stays roughly constant with time, initially large (40°-60°) misalignment between the halo and disk bars quickly decreases with time as the bar grows. The halo bar is nearly aligned with the stellar bar (~10° lag for the halo) after ~2 Gyr. The torque, which the halo bar exerts on the stellar bar, can serve as a mechanism to regulate the angular momentum transfer from the disk to the halo.