Secular bar formation in galaxies with a significant amount of dark matter

Using high-resolution N-body simulations of stellar discs embedded in cosmologically motivated dark matter haloes, we study the evolution of bars and the transfer of angular momentum between haloes and bars. We find that dynamical friction results in some transfer of angular momentum to the halo, but the effect is much smaller than previously found in low-resolution simulations and is incompatible with early analytical estimates. After 5 Gyr of evolution the stellar component loses only 5-7 per cent of its initial angular momentum.

Mass and force resolutions are crucial for the modelling of bar dynamics. In low-resolution (300-500 pc) simulations we find that the bar slows down and angular momentum is lost relatively fast. In simulations with millions of particles reaching a resolution of 20-40 pc, the pattern speed may not change over billions of years. Our high-resolution models produce bars that are fast rotators, where the ratio of the corotation radius to the bar semi-major axis lies in the range , marginally compatible with observational results. In contrast to many previous simulations, we find that bars are relatively short. As in many observed cases, the bar semi-major axis is close to the exponential length of the disc.

The transfer of angular momentum between inner and outer parts of the disc plays a very important role in the secular evolution of the disc and the bar. The bar formation increases the exponential length of the disc by a factor of 1.2-1.5. The transfer substantially increases the stellar mass in the centre of the galaxy and decreases the dark matter to baryon ratio. As the result, the central 2-kpc region is always strongly dominated by the baryonic component. At intermediate (3-10 kpc) scales the disc is sub-dominant. These models demonstrate that the efficiency of angular momentum transfer to the dark matter has been greatly overestimated. More realistic models produce bar structure in striking agreement with observational results.