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

Abstract

Using high resolution N-body simulations of stellar disks embedded in cosmologically motivated dark matter halos, we study the evolution of bars and the transfer of angular momentum between halos 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 Gyrs of evolution the stellar component loses only 5% – 7% of its initial angular momentum. Mass and force resolutions are crucial for the modeling 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 which are fast rotators, where the ratio of the corotation radius to the bar major semi-axis lies in the range R = 1.2 1.7, 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 major semi-axis is close to the exponential length of the disk. The transfer of angular momentum between inner and outer parts of the disk plays a very important role in the secular evolution of the disk and the bar. The bar formation increases the exponential length of the disk by a factor of 1.2 -1.5. The transfer substantially increases the stellar mass in the center of the galaxy and decreases the dark matter-to-baryons ratio. As the result, the central 2 kpc region is always strongly dominated by the baryonic component. At intermediate (3 – 10 kpc) scales the disk 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.