Effects of Inner Halo Angular Momentum on the Peanut/X Shapes of Bars

Abstract

Cosmological simulations show that dark matter halos surrounding baryonic disks have a wide range of angular momenta, which is measured by the spin parameter (λ). In this study, we bring out the importance of inner angular momentum (<30 kpc), measured in terms of the halo spin parameter, on the secular evolution of the bar using N-body simulations. We have varied the halo spin parameter λ from 0 to 0.1, for corotating (prograde) spinning halos and one counterrotating (retrograde) halo spin (λ = −0.1) with respect to the disk. We report that as the halo spin increases the buckling is also triggered earlier and is followed by a second buckling phase in high-spin halo models. The timescale for the second buckling is significantly longer than the first buckling. We find that bar strength does not reduce significantly after the buckling in all of our models, which provides new insights about the role of inner halo angular momentum unlike previous studies. Also, the buckled bar can still transfer significant angular momentum to the halo in the secular evolution phase, but it reduces with increasing halo spin. In the secular evolution phase, the bar strength increases and saturates to nearly equal values for all the models irrespective of halo spin and the sense of rotation with respect to the disk. The final boxy/peanut shape is more pronounced (∼20%) in high-spin halos having higher angular momentum in the inner region compared to nonrotating halos. We explain our results with angular momentum exchanges between the disk and halo.