The Effect of Bulge Mass on Bar Pattern Speed in Disk Galaxies

Abstract

We present a study of the effect of bulge mass on the evolution of bar pattern speed in isolated disk galaxies using N -body simulations. Earlier studies have shown that disk stars at the inner resonances can transfer a signi fi cant amount of angular momentum to the dark matter halo and this results in the slowdown of the bar pattern speed. In this paper we investigate how the mass of the other spheroidal component, the bulge, affects bar pattern speeds. In our galaxy models the initial bars are all rotating fast as the

parameter. The ratio of the corotation radius to bar radius is less than 1.4, which is typical of fast bars. However, as the galaxies evolve with time, the bar pattern speed ( Ω p ) slows down, leading to >  1. 4 for all the models except for the model with the most massive bulge, in which the bar formed late and did not have time to evolve. The rapid slowdown of Ω p is due to the larger angular momentum transfer from the disk to the bulge and to interactions between stars at the inner resonances and those in the bar. Hence we conclude that the decrease in Ω p clearly depends on bulge mass in barred galaxies and decreases faster for galaxies with more massive bulges. We discuss the implications of our results for observations of bar pattern speeds in galaxies.