The destruction of bars by central mass concentrations

More than two-thirds of disk galaxies are barred to some degree. Many today harbor massive concentrations of gas in their centers, and some are known to possess supermassive black holes (SMBHs) and their associated stellar cusps. Previous theoretical work has suggested that a bar in a galaxy could be dissolved by the formation of a mass concentration in the center, although the precise mass and degree of central concentration required is not well established. We report an extensive study of the effects of central masses on bars in high-quality N-body simulations of galaxies. We have varied the growth rate of the central mass, its final mass, and its degree of concentration to examine how these factors affect the evolution of the bar. Our main conclusions are the following: ( 1) Bars are more robust than previously thought. The central mass has to be as large as several percent of the disk mass to completely destroy the bar on a short timescale. ( 2) For a given mass, dense objects cause the greatest reduction in bar amplitude, while significantly more diffuse objects have a lesser effect. ( 3) The bar amplitude always decreases as the central mass is grown and continues to decay thereafter on a cosmological timescale. ( 4) The first phase of bar weakening is due to the destruction by the central mass concentration (CMC) of lower energy, bar-supporting orbits, while the second phase is a consequence of secular changes to the global potential that further diminish the number of bar-supporting orbits. We provide detailed phase-space and orbit analysis to support this suggestion. Thus, current masses of SMBHs are probably too small, even when dressed with a stellar cusp, to affect the bars in their host galaxies. The molecular gas concentrations found in some barred galaxies are also too diffuse to affect the amplitude of the bar significantly. These findings reconcile the apparent high percentage of barred galaxies with the presence of CMCs and have important implications for the formation and survival of bars in such galaxies.