The Destruction of Bars by Central Mass Concentrations

More than two thirds of disk galaxies are barred to some degree. Many todayharbor massive concentrations of gas in their centers, and some are known topossess supermassive black holes (SMBHs) and their associated stellar cusps.Previous theoretical work has suggested that a bar in a galaxy could bedissolved by the formation of a mass concentration in the center, although theprecise mass and degree of central concentration required is notwell-established. We report an extensive study of the effects of central masseson bars in high-quality N-body simulations of galaxies. We have varied thegrowth rate of the central mass, its final mass and degree of concentration toexamine how these factors affect the evolution of the bar. Our main conclusionsare: (1) Bars are more robust than previously thought. The central mass has tobe as large as several percent of the disk mass to completely destroy the baron a short timescale. (2) For a given mass, dense objects cause the greatestreduction in bar amplitude, while significantly more diffuse objects have alesser effect. (3) The bar amplitude always decreases as the central mass isgrown, and continues to decay thereafter on a cosmological time-scale. (4) Thefirst phase of bar-weakening is due to the destruction by the CMC oflower-energy, bar-supporting orbits, while the second phase is a consequence ofsecular changes to the global potential which further diminish the number ofbar-supporting orbits. We provide detailed phase-space and orbit analysis tosupport this suggestion. Thus current masses of SMBHs are probably too small,even when dressed with a stellar cusp, to affect the bar in their hostgalaxies. The molecular gas concentrations found in some barred galaxies arealso too diffuse to affect the amplitude of the bar significantly.