The Destruction of Bars by Central Mass Concentrations

Abstract

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 degree of concentration to examine how these factors affect the evolution of the bar. Our main conclusions are: (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 time-scale. (4) The first phase of bar-weakening is due to the destruction by the CMC of lower-energy, bar-supporting orbits, while the second phase is a consequence of secular changes to the global potential which 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 bar 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.Comment: AASTeX v5.0 preprint; 44 pages, including 1 table and 16 figures. To appear in ApJ. High resolution version can be found at http://www.physics.rutgers.edu/~shen/bar_destruct/paper_high_res.pd

Boxy/Peanut/X-shaped Bulges: Steep Inner Rotation Curve Leads to Barlens Face-on Morphology

“…In Debattista et al (2004), the problem of not attaining round bar-related bulges in simulations was acknowledged, and it was speculated that extended central objects with = B D 0.1 0.2 -might be needed, based on the simulations of Shen & Sellwood (2004) that studied the robustness of bars against central black hole masses. In Shen & Sellwood (2004) simulations, a rigid halo was assumed; however, very similar conclusions were reached in Athanassoula et al (2005) using live halos. Our current simulations demonstrate that a substantially smaller B/D (of a few percents)is sufficient, provided that realistic, small, effective bulge radii from observations (  r h 10% r eff ) are adopted.…”

A Unified Framework for the Orbital Structure of Bars and Triaxial Ellipsoids

“…For two-dimensional bars it is customary to construct an SoS by plotting the velocity component V y as a function of y every time an orbit intersects the x-axis with a negative value of V x (e.g., Sellwood & Wilkinson 1993). In three-dimensional bars one might choose to restrict orbits plotted on an SoS to those that are confined to the equatorial (i.e., x-y) plane of the model (e.g., Shen & Sellwood 2004). When orbits of a single value of E J are plotted on an SoS, regular orbits follow thin closed or broken curves, resonant orbits form groups of islands, while chaotic orbits tend to fill larger areas.…”

The Fueling and Evolution of AGN: Internal and External Triggers

“…Within this framework, bars can dissolve and reform recurrently if the galaxy accretes sufficient cold gas over a Hubble time. In fact, Shen & Sellwood (2004) find that in purely stellar Nbody simulations, the bar is quite robust to CMCs. Ongoing studies (Jogee et al 2004a,b) of the bar properties and CMCs of galaxies over lookback times of 9 Gyr (out to z ∼ 1), based on the GEMS survey, will help provide discriminant tests on the evolution and lifetime of bars out to z ∼ 1.…”