Suppression of Stellar Bar Buckling by Supermassive Black Hole Growth

Using N-body simulations we study the effects of growing a central mass concentration representing a supermassive black hole (SMBH) in a disc galaxy unstable to bar formation. The final SMBH has the same mass in all simulations (0.14% of the disk mass) but the start time of SMBH growth and the duration over which growth occurs is varied. Previous simulations have shown that when a SMBH is grown after the formation of a bar, the bar is weakened. In this previous work the bar always buckled and transients were allowed to dissipate before SMBH growth. As expected and consistent with previous work, in simulations where the SMBH starts to grow after the bar has buckled, the bar is weakened. However, we find that when the SMBH is introduced early such that it co-evolves with the growing bar, the growing SMBH strongly suppresses bar buckling. This leads to a bar that is stronger than the bar in the control simulation (without a SMBH) and significantly stronger than the bar in which the SMBH grows after the bar forms. We find that the buckling instability is suppressed by a decreasing velocity anisotropy characterized by an increasing ratio of out of plane to radial velocity dispersion, Σ_z / Σ_r. The growing black hole causes this ratio to increase in the central regions relative to the control simulation. This increase occurs over a much longer period compared to the rapid increase that coincides with buckling in the control simulation. Interestingly, we also find that contrary to previous expectations that bar buckling is primarily responsible for the formation of X-shaped box/peanut bulges, bars which co-evolve with growing SMBHs buckle very little, yet they have significantly more prominent X-shapes. This work was funded in part by the National Science Foundation (grants NSF-AST-1515001, NSF-AST-2009122), and the Space Telescope Science Institute (grant JWST-ERS-01364.002-A).