The origin of buckling instability in galactic bars: Searching for the scapegoat

Abstract

ABSTRACT We analyse the origin of the buckling instability in stellar bars using high-resolution N-body simulations. Previous studies have promoted the non-resonant firehose instability to be responsible for the vertical buckling. We have analysed the buckling process following the resonant excitation of stellar orbits in the bar. We find that (1) the buckling is associated with an abrupt increase in the central mass concentration and triggers velocities within the bar. The velocity field forms circulation cells, increasing vorticity, which is absent in classical firehose instability; (2) The bending amplitude is non-linear when measured by isodensity contours or curvature of the Laplace plane, which has a substantial effect on the stellar motions; (3) The planar and vertical 2:1 resonances appear only with the buckling and quickly reach the overlapping phase, thus supporting the energy transfer from horizontal to vertical motions; (4) Using non-linear orbit analysis, we analyse the stellar oscillations and find that stars cross the vertical 2:1 resonance simultaneously with the buckling. The overlapping planar and vertical 2:1 resonances trapping more than 25 per cent of the bar particles provide the ‘smoking gun’ pointing to a close relationship between the bending of stellar orbits and the resonant action – these particles are assuring the cohesive response in the growing vertical asymmetry. We conclude that resonant excitation is important in triggering the buckling instability, and the contribution from the non-resonant firehose instability should be re-evaluated. Finally, we discuss some observational implications of buckling on the kinematics in face-on and inclined galactic discs.