# The Origin of Buckling Instability in Galactic Bars: Searching for the
Scapegoat

Xingchen Li, Isaac Shlosman, Daniel Pfenniger, Clayton Heller

Submitted on 2 November 2022

## Abstract

The buckling process in stellar bars is full of unsolved issues. We analyze
the origin of the buckling instability in stellar bars using high-resolution
N-body simulations. Previous studies have promoted the nonresonant firehose
instability to be responsible for the vertical buckling. We have analyzed the
buckling process in terms of the resonant excitation of stellar orbits in the
bar, which pumps energy into vertical oscillations. We find that (1) the
buckling is associated with an abrupt increase in the central mass
concentration and triggers velocities along the bar and along its rotation
axis. The velocity field projected on one of the main axes forms circulation
cells and increases vorticity, which are absent in firehose instability; (2)
The bending amplitude is nonlinear when measured by isodensity contours or
curvature of the Laplace plane, which has a substantial effect on the stellar
motions; (3) In the linear description, the planar and vertical 2:1 resonances
appear only with the buckling and quickly reach the overlapping phase, thus
supporting the energy transfer; (4) Using nonlinear orbit analysis, we analyze
the stellar oscillations along the bar and along the rotation axis 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% 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 provide the necessary ingredient 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
firehose instability should be reevaluated. Finally, we discuss some
observational implications of buckling.