Buckling instability in tidally induced galactic bars
  Strong galactic bars produced in simulations tend to undergo a period of
buckling instability that weakens and thickens them and forms a boxy/peanut
structure in their central parts. This theoretical prediction has been
confirmed by identifying such morphologies in real galaxies. The nature and
origin of this instability remains however poorly understood with some studies
claiming it to be due to fire-hose instability while others relating it to
vertical instability of stellar orbits supporting the bar. One of the channels
for the formation of galactic bars is via the interaction of disky galaxies
with perturbers of significant mass. Tidally induced bars offer a unique
possibility of studying buckling instability because their formation can be
controlled by changing the strength of the interaction while keeping the
initial structure of the galaxy the same. We use a set of four simulations of
flyby interactions where a galaxy on a prograde orbit forms a bar, which is
stronger for stronger tidal forces. We study their buckling by calculating
different kinematic signatures, including profiles of the mean velocity in
vertical direction, as well as distortions of the bars out of the disk plane.
Although our two strongest bars buckle most strongly, there is no direct
relation between the ratio of vertical to horizontal velocity dispersion and
the bar's susceptibility to buckling, as required by the fire-hose instability
interpretation. While our weakest bar buckles, a stronger one does not, its
dispersion ratio remains low and it grows to become the strongest of all at the
end of evolution. Instead, we find that during buckling the resonance between
the vertical and radial orbital frequencies becomes wide and therefore able to
modify stellar orbits over a significant range of radii. We conclude that the
vertical orbital instability is the more plausible explanation for the origin
of buckling.
