Buckling instability in galactic bars (Ewa L. Lokas, 16/7/2024)

Buckling instability in galactic bars
Ewa L. Lokas
Nicolaus Copernicus Astronomical Center, Warsaw
Abstract:
Many simulated galactic bars experience a period of buckling when their bar is distorted vertically out of the disk plane. The nature of buckling instability
remains unclear and it is still debated if it results from trapping x1 orbits
of the bar at vertical resonances or is rather related to the fire-hose
instability and controlled by the ratio of the vertical to horizontal velocity
dispersion of the stars in the bar. I will describe the results of the studies
of the buckling instability performed with controlled N-body/hydro simulations of a bar forming in an isolated Milky Way-like galaxy. The properties of the bar at the time of buckling are quantified in terms of the mean acceleration, velocity and distortion in the vertical direction. The maps of these quantities in face-on projections reveal characteristic quadrupole patterns which wind up over a short timescale. From the study of the orbital structure of the bar in periods before and after buckling I find that most of the buckling orbits originate from x1 orbits supporting the bar. During buckling the ratio of the vertical to horizontal frequency of stellar orbits decreases dramatically and after buckling the orbits obey a very tight relation between the vertical and circular frequency. I propose that buckling is initiated by the vertical resonance of the x1 orbits creating the initial distortion of the bar that later evolves as kinematic bending waves. I will also discuss the effect of the gas component on buckling and the buckling phenomenon in tidally induced bars.