Meridional tilt of the stellar velocity ellipsoid during bar buckling instability

TL;DR

This study uses N-body simulations to link the maximum meridional tilt of the stellar velocity ellipsoid with the onset of bar buckling instability, providing new insights into galaxy evolution and bulge formation.

Contribution

It demonstrates that the peak of the meridional tilt correlates with bar buckling onset, offering a new diagnostic for gravitational instability in barred galaxies.

Findings

Maximum meridional tilt coincides with bar buckling onset

Meridional shear stress peaks during bulge formation

Provides observational diagnostics for buckling instability

Abstract

The structure and evolution of the stellar velocity ellipsoid plays an important role in shaping galaxies undergoing bar driven secular evolution and the eventual formation of a boxy/peanut bulge such as present in the Milky Way. Using collisionless N-body simulations, we show that during the formation of such a boxy/peanut bulge, the meridional shear stress of stars, which can be measured by the meridional tilt of the velocity ellipsoid, reaches a characteristic peak in its time evolution. It is shown that the onset of a bar buckling instability is closely connected to the maximum meridional tilt of the stellar velocity ellipsoid. Our findings bring new insight to this complex gravitational instability of the bar which complements the buckling instability studies based on orbital models. We briefly discuss the observed diagnostics of the stellar velocity ellipsoid during such a phenomenon.