The vertical effects of disc non-axisymmetries from perturbation theory: the case of the Galactic bar

TL;DR

This paper develops an analytical framework using perturbation theory to compute vertical stellar motions caused by internal galactic structures like bars and spirals, comparing predictions with simulations and discussing implications for observed vertical motions.

Contribution

It introduces a general analytical method to estimate vertical bulk motions from internal non-axisymmetric perturbations in galactic discs, validated against numerical simulations.

Findings

Predicted vertical motions from the Galactic bar are small, less than 1 km/sec.

Analytical results align well with numerical simulations near the galactic plane.

Bar-induced motions alone cannot fully explain the observed breathing mode.

Abstract

Evidence for non-zero mean stellar velocities in the direction perpendicular to the Galactic plane has been accumulating from various recent large spectroscopic surveys. Previous analytical and numerical work has shown that a "breathing mode" of the Galactic disc, similar to what is observed in the Solar vicinity, can be the natural consequence of a non-axisymmetric internal perturbation of the disc. Here we provide a general analytical framework, in the context of perturbation theory, allowing us to compute the vertical bulk motions generated by a single internal perturber (bar or spiral pattern). In the case of the Galactic bar, we show that these analytically predicted bulk motions are well in line with the outcome of a numerical simulation. The mean vertical motions induced by the Milky Way bar are small (mean velocity of less than 1 km/sec) and cannot be responsible alone for the observed breathing mode, but they are existing. Our analytical treatment is valid close to the plane for all the non-axisymmetric perturbations of the disc that can be described by small-amplitude Fourier modes. Further work should study how the coupling of multiple internal perturbers and external perturbers is affecting the present analytical results.