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

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 s−1) 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.