The effects of bar-spiral coupling on stellar kinematics in the Galaxy

TL;DR

This study models the Milky Way's disc considering both the bar and spiral arms, showing their combined effects on stellar motions, resonance overlaps, and vertical breathing modes, with implications for interpreting spectroscopic survey data.

Contribution

It demonstrates how the combined influence of the Galactic bar and spiral arms affects stellar kinematics, resonance interactions, and vertical motions, providing insights for disentangling their individual effects.

Findings

Mean radial motions are mainly influenced by the bar when it is strong.

Resonance overlaps enhance stellar churning within the disc.

Combined effects produce non-linear superpositions in vertical breathing modes.

Abstract

We investigate models of the Milky Way disc taking into account simultaneously the bar and a two-armed quasi-static spiral pattern. Away from major resonance overlaps, the mean stellar radial motions in the plane are essentially a linear superposition of the isolated effects of the bar and spirals. Thus, provided the bar is strong enough, even in the presence of spiral arms, these mean radial motions are predominantly affected by the Galactic bar for large scale velocity fluctuations. This is evident when comparing the peculiar line-of-sight velocity power spectrum of our coupled models with bar-only models. However, we show how forthcoming spectroscopic surveys could disentangle bar-only non-axisymmetric models of the Galaxy from models in which spiral arms have a significant amplitude. We also point out that overlaps of low-order resonances are sufficient to enhance stellar churning within the disc, even when the spirals amplitude is kept constant. Nevertheless, for churning to be truly non-local, stronger or (more likely) transient amplitudes would be needed: otherwise the disc is actually mostly unaffected by churning in the present models. Finally, regarding vertical breathing modes, the combined effect of the bar and spirals on vertical motions is a clear non-linear superposition of the isolated effects of both components, significantly superseding the linear superposition of modes produced by each perturber separately, thereby providing an additional effect to consider when analysing the observed breathing mode of the Galactic disc in the extended Solar neighbourhood.