Bending Waves in the Milky Way's disc from halo substructure

TL;DR

This study uses N-body simulations to analyze how halo substructure excites bending waves in the Milky Way's disc, revealing that such waves are more vigorous in thin discs and can be described as superpositions of resonant waves.

Contribution

It demonstrates that halo substructure significantly influences bending wave excitation and provides a detailed Fourier analysis linking these waves to vertical resonances.

Findings

Bending waves are stronger in the thin disc than in the thick disc.

Waves are most prominent in the outer regions of the disc.

Simulation results suggest waves oscillate as simple plane waves.

Abstract

We use $N$-body simulations to investigate the excitation of bending waves in a Milky Way-like disc-bulge-halo system. The dark matter halo consists of a smooth component and a population of subhaloes while the disc is composed of thin and thick components. Also considered is a control simulation where all of the halo mass is smoothly distributed. We find that bending waves are more vigorously excited in the thin disc than the thick one and that they are strongest in the outer regions of the disc, especially at late times. By way of a Fourier decomposition, we find that the complicated pattern of bending across the disc can be described as a superposition of waves, which concentrate along two branches in the radius-rotational frequency plane. These branches correspond to vertical resonance curves as predicted by a WKB analysis. Bending waves in the simulation with substructure have a higher amplitude than those in the smooth-halo simulation, though the frequency-radius characteristics of the waves in the two simulations are very similar. A cross correlation analysis of vertical displacement and bulk vertical velocity suggests that the waves oscillate largely as simple plane waves. We suggest that the wave-like features in astrometric surveys such as the Second Data Release from \textit{Gaia} may be due to long-lived waves of a dynamically active disc rather than, or in addition to, perturbations from a recent satellite-disc encounter.