Spontaneous generation of bending waves in isolated Milky Way-like discs

TL;DR

This study demonstrates that bending waves in Milky Way-like galaxy discs can spontaneously form from shot noise, persist for billions of years, and are self-gravitating, with implications for understanding galactic warps and vertical structures.

Contribution

It provides the first detailed analysis showing that bending waves are self-gravitating, long-lived, and originate from internal noise, advancing the understanding of galactic disc dynamics.

Findings

Bending waves form rapidly from shot noise and persist for billions of years.

Waves are dominated by the m=1 azimuthal component and are self-gravitating.

Power in wave modes migrates outward, supporting an inside-out warp formation scenario.

Abstract

We study the spontaneous generation and evolution of bending waves in $N$-body simulations of two isolated Milky Way-like galaxy models. The models differ by their disc-to-halo mass ratios, and hence by their susceptibility to the formation of a bar and spiral structure. Seeded from shot noise in the particle distribution, bending waves rapidly form in both models and persist for many billions of years. Waves at intermediate radii manifest as corrugated structures in vertical position and velocity that are tightly wound, morphologically leading, and dominated by the $m=1$ azimuthal Fourier component. A spectral analysis of the waves suggests they are a superposition of modes from two continuous branches in the Galactocentric radius-rotational frequency plane. The lower-frequency branch is dominant and is responsible for the corrugated, leading, and warped structure. Over time, power in this branch migrates outward, lending credence to an inside-out formation scenario for the warp. Our power spectra qualitatively agree with results from linear perturbation theory and a WKB analysis, both of which include self-gravity. Thus, we conclude that the waves in our simulations are self-gravitating and not purely kinematic. These waves are reminiscent of the wave-like pattern recently found in Galactic star counts from the Sloan Digital Sky Survey and smoothly transition to a warp near the disc's edge. Velocity measurements from \textit{Gaia} data will be instrumental in testing the true wave nature of the corrugations. We also compile a list of "minimum requirements" needed to observe bending waves in external galaxies.