Swing amplification in star-gas disks

Abstract

Recent JWST and ALMA observations have revealed stellar bars and spirals in gas-rich galactic disks at redshifts as high as $z\simeq 4$. The simplest theoretical paradigm we have for understanding such non-axisymmetric features is the linear theory of swing amplification (SA) in the 2D shearing sheet. However, while the SA mechanism in a gaseous shearing sheet was first studied in 1965, and that in a collisionless stellar sheet in 1966, the coupled star-gas linear SA equations have never been solved explicitly. Here we write down these equations and use them to study the evolution of non-axisymmetric swinging waves in stable disks. We find that waves are often amplified temporarily by factors of $10-100$ or more, and that the maximum \textit{non-axisymmetric} amplification factor is tightly correlated with the system's distance from the \textit{axisymmetric} stability boundary in the $(1/Q_{\mathrm{s}},1/Q_{\mathrm{g}})$ plane. The true star-gas behavior differs significantly from the `two-fluid' idealizations used in the past, because phase mixing of the collisionless stellar component acts as a sink of perturbation energy. Closely analogous results hold for finite-thickness disks except for the shifting of the stability boundary. We provide a \texttt{python} code that calculates the SA factors and maximally-amplified wavelength given the background disk parameters.

13 pages, 5 figures