Galactic echoes

TL;DR

This paper explores the concept of phase space echoes in galactic dynamics, deriving a theoretical framework and testing it with simulations, suggesting echoes could be common in disc galaxies but unlikely to explain the Gaia Snail feature.

Contribution

It introduces the galactic analogue of plasma echo theory using angle-action variables and applies it to a model of vertical stellar motion, verified through simulations.

Findings

Derived the galactic echo theory using angle-action variables.

Simulations confirmed the predicted echo behavior.

Gaia Snail is unlikely a pure echo effect.

Abstract

Gaia has revealed a variety of substructures in the phase space of stars in the Solar neighborhood, including the vertical `Snail' in $(z,v_z)$ space. Such substructures are often interpreted as the incompletely phase-mixed response of the disc stars to a single perturbation, such as an impulsive encounter with a satellite galaxy. In this paper we consider the possibility that such structures contain manifestations of phase space echoes. First established in plasma physics in the 1960s, echoes arise when a collisionless system is perturbed twice: the macroscopic responses to both perturbations mix to small scales in phase space, whereupon they couple nonlinearly, producing a third macroscopic `echo' response without the need for a third perturbation. We derive the galactic analogue of the plasma echo theory using angle-action variables and apply it to a one-dimensional model of vertical motion in the Milky Way. We verify the predicted echo behavior using idealized test particle simulations, both with and without the inclusion of diffusion through orbital scattering off molecular clouds. While we conclude that the Gaia Snail itself is unlikely a (pure) echo effect, the basic physics we uncover is sufficiently generic that we expect phase-space echoes to be common in disc galaxies.