Modelling the Galactic disc: perturbed distribution functions in the presence of spiral arms

TL;DR

This paper develops an analytical model for the perturbed distribution function of stars in the Milky Way's disc due to spiral arms, predicting stellar motions and bulk flows consistent with observations and simulations.

Contribution

It introduces an explicit, analytical form of the perturbed distribution function for a stellar disc in the presence of spiral arms, valid away from resonances.

Findings

Predicted non-zero radial and vertical bulk flows match observed breathing modes.

Analytical response agrees with test-particle simulations.

First estimation of vertical bulk motion reduction factor for stellar populations.

Abstract

Starting from an axisymmetric equilibrium distribution function (DF) in action space, representing a Milky Way thin disc stellar population, we use the linearized Boltzmann equation to explicitly compute the response to a three-dimensional spiral potential in terms of the perturbed DF. This DF, valid away from the main resonances, allows us to investigate a snapshot of the velocity distribution at any given point in three-dimensional configuration space. Moreover, the first order moments of the DF give rise to non-zero radial and vertical bulk flows -- namely breathing modes -- qualitatively similar to those recently observed in the extended Solar neighbourhood. We show that these analytically predicted mean stellar motions are in agreement with the outcome of test-particle simulations. Moreover, we estimate for the first time the reduction factor for the vertical bulk motions of a stellar population compared to the case of a cold fluid. Such an explicit expression for the full perturbed DF of a thin disc stellar population in the presence of spiral arms will be helpful in order to dynamically interpret the detailed information on the Milky Way disc stellar kinematics that will be provided by upcoming large astrometric and spectroscopic surveys of the Galaxy.