Vertical kinematics of the thick disc at 4.5 < R < 9.5 kpc

TL;DR

This study reconstructs the vertical velocity dispersion profile of the Milky Way's thick disc using action space analysis of nearby stars, revealing an exponential decline with radius and weak velocity dependencies.

Contribution

It introduces a method to reconstruct the thick disc's distribution function in action space and applies it to observational data, providing new insights into the disc's radial structure.

Findings

Vertical velocity dispersion declines exponentially with radius.

Weak dependency of velocity dispersion on radial and azimuthal velocities.

Estimated scale length of the dispersion profile is approximately 8.3 kpc.

Abstract

We explored a method to reconstruct the distribution function of the Galactic thick disc within the action space where nearby thick-disc stars are distributed. By applying this method to 127 chemically-selected thick-disc stars in the Solar neighbourhood, we found that the vertical velocity dispersion that corresponds to the reconstructed distribution function declines approximately as $\exp (-R/R_s)$ at 4.5 kpc < R < 9.5 kpc, with $R_s$ = 8.3 $\pm$ 1.1 (rand.) $\pm$ 1.6 (sys.) kpc. Also, we found that the vertical velocity dispersion $\sigma_z$ of our local thick-disc stars shows only weak dependency on radial and azimuthal velocities $(v_R, v_\phi)$. We discuss possible implications of these results on the global structure of the Milky Way thick disc.