The imprints of the Galactic bar on the thick disk with RAVE

TL;DR

This study reveals that the Galactic bar influences the kinematics of the thick disk stars, showing asymmetries similar to those in the thin disk, challenging traditional velocity distribution models and suggesting a need to revisit the origins of thick disk substructures.

Contribution

First detection of Galactic bar-induced kinematic asymmetries in the thick disk, highlighting the need for dynamical models to incorporate full 3D velocity distributions.

Findings

Asymmetries in thick disk star velocities linked to the Galactic bar.

Traditional velocity models are insufficient for the thick disk.

Reevaluation of thick disk substructure origins, favoring dynamical resonances.

Abstract

We study the kinematics of a local sample of stars, located within a cylinder of 500 pc radius centered on the Sun, in the RAVE dataset. We find clear asymmetries in the $v_R$-$v_\phi$ velocity distributions of thin and thick disk stars: here are more stars moving radially outwards for low azimuthal velocities and more radially inwards for high azimuthal velocities. Such asymmetries have been previously reported for the thin disk as being due to the Galactic bar, but this is the first time that the same type of structures are seen in the thick disk. Our findings imply that the velocities of thick disk stars should no longer be described by Schwarzschild's, multivariate Gaussian or purely axisymmetric distributions. Furthermore, the nature of previously reported substructures in the thick disk needs to be revisited as these could be associated with dynamical resonances rather than to accretion events. It is clear that dynamical models of the Galaxy must fit the 3D velocity distributions of the disks, rather than the projected 1D, if we are to understand the Galaxy fully.