# Identifying resonances of the Galactic bar in Gaia DR2: I. Clues from action space

## TL;DR

This study uses Gaia DR2 data and simulations to identify the Galactic bar's resonances in action space, revealing their effects on stellar kinematics and providing constraints on the bar's pattern speed.

## Contribution

It demonstrates that axisymmetric action estimates can reveal bar resonances and their effects in the Milky Way, offering new diagnostics for Galactic dynamics.

## Key findings

- Identified three candidate pattern speeds for the Galactic bar's OLR.
- Discovered a gradient in vertical action related to bar resonances.
- Confirmed that resonant orbit behavior aligns with scattering and oscillation models.

## Abstract

Action space synthesizes the orbital information of stars and is well-suited to analyse the rich kinematic substructure of the disc in the \emph{Gaia} DR2 radial velocity sample (RVS). We revisit the strong perturbation induced in the Milky Way (MW) disc by an $m=2$ bar, using test particle simulations and the actions $(J_R,L_z,J_z)$ estimated in an axisymmetric potential. These make three useful diagnostics cleanly visible. (1.) We use the well-known characteristic flip from outward to inward motion at the Outer Lindblad Resonance (OLR, $l=+1,m=2$), which occurs along the axisymmetric resonance line (ARL) in $(L_z,J_R)$, to identify in the \emph{Gaia} action data three candidates for the bar's OLR and pattern speed $\Omega_\text{bar}$: $1.85\Omega_0$, $1.20\Omega_0$, and $1.63\Omega_0$ (with $\sim0.1\Omega_0$ systematic uncertainty). The \emph{Gaia} data is therefore consistent with both slow and fast bar models in the literature, but disagrees with recent measurements of $\sim1.45\Omega_0$. (2.) For the first time, we demonstrate that bar resonances -- especially the OLR -- cause a gradient in vertical action $\langle J_z \rangle$ with $L_z$ around the ARL via "$J_z$-sorting" of stars. This could contribute to the observed coupling of $\langle v_R \rangle$ and $\langle | v_z | \rangle$ in the Galactic disc. (3.) We confirm prior results that the behaviour of resonant orbits is well approximated by scattering and oscillation in $(L_z,J_R)$ along a slope $\Delta J_R/\Delta L_z = l/m$ centered on the $l$:$m$ ARL. Overall, we demonstrate that axisymmetrically estimated actions are a powerful diagnostic tool even in non-axisymmetric systems.