Identifying resonances of the Galactic bar in Gaia DR2: II. Clues from angle space

TL;DR

This study uses Gaia DR2 data and angle space analysis to identify the Galactic bar's Outer Lindblad Resonance, revealing candidate pattern speeds and discussing implications for the bar's dynamical properties.

Contribution

It demonstrates that Gaia data can be used to detect the Galactic bar's resonance in angle space and identifies multiple candidate pattern speeds, advancing understanding of Galactic dynamics.

Findings

Four candidate pattern speeds for the bar's OLR identified.

The strongest candidate aligns with the pattern speed of ~1.4 times the local standard of rest.

Stellar angles do not support the Hercules/Horn features being caused by the OLR of a fast bar.

Abstract

The Milky Way disk exhibits intricate orbit substructure of still-debated dynamical origin. The angle variables $(\theta_\phi,\theta_R)$ -- which are conjugates to the actions $(L_z,J_R)$, and describe a star's location along its orbit -- are a powerful diagnostic to identify $l$:$m$ resonances via the orbit shape relation $\Delta \theta_R / \Delta \theta_\phi = -m/l$. In the past, angle signatures have been hidden by survey selection effects (SEs). Using test particle simulations of a barred galaxy, we demonstrate that \emph{Gaia} should allow us to identify the Galactic bar's Outer Lindblad Resonance ($l=+1, m=2$, OLR) in angle space. We investigate strategies to overcome SEs. In the angle data of the \emph{Gaia} DR2 RVS sample, we independently identify four candidates for the OLR and therefore for the pattern speed $\Omega_\text{bar}$. The strongest candidate, $\Omega_\text{bar}\sim1.4\Omega_0$, positions the OLR above the `Sirius' moving group, agrees with measurements from the Galactic center, and might be supported by higher-order resonances around the `Hercules/Horn'. But it misses the classic orbit orientation flip, as discussed in the companion study on actions. The candidate $\Omega_\text{bar}\sim1.2\Omega_0$ was also suggested by the action-based study, has the OLR at the `Hat', is consistent with \emph{slow bar} models, but still affected by SEs. Weaker candidates are $\Omega_\text{bar}=1.6\Omega_0$ and $1.74\Omega_0$. In addition, we show that the stellar angles do not support the `Hercules/Horn' being created by the OLR of a \emph{fast bar}. We conclude that -- to resolve if `Sirius' or `Hat' is related to the bar's OLR -- more complex dynamical explanations and more extended data with well-behaved SEs are required.