The Milky Way Bar Pattern Speed using Hercules and Gaia DR3

TL;DR

This study uses Gaia DR3 data to analyze the Hercules moving group, providing evidence that supports a low pattern speed bar model for the Milky Way, aligning with recent kinematic studies.

Contribution

It offers an independent confirmation of the low pattern speed bar model by examining Hercules' variation across Galactic azimuth using Gaia DR3 data.

Findings

Hercules' velocity increases towards the bar's minor axis

Hercules' strength varies with position relative to the bar

Results support the low pattern speed bar model with corotation resonance

Abstract

The distribution of moving groups in the solar neighborhood has been used to constrain dynamical properties of the Milky Way for decades. The kinematic bimodality between the main mode (Hyades, Pleiades, Coma Berenices, and Sirius) and Hercules can be explained by two different bar models -- via the outer Lindblad resonance of a bar with a high pattern speed ($\sim$55 km s$^{-1}$ kpc$^{-1}$), or via the corotation resonance of a bar with a low pattern speed ($\sim$40 km s$^{-1}$ kpc$^{-1}$). Recent works directly studying the kinematics of bar stars and gas flows near the center of the Galaxy have converged on the low pattern speed model. In this paper, we independently confirm this result by using Gaia DR3 to directly study the variation of Hercules across Galactic azimuth. We find that Hercules increases in $V_\phi$ and becomes stronger as we move towards the minor axis of the bar, and decreases in $V_\phi$ and becomes weaker as we move towards the major axis of the bar. This is in direct agreement with theoretical predictions of a low pattern speed bar model in which Hercules is formed by the corotation resonance with stars orbiting the bar's L4/L5 Lagrange points.