The Milky Way Bar Pattern Speed using Hercules and Gaia DR3

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 $V_{\varphi }$ and becomes stronger as we move towards the minor axis of the bar, and decreases in $V_{\varphi }$ 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.

6 pages, 5 figures, accepted in MNRAS Letters

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• GaiaUnlimited: The old stellar disc of the Milky Way as traced by the Red Clump
• From moving groups to star formation in the Solar Neighborhood
• Comparing radial migration in dark matter and MOND regimes
• Mapping the Milky Way with Gaia Bp/Rp spectra-IV: the broken and asymmetric density profile of the stellar disk traced by a large sample of red clumps
• Deep chemical tagging -- Identifying open clusters and moving groups in chemical space with graph attention networks