Observational Constraints of Radial Migration in the Galactic Disc Driven by the Slowing Bar

Abstract

Radial migration is an important dynamical effect that has reshaped the Galactic disc, but its origin has yet to be elucidated. In this work, we present evidence that resonant dragging by the corotation of a decelerating bar could be the main driver of radial migration in the Milky Way disc. Using a test particle simulation, we demonstrate this scenario explains the two distinct age-metallicity sequences observed in the solar vicinity: the plateauing upper sequence is interpreted as stars dragged outwards by the expanding corotation of the decelerating bar and the steeper lower sequence as stars formed locally around the solar circle. The upper migrated sequence dominates at guiding radii around the current corotation radius of the bar, $R\sim 7\mathrm{kpc}$, but rapidly dies away beyond this where the mechanism cannot operate. This behaviour naturally explains the radial dependence of the $[\alpha /Fe]$-bimodality, in particular the truncation of the high-$[\alpha /Fe]$ disc beyond the solar circle. Under our proposed radial migration scenario, we constrain the Milky Way bar's pattern speed evolution using the age-metallicity distribution of stars currently trapped at corotation. We find the bar likely formed with an initial pattern speed of $60-100$ km s${}^{-1}$ kpc${}^{-1}$ and began decelerating $6-8$ Gyr ago at a rate $-\dot{\Omega }/{\Omega }^2\sim 0.0025-0.0040$ (where the quoted ranges include systematic uncertainties).

17 pages, 4 figures, 4 appendices, Accepted by ApJL, https://doi.org/10.3847/2041-8213/adc261

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