The Milky Way in motion: gauging stellar trajectories that shape the Galactic thin disc

TL;DR

The paper addresses how radial migration shapes the Milky Way's thin disc by separating churning, which alters angular momentum, from blurring, which increases orbital eccentricities. It advances a data-driven approach by refining Galactic chemical-evolution grids and applying a generalized additive model (GAM) to infer birth radii $R_{ m b} = f([Fe/H], t)$ from metallicity and age, then comparing them to present-day guiding radii $R_{ m g}$ for Gaia-ESO stars, aided by hierarchical clustering of 21 abundances and orbital data from Gaia and Galpy. The key finding is that roughly $75\%$ of stars show evidence of churning, with outward migration dominating among metal-rich inner-disc populations and inward migration among metal-poor groups, with varying fractions across chemical groups due to bar and spiral-arm interactions. This work provides a robust, chemo-dynamical framework to reconstruct birth radii and quantify radial migration, offering insights into thin-disc assembly and the influence of non-axisymmetric Galactic structures.

Abstract

As stars traverse the Milky Way, their orbits evolve through perturbations that alter their orbital radii. These changes arise from two mechanisms: churning, which modifies angular momentum, and blurring, which induces eccentric orbits without major angular momentum change. To assess whether churning or blurring dominates the dynamical evolution of Gaia-ESO stars, we refine Galactic chemical-evolution models by constructing finer grids that span a wider age range. Using a generalised additive model (GAM), we estimate stellar birth radii beyond the limits of binned metallicity models and compare them with dynamical parameters derived from Gaia parallaxes and proper motions, and Galpy. Our metallicity-stratified sample, grouped through hierarchical clustering of 21 chemical abundances, reveals clear migratory signatures: metal-rich stars formed in the inner disc preferentially move outwards, while more metal-poor stars formed at larger radii tend to migrate inwards. About 75% of stars show signs of churning, while the remainder are largely undisturbed or shaped by blurring. These patterns vary among chemical groups, likely reflecting interactions with the Galactic bar and spiral arms.

Figures (2)

• Figure 1: Letter-value plots showing the distribution of inferred birth radii ($\langle R_{\rm b} \rangle$, in dark shades, left distribution of each group) and present-day guiding radii ($\langle R_{\rm g} \rangle$, in light shades, right distribution of each group) across the six chemically identified groups.
• Figure 2: Fractional contribution of different migration behaviours across the chemically defined stellar groups. Ribbon plot showing the proportions of outward migrators ($\langle R_{\rm g} \rangle$ > $\langle R_{\rm b} \rangle$), inward migrators ($\langle R_{\rm g} \rangle$ < $\langle R_{\rm b} \rangle$), and stars whose guiding radii are consistent with their inferred birth radii ($\langle R_{\rm g} \rangle$$\approx$$\langle R_{\rm b} \rangle$). The groups are ordered from metal-rich (left) to more metal-poor (right). Outward migration dominates at super-metal-rich abundances, while the metal-poor groups display increasing fractions of inward migrators, illustrating how radial mixing varies systematically across the thin disc metallicity sequence.