A slow spin to win: The gradual kinematic evolution across metallicities of the proto-Galaxy to the high-α disc

Observational studies are identifying stars thought to be remnants from the earliest stages of the Milky Way's hierarchical mass assembly, referred to as the proto-Galaxy. We used red giant stars with kinematics from Gaia DR3 RVS data and α/M and M/H estimates from low-resolution Gaia XP spectra to investigate the relationship between azimuthal velocity and metallicity. Our aim is to understand the transition from a chaotic proto-Galaxy to a well-ordered rotating (high-α) disc-like population. To analyze the structure of the data in M/H –v_ϕ space for both high- and low-α samples with carefully defined α-separation, we developed a model with two Gaussian components in v_ϕ, one representing a disc-like population and the other a halo-like population. This model is designed to capture the conditional distribution P(v_ϕ ∣ M/H ) with a two-component Gaussian mixture model with fixed means and standard deviations in the azimuthal velocities. To quantify the spin-up of the high-α disc population, we extended this two-component model by allowing the mean velocity and velocity dispersion to vary between the spline knots across the metallicity range used. We also compared our findings with existing literature using traditional Gaussian mixture modelling in bins of M/H and investigated using orbital circularity instead of azimuthal velocity. Our findings show that the metal-poor high-α disc gradually spins up across M/H ∼ -1.7 to -1, while the low-α sample exhibits a sharp transition at M/H ≈ -1. This latter result is due to the accreted (mostly Gaia -Enceladus-Sausage) debris dominating the metal-poor end, underscoring the critical role of α/M selection in studying the Milky Way’s (old, high-α) disc evolution. These results indicate that the proto-Galaxy underwent a slow, monotonic spin-up phase over increasing metallicities rather than a rapid, dramatic spin-up at M/H ∼ -1, as previously inferred in the literature.