A slow spin to win -- the gradual evolution of the proto-Galaxy to the old disc

TL;DR

The study tackles how the Milky Way transitioned from a chaotic proto-Galaxy to the old high-$\alpha$ disc by analyzing Gaia DR3 kinematics with XP-derived $[\mathrm{M/H}]$ and $[\alpha/\mathrm{M}]$ abundances. It introduces a two-component GMM in the $[\mathrm{M/H}]-v_{\phi}$ plane, with a later evolution-capable version that lets the means and dispersions evolve with metallicity, complemented by orbital circularity $\eta$ modeling in the $[\mathrm{M/H}]-\eta$ plane. The metal-poor high-$\alpha$ population shows a gradual spin-up across $-1.7 \lesssim [\mathrm{M/H}] \lesssim -1$, while the low-$\alpha$ population exhibits a sharp transition near $[\mathrm{M/H}] \approx -1$, driven by GES debris; this underscores the importance of $[\alpha/\mathrm{M}]$ selection. The results support a gradual build-up of rotational support rather than a rapid, singular spin-up event, aligning with recent APOGEE-based findings and offering a framework for integrating kinematic and chemical evolution in Galactic archaeology.

Abstract

Observational studies are identifying stars thought to be remnants from the earliest stages of the hierarchical mass assembly of the Milky Way, referred to as the proto-Galaxy. We use red giant stars with kinematics and [$α$/M] and [M/H] estimates from Gaia DR3 data to investigate the relationship between azimuthal velocity and metallicity, aiming to understand the transition from a chaotic proto-Galaxy to a well-ordered, rotating (old) disc-like population. To analyse the structure of the data in [M/H]-v$_φ$ space for both high- and low-$α$ samples with carefully defined $α$-separation, we develop 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$_φ$ $\mid$ [M/H]) with a 2-component Gaussian Mixture Model with fixed azimuthal velocities means and standard deviations. To quantify the spin-up of the high-$α$ disc population, we extend 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 compare our findings with existing literature using traditional Gaussian Mixture Modelling in bins of [M/H] and investigate using orbital circularity instead of azimuthal velocity. Our findings show that the metal-poor high-$α$ disc gradually spins up across [M/H] $\sim$ -1.7 to -1.0, while the low-$α$ sample exhibits a sharp transition at [M/H] $\sim$ -1.0. This latter result is due to the accreted debris dominating the metal-poor end, underscoring the critical role of [$α$/M] selection in studying the (old) disc evolution of the Milky Way. These results indicate that the proto-Galaxy underwent a slow, monotonic spin-up phase rather than a rapid, dramatic spin-up at [M/H] $\sim$ -1.0, as previously inferred.

Figures (16)

• Figure 1: Logarithmic density of [$\alpha$/M] versus [M/H]. The purple band represents the high- and low-$\alpha$ sequence separation defined in this work (see text for details). Stars in the purple band are excluded. The bulk of accreted last major merger (GES) is primarily restricted to the low-$\alpha$ population with our selection.
• Figure 2: Distribution of stars in cylindrical Galactic coordinates (R-z plane) colour-coded by their mean metallicities for all the stars in our sample (top), high-$\alpha$ selection (bottom left), and low-$\alpha$ selection (bottom right). We can see that dust near the Milky Way's midplane significantly impacts our survey selection. We see a sharp negative metallicity gradient w.r.t height above the disc plane in all stars and low-$\alpha$ stars. High-$\alpha$ stars have a shallower negative metallicity gradient.
• Figure 3: Column-normalised (by sum) 2D histogram of stars in the [M/H]-v$_\phi$ plane (azimuthal velocity versus metallicity) for all the stars (top), high-$\alpha$ selection (bottom left), and low-$\alpha$ selection (bottom right). The running median track is shown as dashed black line and the 16$^{th}$ and 84$^{th}$ percentile tracks are shown as black lines in all panels. The running median tracks for the low-$\alpha$ panel look more like a step-function, while high-$\alpha$ tracks is shallower, supporting the gradual monotonic increase, that can be interpreted as a gradual spin-up from old proto-Galactic populations to the present day high-$\alpha$ disc.
• Figure 4: Column-normalised (by sum) 2D histogram of stars in the [M/H]-v$_\phi$ plane for APOGEE DR17 giants (with the same high- and low-$\alpha$ separation as shown in Figure \ref{['fig:tinsley']}) for all the stars (left), high-$\alpha$ selection (center), and low-$\alpha$ selection (right). The running median track is shown as dashed black line and the 16$^{th}$ and 84$^{th}$ percentile tracks are shown as black lines in all panels. The median, 16$^{th}$ and 84$^{th}$ percentile tracks for our Gaia XP sample are shown in gray for comparison. The tracks between Gaia XP and APOGEE samples show remarkable resemblance in high-$\alpha$ and small differences for low-$\alpha$ and all stars due to lower contamination in low-$\alpha$ selection in APOGEE (see Appendix \ref{['A']}).
• Figure 5: Graphical model representation of the evolving means model, modeling the conditional distribution P(v$_\phi$|[M/H]) with a 2-component GMM, wherein the means and standard deviations are fixed (left) and varying (right) with respect to the underlying metallicities. The evolving/varying means model is also implemented for circularity as the conditional distribution or P($\eta$|[M/H]). The subscript 1 and 2 stands for the disc-like and halo-like components respectively, with $\mu$ and $\sigma$ as the Gaussian mean and standard deviations in v$_\phi$ or $\eta$, conditioned on the value of [M/H]. $w$ stands for the relative contribution of the disc-like component with the relative contribution of the halo-like component defined as (1-$w$). A list of the model parameters and their priors/functional forms are given in Table \ref{['func-form']}.