Quantitative physical modelling of the Milky Way's two-phase age-metallicity structure: radial migration and the Gaia-Sausage-Enceladus star-formation peak

Xiang & Rix (Nature 603, 599-603, 2022) used ~250,000 LAMOST-Gaia subgiant stars (LAMOST: the Large Sky Area Multi-Object Fiber Spectroscopic Telescope) with precise isochrone ages to show that the Galactic stellar age-metallicity distribution $p(\tau,\mathrm{[Fe/H]})$ splits into two almost disjoint phases at $\tau\simeq 8$ Gyr: a late, dynamically quiescent phase whose V-shaped $p(\tau|\mathrm{[Fe/H]})$ bears manifest evidence of radial orbit migration, and an early phase culminating in a prominent old-disk star-formation maximum ~11.2 Gyr ago, apparently coinciding with the Gaia-Sausage-Enceladus (GSE) merger. The paper left the physical interpretation of both phases as open, explicitly model-dependent questions: whether secular radial migration alone can quantitatively reproduce the late-phase V-shape and its age-metallicity slope, and whether the GSE merger causally drove the old-disk star-formation peak rather than coinciding with it. Later work has advanced both fronts - a slowing-bar migration model reproducing the two age-metallicity sequences (which does not propagate the survey selection function or age-measurement uncertainties), CMD-fitting support for an ~11 Gyr burst, a revised GSE epoch of $10.1^{+0.2}_{-0.2}$ Gyr, and simulations in which GSE-like mergers suppress rather than enhance disk star formation - but no quantitative, systematics-controlled, model-based account of the observed $p(\tau,\mathrm{[Fe/H]})$ exists. Establishing what shaped the late-phase structure and what ended the thick-disk epoch remains a central open problem in near-field cosmology.

Background

The Milky Way's disk has two structural components: a geometrically and dynamically hot 'thick' disk of old, $\alpha$-element-enhanced stars ($\alpha$ elements such as Mg, Si, Ca, Ti are produced mainly by core-collapse supernovae, so the abundance ratio $[\alpha/\mathrm{Fe}]$ traces the star-formation timescale of a stellar population), and a cool, younger 'thin' disk. Reconstructing when and how these components formed requires precise ages for large stellar samples. Subgiant stars - stars that have just exhausted core hydrogen and are brightening across the Hertzsprung-Russell diagram - are the best age clocks for field stars, because their position in a color-magnitude diagram maps almost one-to-one onto age when compared with stellar-evolution models ('isochrones', e.g. the Yonsei-Yale (YY) or MIST grids). Xiang & Rix (2022) combined LAMOST spectroscopy with Gaia astrometry for ~250,000 subgiants, achieving a median relative age precision of about 7.5%, and measured the joint distribution of stellar age $\tau$ and iron abundance $\mathrm{[Fe/H]}$ (a metallicity tracer). They found that $p(\tau,\mathrm{[Fe/H]})$ splits into two almost disjoint parts at $\tau\simeq 8$ Gyr. In the late phase, $p(\tau|\mathrm{[Fe/H]})$ forms a V-shaped locus with two branches per metallicity, which they attributed to radial orbit migration - the churning of stars between galactocentric radii by transient spiral structure or bar resonances, changing stars' orbital guiding radii without heating them. The upper (more metal-rich, older) branch would be stars born inward of the Sun and dragged outward; the lower branch stars born further out and migrated inward. Whether secular evolution models of the disk reproduce this V-shape and its age-metallicity slope quantitatively was left as 'separate ongoing work'. In the early phase, the star-formation rate of the old (thick) disk peaked strongly around 11.2 Gyr ago, apparently just when the last major merger - that of the Gaia-Sausage-Enceladus (GSE) dwarf galaxy, whose debris forms much of the inner stellar halo and which dynamically heated the early disk, scattering disk stars onto low-angular-momentum 'Splash' orbits - is thought to have been completed. The authors called the merger-triggered starburst 'the most obvious interpretation of this coincidence', not a demonstrated causal connection. Two further systematics gate any definitive answer: absolute ages depend on the adopted isochrone family (public MIST grids then lacked $[\alpha/\mathrm{Fe}]$-enhanced variants, with impacts up to ~0.5-2 Gyr in some regimes), and the survey selection function and spectroscopic signal-to-noise dependence must be propagated when comparing models to the observed $p(\tau,\mathrm{[Fe/H]})$.

Problem Statement

Provide a quantitative, model-based physical account of the observed two-phase stellar age-metallicity distribution $p(\tau,\mathrm{[Fe/H]})$ of the Milky Way disk established by Xiang & Rix (2022), jointly resolving its two outstanding facets: (1) Late phase - determine quantitatively whether secular radial migration alone (bar-resonance dragging, spiral-arm churning, or a stated combination) can reproduce the observed V-shaped $p(\tau|\mathrm{[Fe/H]})$ for ages $\tau\lesssim 8$ Gyr and its age-metallicity slope, or whether additional physics is required, with the survey selection function and age-measurement uncertainties propagated; (2) Early phase - determine whether the Gaia-Sausage-Enceladus merger causally produced the ~11.2 Gyr old-disk star-formation peak and ended the thick-disk star-formation epoch, as opposed to the peak preceding a later merger completion (e.g. the inferred ~10.1 Gyr epoch), a merger-induced suppression or quiescence, or a merger-free origin; establishing causality versus coincidence turns on whether the star-formation-peak timing and the kinematic signatures of disk heating (the low-angular-momentum Splash population) are jointly reproduced. Any definitive answer must quantify the impact of isochrone-systematic age uncertainties on the absolute timing of these early events. Both facets bear on the same open problem - what shaped the late-phase structure of the observed $p(\tau,\mathrm{[Fe/H]})$ and what ended the thick-disk star-formation epoch.

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