Revisiting the Radial Metallicity Gradient-Age Relation in the Milky Way's Thin and Thick Disks

TL;DR

The study investigates the metallicity gradient–age relation (MGAR) in the Milky Way's thin and thick disks using LAMOST DR8 spectroscopy and asteroseismic ages, enabling separate MGAR measurements for chemically defined disk components. It finds a steadily flattening MGAR in the thin disk, with the youngest stars around a gradient of $-0.07$ dex kpc$^{-1}$ and a flattening rate of $0.0016$ dex kpc$^{-1}$ Gyr$^{-1}$, while the thick disk maintains a globally positive gradient near $0.013$ dex kpc$^{-1}$, peaking near $5.4$ Gyr at $ oughly 0.019$ dex kpc$^{-1}$. The results imply distinct chemodynamical evolution histories for the two disks, with the thin disk potentially shaped by radial migration and evolving ISM gradients, and the thick disk reflecting early turbulent feedback and accretion processes. These measurements provide robust constraints for Galactic chemical-evolution models and offer benchmarks for future simulations, including FIRE2-based comparisons.

Abstract

Galactic disks typically exhibit a negative radial metallicity gradient, indicating faster enrichment in the inner regions. Recent studies report that this gradient becomes flatter with increasing stellar age in the Milky Way's (MW) thin disk, while the thick disk exhibits a mildly positive gradient across all ages. In this work, we revisit the metallicity gradient-age relation (MGAR) in both the thin and thick disks of the MW. We use spectroscopic data from LAMOST DR8 and stellar ages calibrated with asteroseismology. Our results show a steadily flattening MGAR in the thin disk and confirm a positive gradient $\sim0.013\,\mathrm{dex\,kpc^{-1}}$ in the thick disk. The flattening in the thin disk may be caused by large-scale radial migration induced by transient spiral arms, or by a time-dependent steepening of the interstellar medium (ISM) metallicity gradient as suggested by recent FIRE2 simulations. The positive gradient in the thick disk may reflect early enrichment of the outer regions by strong feedback or starburst-driven outflows in a turbulent, gas-rich proto-disk. These findings suggest distinct chemodynamical evolution paths for the MW's thin and thick disks and provide valuable constraints for future models of Galactic chemical evolution.

Figures (9)

• Figure 1: Spatial and chemical distributions of the selected clean LAMOST DR8 sample. (a): Galactic meridional distribution. (b): Face-on view of the sample. (c): Number density in the [$\alpha$/Fe]--[Fe/H] plane. The black line shows our chemical separation between the thin and thick disks ([$\alpha$/Fe] $= 0.12$ for [Fe/H] $< -0.3$, otherwise [$\alpha$/Fe] $= 0.12 - 0.14(\mathrm{[Fe/H]} + 0.3)$). The gray line indicates the V21 separation, which is not optimal for our sample. (d): Number density in the $Z_\mathrm{max}$--$R_\mathrm{g}$ plane. The blue line at $Z_\mathrm{max}=0.25\,\mathrm{kpc}$ and red line at $Z_\mathrm{max}=2\,\mathrm{kpc}$ mark our geometric cuts for the thin and thick disk selections, respectively.
• Figure 2: [Fe/H]--$R_\mathrm{g}$ and age distributions for the selected thin (blue) and thick (red) disk stars. Upper panel: [Fe/H] vs. guiding center radius $R_\mathrm{g}$. Thin disk stars with near-solar metallicity show a gradient decreasing outward, while thick disk stars are more metal-poor (typical [Fe/H] $\sim-0.6$ dex) and exhibit a mildly positive gradient; Lower panel: Age distribution. Thin disk stars are generally younger than thick disk stars. The presence of old, metal-poor stars concentrated in the inner disk, and young, solar-metallicity stars in the outer disk, is consistent with inside-out disk formation.
• Figure 3: Metallicity gradient–age relation (MGAR) in the thin (top) and thick (bottom) disks, compared to the results of V21 (black dots). Error bars are derived from the covariance matrix of the weighted OLS fits. Upper panel: MGAR in the thin disk. The blue line shows our fitted trend, with a flattening rate of $0.0016\,\mathrm{dex \, kpc^{-1}\, Gyr^{-1}}$; the shaded region denotes the 98% highest density interval (HDI). The black line and gray shade show the fit for V21. Check the caption of Figure \ref{['fig:all_uncertainties']} in the Appendix for prior specifications. Lower panel: MGAR in the thick disk. Our results yield a mean positive gradient of $\sim0.013\,\mathrm{dex\,kpc^{-1}}$.
• Figure 4: Metallicity gradient fits for mono-age populations in the thin (top three rows) and thick (bottom three rows) disks. Each panel in the right three columns shows the [Fe/H]--$R_\mathrm{g}$ distribution for a specific age bin, with 20%, 80%, and 95% contours and the size of the mono-age population. The first column displays the MGARs, [$\alpha$/Fe]--[Fe/H], and $Z_\mathrm{max}$--$R_\mathrm{g}$ distributions for the two disk populations. Notably, spurred contours appear in the fourth row at $R_\mathrm{g}>10\,\mathrm{kpc}$, which tend to suppress the positive gradient. After removing these stars, the thick disk gradient increases from $\sim 0.013\,\mathrm{dex\,kpc^{-1}}$ to $\sim 0.02\,\mathrm{dex\,kpc^{-1}}$ with reduced age-dependent fluctuation.
• Figure 5: Comparison between our observed MGAR in the thin disk and literature results (solid lines for MGARs, dashed lines for stellar birth/ISM gradient evolutions).Upper panel: MGAR in the thin disk compared with reconstructed birth gradients from 2023MNRAS.525.2208R and 2024MNRAS.535..392L. The divergence between the MGAR and birth gradient at old ages may reflect the cumulative effect of churning. The gradient of the youngest stars aligns with the birth gradients ($\sim -0.07\,\mathrm{dex\,kpc^{-1}}$), as little migration is expected to occur. Lower panel: MGAR compared with two results predicting steepening gradients: the gas-phase evolution from 2021MNRAS.502.5935S and the birth gradient from FIRE2 simulations 2024arXiv241021377G, we also plot the MGAR from the FIRE2 simulations. See the text for further discussion.