Revisiting the radial metallicity gradient–age relation in the Milky Way’s thin and thick discs

ABSTRACT Stellar discs in galaxies 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 disc, while the thick disc 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 discs of the MW, using spectroscopic data from LAMOST DR8 and stellar ages calibrated with asteroseismology. Our results show a steadily flattening MGAR in the thin disc and confirm a positive gradient $\sim 0.013\, \mathrm{dex\, kpc^{-1}}$ in the thick disc. The flattening in the thin disc 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 disc may reflect early enrichment of the outer regions by strong feedback or starburst-driven outflows in a turbulent, gas-rich proto-disc. These findings suggest distinct chemodynamical evolution paths for the MW’s thin and thick discs and provide valuable constraints for future models of Galactic chemical evolution.