Characterizing epochs of star formation across the Milky Way disc using age-metallicity distributions of GALAH stars

We provide a detailed map of the ages and metallicities of turn-off stars in the Milky Way disc based on data from GALAH DR3 and Gaia EDR3. From this map, we identify previously undetected features in the age-metallicity distribution of disc stars and interpret these results as indicating a three-phase formation history of the Milky Way. In the first phase, inner disc stars form along a single age-metallicity sequence and are today kinematically hot. The end of this phase is marked by a local minimum in the inner disc age distribution 10 Gyr ago. At this time, we find the stellar populations to transition from high to low alpha-element abundances and from high to low vertical velocity dispersion. In the second phase, stars form across the disc with outwardly decreasing metallicity. In this phase, inner disc stars form at supersolar metallicities in a continuation of the early age-metallicity relation, while outer disc stars begin forming at metallicities at least 0.5 dex lower. Finally, the third phase is associated with a recent burst of star formation across the local disc marked by a local minimum in the age-metallicity distribution 4-6 Gyr ago. Future quantitative comparisons between the observed age-metallicity distribution and those of simulated galaxies could help constrain the processes driving each of the star formation phases.