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 turnoff 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 super-solar
metallicites 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 to 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.
