Spatial variations in the Milky Way disc metallicity–age relation

ABSTRACT Stellar ages are a crucial component to studying the evolution of the Milky Way. Using Gaia DR2 distance estimates, it is now possible to estimate stellar ages for a larger volume of evolved stars through isochrone matching. This work presents M/H–age and α/M–age relations derived for different spatial locations in the Milky Way disc. These relations are derived by hierarchically modelling the star formation history of stars within a given chemical abundance bin. For the first time, we directly observe that significant variation is apparent in the M/H–age relation as a function of both Galactocentric radius and distance from the disc mid-plane. The M/H–age relations support claims that radial migration has a significant effect in the plane of the disc. Using the M/H bin with the youngest mean age at each radial zone in the plane of the disc, the present-day metallicity gradient is measured to be −0.059 ± 0.010 dex kpc−1, in agreement with Cepheids and young field stars. We find a vertically flared distribution of young stars in the outer disc, confirming predictions of models and previous observations. The mean age of the M/H–α/M distribution of the solar neighbourhood suggests that the high-M/H stars are not an evolutionary extension of the low-α sequence. Our observational results are important constraints to Galactic simulations and models of chemical evolution.