# Spatial variations in the Milky Way disc metallicity-age relation

## TL;DR

This study maps how the relationship between metallicity and age varies across different regions of the Milky Way disc, revealing the effects of radial migration and vertical structure on stellar populations.

## Contribution

It provides the first direct observations of spatial variations in the metallicity-age relation in the Milky Way disc using Gaia data and hierarchical modeling.

## Key findings

- Significant variation in [M/H]-age relation with Galactocentric radius and height.
- Measured the present-day metallicity gradient as -0.059 dex/kpc.
- Confirmed a vertically flared distribution of young stars in the outer disc.

## 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 [$\alpha$/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 $-0.059 \pm 0.010$ 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]-[$\alpha$/M] distribution of the solar neighborhood suggests that the high-[M/H] stars are not an evolutionary extension of the low-$\alpha$ sequence. Our observational results are important constraints to Galactic simulations and models of chemical evolution.