Empirical derivation of the metallicity evolution with time and radius using TNG50 Milky Way/Andromeda analogues

Abstract

Recent works have used a linear birth metallicity gradient to estimate the evolution of the [Fe/H] profile in the Galactic disk over time, and infer stellar birth radii (R${}_{\text{birth}}$) from [Fe/H] and age measurements. These estimates rely on the evolution of [Fe/H] at the Galactic center ([Fe/H](0, $\ddot{I}„$)) and the birth metallicity gradient ($\nabla$[Fe/H]($\ddot{I}„)$) over time -- quantities that are unknown and inferred under key assumptions. In this work, we use the sample of Milky Way/Andromeda analogues from the TNG50 simulation to investigate the ability to recover [Fe/H](R, $\ddot{I}„$) and R${}_{\text{birth}}$ in a variety of galaxies. Using stellar disk particles, we test the assumptions required in estimating R${}_{\text{birth}}$, [Fe/H](0, $\ddot{I}„$), and $\nabla$[Fe/H]($\ddot{I}„)$ using recently proposed methods to understand when they are valid. We show that $\nabla$[Fe/H]($\ddot{I}„)$ can be recovered in most galaxies to within 26% from the range in [Fe/H] across age, with better accuracy for more massive and stronger barred galaxies. We also find that the true central metallicity is unrepresentative of the genuine disk [Fe/H] profile; thus we propose to use a projected central metallicity instead. About half of the galaxies in our sample do not have a continuously enriching projected central metallicity, with a dilution in [Fe/H] correlating with mergers. Most importantly, galaxy-specific [Fe/H](R, $\ddot{I}„$) can be constrained and confirmed by requiring the R${}_{\text{birth}}$ distributions of mono-age, solar neighborhood populations to follow inside-out formation. We conclude that examining trends with R${}_{\text{birth}}$ is valid for the Milky Way disk and similarly structured galaxies, where we expect R${}_{\text{birth}}$ can be recovered to within 20% assuming today's measurement uncertainties in TNG50.

Version accepted at A&A