APOGEE DR16: a multi-zone chemical evolution model for the Galactic disc based on MCMC methods

TL;DR

This paper uses Bayesian MCMC methods to fit a multi-zone chemical evolution model to APOGEE DR16 data, revealing inside-out galaxy formation, delayed gas infall, and distinct chemical evolution pathways for different Galactic disc regions.

Contribution

It introduces a Bayesian multi-zone two-infall chemical evolution model constrained by APOGEE data, highlighting the inside-out formation and delayed gas infall in the Milky Way's disc.

Findings

Inner regions formed faster than outer regions.

Outer disc influenced by late primordial gas accretion.

Delay of 3.0 to 4.7 Gyr between gas infall events.

Abstract

The analysis of the APOGEE DR16 data suggests the existence of a clear distinction between two sequences of disc stars at different Galactocentric distances in the [$\alpha$/Fe] vs. [Fe/H] abundance ratio space: the so-called high-$\alpha$ sequence, classically associated to an old population of stars in the thick disc, and the low-$\alpha$ sequence, which mostly comprises relatively young stars in the thin disc. We perform a Bayesian analysis based on a Markov Chain Monte Carlo method to constrain a multi-zone two-infall chemical evolution model designed for regions at different Galactocentric distances using measured chemical abundances from the APOGEE DR16 sample. An inside-out formation of the Galaxy disc naturally emerges from the best fit of our two-infall chemical-evolution model to APOGEE-DR16: inner Galactic regions are assembled on shorter time-scales compared to the external ones. In the outer disc (with radii $R>6$ kpc), the chemical dilution due to a late accretion event of gas with primordial chemical composition is the main driver of the [Mg/Fe] vs. [Fe/H] abundance pattern in the low-$\alpha$ sequence. In the inner disc, in the framework of the two-infall model, we confirm the presence of an enriched gas infall in the low-$\alpha$ phase as suggested by chemo-dynamical models. Our Bayesian analysis of the recent APOGEE DR16 data suggests a significant delay time, ranging from $\sim$3.0 to 4.7 Gyr, between the first and second gas infall events for all the analyzed Galactocentric regions. Our results propose a clear interpretation of the [Mg/Fe] vs. [Fe/H] relations along the Galactic discs. The signatures of a delayed gas-rich merger which gives rise to a hiatus in the star formation history of the Galaxy are impressed in the [Mg/Fe] vs. [Fe/H] relation, determining how the low-$\alpha$ stars are distributed in the abundance space at different Galactocentric distances.