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

Context. The analysis of the latest release of the Apache Point Observatory Galactic Evolution Experiment project (APOGEE DR16) data suggests the existence of a clear distinction between two sequences of disc stars at different Galactocentric distances in the [alpha /Fe] versus [Fe/H] abundance ratio space: the so-called high-alpha sequence, classically associated with an old population of stars in the thick disc with high average [alpha /Fe], and the low-alpha sequence, which mostly comprises relatively young stars in the thin disc with low average [alpha /Fe].Aims. We aim 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.Methods. We performed a Bayesian analysis based on a Markov chain Monte Carlo method to fit our multi-zone two-infall chemical evolution model to the APOGEE DR16 data.Results. 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 timescales 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 a primordial chemical composition is the main driver of the [Mg/Fe] versus [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 similar to 3.0 to 4.7 Gyr, between the first and second gas infall events for all the analysed Galactocentric regions. The best fit model reproduces several observational constraints such as: (i) the present-day stellar and gas surface density profiles; (ii) the present-day abundance gradients; (iii) the star formation rate profile; and (iv) the solar abundance values.Conclusions. Our results propose a clear interpretation of the [Mg/Fe] versus [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] versus [Fe/H] relation, determining how the low-alpha stars are distributed in the abundance space at different Galactocentric distances, which is in agreement with the finding of recent chemo-dynamical simulations.

Automated summary

The analysis of APOGEE DR16 data points towards distinct sequences of disc stars at different Galactocentric distances, with an inside-out formation of the Galaxy disc being suggested. Different chemical evolution models for regions at various distances are examined, with a significant delay time between gas infall events being identified. The best fit model reproduces key observational constraints, indicating how low-alpha stars are distributed in the abundance space at different Galactocentric distances.