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The analysis of the APOGEE DR16 data suggests the existence of a cleardistinction between two sequences of disc stars at different Galactocentricdistances in the [$\alpha$/Fe] vs. [Fe/H] abundance ratio space: the so-calledhigh-$\alpha$ sequence, classically associated to an old population of stars inthe thick disc, and the low-$\alpha$ sequence, which mostly comprisesrelatively young stars in the thin disc. We perform a Bayesian analysis basedon a Markov Chain Monte Carlo method to constrain a multi-zone two-infallchemical evolution model designed for regions at different Galactocentricdistances using measured chemical abundances from the APOGEE DR16 sample. Aninside-out formation of the Galaxy disc naturally emerges from the best fit ofour two-infall chemical-evolution model to APOGEE-DR16: inner Galactic regionsare assembled on shorter time-scales compared to the external ones. In theouter disc (with radii $R>6$ kpc), the chemical dilution due to a lateaccretion event of gas with primordial chemical composition is the main driverof the [Mg/Fe] vs. [Fe/H] abundance pattern in the low-$\alpha$ sequence. Inthe inner disc, in the framework of the two-infall model, we confirm thepresence of an enriched gas infall in the low-$\alpha$ phase as suggested bychemo-dynamical models. Our Bayesian analysis of the recent APOGEE DR16 datasuggests a significant delay time, ranging from $\sim$3.0 to 4.7 Gyr, betweenthe first and second gas infall events for all the analyzed Galactocentricregions. Our results propose a clear interpretation of the [Mg/Fe] vs. [Fe/H]relations along the Galactic discs. The signatures of a delayed gas-rich mergerwhich gives rise to a hiatus in the star formation history of the Galaxy areimpressed in the [Mg/Fe] vs. [Fe/H] relation, determining how the low-$\alpha$stars are distributed in the abundance space at different Galactocentricdistances.

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