The LEGARE Project I. Chemical evolution model of the Nuclear Stellar Disc in a Bayesian framework

Context. The nuclear stellar disc (NSD) of the Milky Way is a dense, rotating stellar system in the central ~200 pc. The NSD is thought to be primarily fuelled by bar-driven gas inflows from the inner Galactic disc. Aims. As part of the LEGARE project, we aim to construct the first chemical-evolution models for the NSD using a Bayesian approach tailored to reproducing the observed metallicity-distribution functions and compared with the available abundance ratios for Mg, Si, and Ca relative to Fe. In particular, we intend to test whether the flowing gas from the inner Galactic disc, which feeds the NSD, can reproduce the observed abundances. Methods. We adopted a state-of-the-art chemical-evolution model in which the gas responsible for the formation of the NSD is assumed to be driven by the Galactic-bar-induced inflows. The chemical composition of the accreted material is assumed to reflect that of the Galactic disc at a radius of ~4 kpc. A Bayesian framework based on Markov Chain Monte Carlo (MCMC) techniques was then employed to fit the metallicity-distribution functions of different samples of NSD stars. Results. If we take the NSD data at face value, without considering possible contamination from bulge stars, we find that a formation scenario based on the inner disc’s flowing gas is inconsistent with the low-metallicity tail of the observed metallicity-distribution function. This is because the inner disc’s metallicity, at the epoch of bar formation, was already near solar. On the other hand, models invoking dilution from additional metal-poor inflows successfully reproduce the observations. Models with different levels of gas dilution share similar gas infall timescales (ranging from 3.7 to 5.2 Gyr) and negligible galactic winds (mass-loading factors, ω, between 0.001 and 0.030). The best-fit model corresponds to an inflow with a metallicity five times lower than that of the inner disc and a moderate star-formation efficiency. The same model successfully reproduces the observed [α/Fe] − [Fe/H] abundance trends and predicts a star formation history consistent with the most recent estimates. However, if we assume that the metallicity distribution function is contaminated by metal-poor bulge stars and is restricted to stars with [Fe/H] > −0.3 dex, there is no longer any need for gas dilution. In this case, the best-fit model is characterised by a very low star formation efficiency, coupled with a mild galactic wind. Conclusions. Our analysis indicates that dilution of the inflowing gas forming the NSD is necessary to reproduce its observed chemical properties, if bulge contamination in the data is not considered. This implies that, in addition to bar-driven inflows from the inner thin disc, lower metallicity gas – possibly originating from the thick disc or from more recent accretion events – contributed to the formation of the NSD. On the other hand, when contamination by bulge stars is assumed, dilution is no longer required.