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

TL;DR

The paper develops the first NSD-specific chemical-evolution models within a Bayesian framework, using MCMC to fit NSD MDFs and compare to $[\alpha/Fe]$ trends. It tests bar-driven inflows from the inner disc with varying dilution levels and finds that significant dilution (inflow metallicity ~$1/5$ of the inner-disc at bar formation) is needed to reproduce the NSD MDF without bulge contamination, while mixing with purely inner-disc gas is disfavored. The preferred model (ENR_D05) also yields a plausible SFR history and abundance-ratio behavior consistent with Ryde et al. (2025), and it implies a mixed inflow history including metal-poor gas possibly from the thick disc or external accretion; bulge contamination can remove the need for dilution. The results highlight the NSD's complex assembly and the impact of data contamination on inferences, with future surveys (MOONS) expected to sharpen constraints on inflow compositions and star-formation efficiencies.

Abstract

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. As part of the LEGARE project, we construct the first chemical evolution models for the NSD using a Bayesian approach tailored to reproduce the observed metallicity distribution functions (MDFs) and compared with the available abundance ratios for Mg, Si, Ca relative to Fe. We adopt 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 MCMC framework is used to fit the MDFs of different samples of NSD stars. If we take the NSD data at face value, without considering a possible contamination from bulge stars, we find that a formation scenario based on the inner disc flowing gas is inconsistent with the low metallicity tail of the observed MDF. This is because the inner disc 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. The best-fit model requires inflow metallicity 5 times lower than the inner disc and a moderate star formation efficiency. The same model successfully reproduces the observed [$α$/Fe] vs. [Fe/H] ratios and predicts a star formation history consistent with the most recent estimates. However, if we assume that the MDF is contaminated by metal poor bulge stars and restricted to [Fe/H] > -0.3 dex, gas dilution is no longer required. In this case, the best-fit model has a very low star formation efficiency and a mild galactic wind.

Figures (12)

• Figure 1: Chemical evolution of the inner Galactic disc at 4 kpc, computed with the same model parameters adopted in spitoni2021 but using the stellar yields of romano2010. Left panel: Predicted age-metallicity relation highlighting the minimum [Fe/H] value after the bar formation (Age $\leq$ 8 Gyr). Right panel: Evolution of the abundance ratios [Fe/H] versus [X/Fe] (with X = Mg, Si, Ca). Large black-edged circles connected by the vertical green line indicate the predicted chemical composition of the ISM at the epoch of bar formation.
• Figure 2: Evolution of the abundance by mass $X_{4D,i}(t)$ of the elements considered in this study ($i$=Mg, Si, Ca and Fe) as predicted by the model of the inner Galactic disc at 4 kpc after the formation of the Galactic bar (Age< 8 Gyr).
• Figure 3: Comparison between the observed NSD MDF (Sample A, filled distributions, see Section \ref{['sec_data_MDF']} for details) and the predictions of the best-fit models (with parameters listed in Table \ref{['tab_mcmc']}), after applying a Gaussian convolution with $\sigma_{\rm [Fe/H]}$=0.2 dex (“Model smooth”). The analytical fit to this distribution, indicated as “smooth (fit)” in the likelihood definition of eq. \ref{['like']}, is also shown.
• Figure 4: Corner plots showing the PDFs of the chemical–evolution model parameters for the different models reported in Table \ref{['tab_mcmc']} (one in each panel). For each parameter, the median and the 16th and 84th percentiles of the posterior PDF are shown with dashed lines above the marginalised PDF. All models have adopted the Sample A data.
• Figure 5: Comparison of the infall time‐scale $\tau$ (red symbols and line, left‐hand y‐axis) and the star‐formation efficiency $\nu$ (black symbols and line, right‐hand y‐axis) for the different models reported in Table \ref{['tab_mcmc']}.