Diluting the Galactic Center: How the Milky Way’s Nuclear Stellar Disc Got Its Chemical Mix

The Milky Way’s Nuclear Stellar Disc (NSD) is a dense, rapidly rotating disc of stars packed into the central few hundred parsecs of our Galaxy. In this paper, Spitoni et al. present the first dedicated chemical evolution model of the NSD built within a Bayesian framework, as part of the LEGARE project. Their goal is to understand where the gas that formed the NSD came from, and whether gas flowing inward from the inner Galactic disc, driven by the Milky Way’s bar, can explain the observed chemical composition of NSD stars.

Observational Motivation and Galactic Environment

The authors begin by outlining why the NSD is such an interesting target. Observations show that it is chemically and kinematically distinct from both the Galactic bulge and the nuclear star cluster. Recent spectroscopic surveys reveal that NSD stars span a wide range of metallicities, including a substantial number of metal-poor stars. Because simulations suggest that most NSD stars formed shortly after the Galactic bar appeared about 8 billion years ago, the NSD offers a way to connect gas dynamics, star formation, and chemical enrichment in the Galaxy’s central regions.

Modeling the Inner Galactic Disc

To tackle this problem, the paper first describes a chemical evolution model for the inner Galactic disc at a radius of 4 kiloparsecs. This model tracks how gas accretion, star formation, and supernova explosions enrich the interstellar medium over time. A key result is that, by the time the Galactic bar formed, the inner disc gas was already close to solar metallicity. This sets an important constraint: if the NSD formed purely from this inner-disc gas, it should not contain many metal-poor stars.

A Chemical Evolution Model for the NSD

The authors then introduce their new chemical evolution model for the NSD itself. In this framework, star formation in the NSD begins only after bar formation, and the disc grows as gas flows inward. The model follows how elements such as iron, magnesium, silicon, and calcium evolve as stars form and die. Several key parameters, such as the gas infall timescale, the star formation efficiency, and the strength of galactic winds, are left free and later constrained using Bayesian Markov Chain Monte Carlo (MCMC) techniques.

Data Samples and Bayesian Fitting

Observational constraints come from metallicity distribution functions (MDFs) of NSD stars observed by Schultheis et al. (2021). The authors consider two samples: one that includes all candidate NSD stars, and another that excludes the most metal-poor stars to reduce contamination from the Galactic bulge. By comparing models with different assumptions about the chemical composition of the inflowing gas, they test scenarios ranging from purely primordial gas to gas diluted relative to the inner disc by factors of three to ten.

Key Results: The Need for Diluted Gas Inflows

The main result is clear. When the full metallicity distribution is used, models in which the NSD forms solely from inner-disc gas fail to reproduce the observed low-metallicity tail. The best-fitting models require dilution of the inflowing gas, with metallicities about one-fifth that of the inner disc. These models naturally explain both the metallicity distribution and the observed trends of element ratios like [α/Fe] versus [Fe/H]. However, if the metal-poor stars are assumed to be bulge contaminants, then dilution is no longer required, and a very different scenario, low star formation efficiency and stronger winds, can also fit the data.

Conclusions and Implications for Galactic Formation

In their conclusions, Spitoni et al. argue that the most likely picture is one in which the NSD formed from bar-driven inflows that were mixed with lower-metallicity gas, possibly from the thick disc or from later accretion events. This work demonstrates how combining detailed chemical models with Bayesian methods can disentangle complex formation histories, and it sets the stage for future spectroscopic surveys to further pin down how the Milky Way’s central regions were assembled.

Source: Spitoni