Mapping the Metals at the Milky Way’s Heart: A New Look at the Nuclear Star Cluster

At the very center of the Milky Way lies the nuclear star cluster (MWNSC), a dense collection of stars packed into just a few parsecs around the supermassive black hole Sgr A. In this paper, Schultheis et al. revisit how this cluster formed by studying its chemical composition, focusing especially on metallicity, the abundance of elements heavier than hydrogen and helium. Because different formation scenarios predict different metallicity patterns, chemical information offers a powerful way to reconstruct the MWNSC’s history. Using improved analysis techniques applied to existing infrared data, the authors aim to clarify whether the cluster formed “inside-out,” through gradual star formation near the center, or through the infall of older star clusters.

Data and Observations: Revisiting KMOS Spectra of M Giants

The study is based on low-resolution infrared spectra taken with the KMOS instrument at the Very Large Telescope, which is well suited for observing the Galactic center despite the extreme dust extinction. The authors reanalyze spectra of cool M giant stars, which dominate the stellar population of the MWNSC but are notoriously difficult to model. A key improvement over earlier work is the use of a new synthetic spectral grid optimized for these cool stars, including updated atomic and molecular line lists and the treatment of non-local thermodynamic equilibrium (NLTE) effects. After careful quality control and the removal of foreground stars and stars belonging to the surrounding nuclear stellar disc, the final sample contains 1140 M giants within the MWNSC.

Method Validation: Testing Against High-Resolution Spectra

Before interpreting the results, Schultheis et al. validate their method by comparing the KMOS-based measurements to high-resolution infrared spectra from the IGRINS instrument. By degrading the high-resolution spectra to match KMOS quality and reanalyzing them in the same way, they show that stellar parameters such as effective temperature, surface gravity, and metallicity can be reliably recovered. Typical uncertainties are about 150 K in temperature, 0.4 dex in surface gravity, and 0.2 dex in metallicity, giving confidence that the low-resolution data still capture meaningful chemical information.

Results I: The Metallicity Distribution of the MWNSC

With this validated approach, the authors examine the metallicity distribution function of the MWNSC. They find that the stars are best described by two distinct populations: a dominant metal-rich component centered at [M/H] ≃ +0.26 dex, and a significant metal-poor component centered at [M/H] ≃ −0.77 dex, making up about 17–20% of the sample. This metal-poor fraction is larger than in previous studies, which the authors attribute to their improved modeling of cool M giants rather than contamination from the surrounding nuclear stellar disc.

Results II: A Clear Metallicity Gradient

Beyond the overall distribution, the most striking result comes from mapping metallicity as a function of distance from Sgr A. The authors construct spatial metallicity maps and find a clear negative radial metallicity gradient of about −0.1 dex per parsec, meaning that stars closer to the center are, on average, more metal-rich than those farther out. This gradient is statistically significant and appears consistently in multiple analyses. Such a pattern is difficult to explain by random cluster infall alone and instead points toward a gradual buildup of the cluster from the inside outward.

Discussion: Inside-Out Formation and Links to the Nuclear Disc

In the discussion, Schultheis et al. connect this gradient to an inside-out formation scenario, where gas driven inward by the Milky Way’s bar fuels repeated episodes of star formation near the center. Over time, newer generations of stars form from increasingly metal-rich gas, naturally producing the observed gradient. This picture also strengthens the possible link between the MWNSC and the surrounding nuclear stellar disc, suggesting they may be parts of the same evolving structure rather than entirely separate components.

Conclusions: Rewriting the Formation History of the MWNSC

Overall, this study shows how revisiting existing data with improved models can significantly change our understanding of the Milky Way’s core. By establishing a robust metallicity gradient in the MWNSC, the authors provide strong chemical evidence that the cluster grew from the inside out, shaped by ongoing gas inflow and star formation rather than solely by the mergers of ancient star clusters.

Source: Schultheis