A Chemical Portrait of the Milky Way’s Heart: Mapping the Elements of the Nuclear Stellar Disc

In the heart of our Galaxy lies a unique and mysterious region: the Nuclear Stellar Disc (NSD). This disc-like structure, only a few hundred light-years across, spins around the Milky Way’s center and is packed with stars, gas, and dust. Yet, its origin and relationship with other central Galactic components, like the Nuclear Star Cluster (NSC) and the Galactic bulge, remain poorly understood. In this study, Ryde et al. conduct a comprehensive survey of chemical abundances in nine red giant stars in the NSD. By carefully comparing these with stars from other parts of the Milky Way, they aim to uncover how the NSD formed and evolved.

Introduction and Background

The NSD is a rotating disc embedded in the central part of the Milky Way, located near the supermassive black hole. Unlike the larger, surrounding bulge, the NSD is a thin, dense structure thought to have formed from gas funneled inward by the Galactic bar. Previous studies hinted at its stars being mostly old and metal-rich, but full chemical abundance patterns had not been extensively measured due to extreme dust that blocks visible light. Ryde and collaborators explain that chemical abundances act as “fingerprints,” revealing where and how stars formed, since different elements come from different types of stars and explode into space on different timescales.

Observations and Data

To overcome the heavy dust obscuration, the team used the IGRINS infrared spectrograph on the Gemini South telescope to observe nine M giant stars in the NSD. Infrared light penetrates dust more effectively, allowing the researchers to collect high-resolution spectra and analyze faint absorption lines that indicate the presence of elements like magnesium, calcium, sodium, and more. These nine stars were carefully selected to ensure they truly belonged to the NSD based on their motion and position. Each star’s light was dissected to determine how much of each element it contained.

Analysis Techniques

Using a method developed in earlier studies, the authors determined each star’s physical parameters like temperature and gravity, which are essential for interpreting spectral lines accurately. Then, they used model atmospheres and a software tool called SME (Spectroscopy Made Easy) to extract elemental abundances. By comparing the NSD stars with stars from other Galactic regions—like the NSC, the inner bulge, and the thin and thick discs—they could identify patterns and potential shared histories. Importantly, all stars were analyzed using the same technique to avoid introducing systematic errors.

Results

The study measured the abundances of 18 elements, including α-elements (like magnesium, silicon, and calcium), iron-peak elements (like nickel and cobalt), odd-Z elements (like sodium and aluminum), and neutron-capture elements (like barium and cerium). The NSD stars showed chemical patterns similar to those in the thick disc at lower metallicities (less iron), and resembled the NSC and inner bulge at higher metallicities. Most notably, sodium stood out—NSD stars showed higher sodium levels than stars from the bulge or thin disc, matching trends seen in the NSC. This peculiar sodium signature could hint at a shared origin or unique enrichment history involving metal-rich clusters like Liller 1.

Discussion

One key takeaway is the remarkable chemical continuity between the NSD, the NSC, and the inner bulge. Despite likely having experienced different star formation episodes, their stars appear chemically similar, suggesting a possibly connected evolutionary path. The absence of typical globular cluster chemical patterns—such as sodium-oxygen anti-correlations—in the NSD stars implies that they likely didn’t originate from disrupted globular clusters. However, the unusual sodium enhancement might still hint at a link to massive, chemically peculiar systems like NGC 6528 or Liller 1.

Conclusion and Future Prospects

This study breaks new ground by showing that even in the dusty, chaotic center of the Galaxy, it is now possible to perform precise chemical fingerprinting of stars. The findings reinforce the idea that different central components of the Milky Way may share a common or intertwined past. Although the current sample is small, future instruments like MOONS will allow astronomers to extend such surveys to more stars and younger populations, potentially unveiling the full history of the Milky Way’s innermost regions.

Source: Ryde