Unveiling the Heart of the Milky Way: Mapping Mass and Motion in the Galactic Centre

In their recent paper, Feldmeier-Krause et al. present a detailed investigation of the inner ~60 parsecs of our Galaxy, focusing on the motion and distribution of stars in the Milky Way’s core. This region includes two major structures: the nuclear star cluster (NSC), a dense grouping of stars within a few parsecs of the central black hole, and the nuclear stellar disc (NSD), a larger, flatter collection of stars extending tens of parsecs outward. By combining stellar velocity data and metallicity (a measure of the chemical composition of stars), the authors used advanced modelling to explore how mass is distributed and how stars move in this densely packed region.

The Study Setup

To conduct this analysis, the authors compiled a catalog of about 4,600 stars, using both their motions—measured as proper motions across the sky and line-of-sight velocities—and their metallicities, which help identify distinct stellar populations. They combined data from various instruments, including KMOS on the Very Large Telescope and Flamingos-2 on Gemini South, allowing for broad spatial coverage of the Galactic centre. Proper motion data from several sources were carefully cross-matched with velocity and chemical data to ensure reliability and exclude younger, dynamically distinct stars.

One-Population Models

The researchers first tested a model assuming that the stars belong to a single population influenced by the gravitational pull of the supermassive black hole (Sgr A*) and surrounding stars. They confirmed that their stellar motion data yielded a black hole mass of about 4.35 million solar masses—very close to the value obtained from tracking individual stellar orbits. Interestingly, they found little evidence for a significant dark matter component within the central ~30 parsecs and concluded that the mass-to-light ratio (used to estimate stellar mass from light) did not vary significantly with distance from the centre.

Two-Population Models

The authors then expanded their approach to consider that the stars may not all share the same origin or properties. By using metallicity as a tracer, they identified two distinct groups: a high-metallicity ([M/H]) population and a low-metallicity one. The high-[M/H] stars, making up over 90% of the total stellar density, rotate faster and follow more circular orbits—suggesting they formed locally from gas funneled inward by the Galactic bar. In contrast, the low-[M/H] stars exhibit more radial motion and may have originated outside the Galactic centre, possibly brought in by merging star clusters or dwarf galaxies.

Mass Distribution Findings

Using these models, the team constructed a detailed map of how mass is distributed in the Milky Way’s centre. They found that the enclosed mass within ~30 parsecs is somewhat lower than previous estimates. This discrepancy likely stems from their use of high-quality data in a region that was previously under-sampled. Their findings refine our understanding of how the central regions of the Milky Way are structured and how they evolved.

Implications and Conclusion

This study highlights the value of combining precise stellar velocity measurements with chemical information to separate different stellar populations in a dense and complex region. It shows that most stars near the Galactic centre likely formed in situ, shaped by the internal dynamics of the Milky Way. The lower metallicity population, however, offers a glimpse into past interactions and the assembly history of the Galaxy’s nucleus. Feldmeier-Krause et al.’s results help bridge the gap between small-scale studies near the black hole and larger-scale views of the Milky Way’s bulge and disc, contributing to a more complete picture of our Galaxy’s central dynamics.

Source: Feldmeier-Krause