Mapping Our Galaxy in Unprecedented Detail: Why the Milky Way Needs a New Stellar Census
This white paper, led by Bergemann and collaborators, argues that understanding how the Milky Way formed and evolved is still limited by gaps in our data. Although astronomers now have vast stellar datasets and improved computer models, they still lack a single, continuous, high-precision “chemo-dynamical” map of the Galaxy, that is, a map that combines stellar motions (dynamics) with detailed chemical compositions (chemistry) across the entire disc and bulge. Without such a map, key questions remain unresolved, including how the earliest discs survived or were destroyed, how the Galactic bulge formed its multiple components, and how mergers and long-term internal processes shaped today’s Milky Way.
The Science Case: Competing Pictures of Galactic Formation
The paper’s science case begins by describing a shift in how astronomers think about Galactic formation. Competing models suggest very different histories: some propose a neat, sequential assembly of the disc and bulge, while others predict a messy early history shaped by mergers and gas accretion. Observations are now revealing stars in the disc and bulge that are extremely low in heavy elements (“very metal-poor”), which may be relics of the earliest star-forming epochs. These discoveries raise exciting possibilities, such as finding surviving stars connected to the first stellar generations, but current and upcoming surveys still struggle to measure their properties precisely enough or in large enough numbers.
Chemo-Dynamics and the Role of Galactic Structure
A major theme of the paper is that Galactic structure is far from simple or static. Features such as ridges, arches, and streaming motions in stellar orbits point to ongoing interactions between the bar, spiral arms, and even nearby satellite galaxies. To untangle these effects, astronomers need extremely accurate chemical abundances and full three-dimensional motions for stars spread across the entire disc, including dusty and distant regions that are hard to observe. Existing and planned facilities cannot provide this combination of precision, depth, and Galaxy-wide coverage, leaving many dynamical questions unanswered.
The Challenge of Understanding the Galactic Bulge
The authors then turn to the Galactic bulge, the dense central region of the Milky Way. The bulge contains stars spanning a wide range of ages and metallicities, suggesting multiple formation pathways, from early disc material to disrupted star clusters or dwarf galaxies. Observing the bulge is particularly challenging because dust and crowding block optical light. While future surveys will improve the situation, they will still miss key regions or lack sufficient depth, preventing a complete reconstruction of how the bulge’s different components are connected.
Star-Formation History and the Galactic Potential
Reconstructing the Galaxy’s full star-formation history also requires knowing where stars were born, not just where they are today. Because stars can migrate large distances over billions of years, this demands precise age measurements and detailed chemical “fingerprints” for millions of stars. Such data would also allow astronomers to measure the Milky Way’s gravitational potential and dark matter distribution by linking stellar chemistry to orbital behavior. According to the paper, achieving the necessary accuracy requires high-resolution spectroscopy on a scale far beyond what any currently planned 2030s survey can deliver.
A New Facility for a Complete Galactic Map
The paper concludes by outlining what is needed: a new, wide-field, 10-meter-class spectroscopic facility capable of observing tens of millions of stars at both high and low spectral resolution over most of the sky. The required technology largely builds on existing instruments, but the scale of data processing would be unprecedented. With such a facility, astronomers could finally produce a complete and precise chemo-dynamical map of the Milky Way, establishing our Galaxy as the definitive benchmark for understanding how disc galaxies form and evolve across cosmic time.
Source: Bergemann
