The Galactic bulge, the dense central region of our Milky Way, is a fascinating and complex system made up of stars of varying ages, metallicities, and dynamics. Researchers Xiao Han and colleagues used data from three major surveys—OGLE, APOGEE, and Gaia—to study this region's structure, stellar populations, and movements. They analyzed specific types of stars, including RR Lyrae stars, red giants, and red clump stars, to learn about the formation history and evolution of the bulge.

Mapping the Bulge with RR Lyrae Stars

RR Lyrae stars, pulsating stars older than 12 billion years, are essential for understanding the Milky Way's oldest structures. The team identified over 2,000 of these stars from the OGLE-IV survey. By studying their orbits, they classified the stars into three groups: central bulge stars close to the Galactic center, inner bulge stars further out, and interlopers from the Milky Way's halo.

The central bulge stars rotated slowly and had low velocity dispersion, while inner bulge stars showed patterns consistent with the bar structure of the Galaxy. Halo stars, which were not confined to the bulge, moved faster and more chaotically. Interestingly, these findings suggested that metallicity—a measure of how much heavy elements a star contains—did not significantly affect the motions of the stars.

Insights from APOGEE Data

Using red giant and red clump stars observed in infrared light by APOGEE, the researchers performed detailed kinematic and chemical analysis. These stars' orbits and chemical compositions revealed that the inner bulge aligned with the Galactic bar, while the central bulge did not. This indicated that different parts of the bulge had distinct formation histories.

The chemical composition of stars provided further clues. Metal-rich stars (with more heavy elements) were associated with the bar structure, while metal-poor stars (older and less chemically enriched) were more evenly distributed. Patterns in elements like magnesium and oxygen hinted at how quickly the stars formed and whether they were influenced by supernova explosions.

Gaia’s Contribution to the Puzzle

Gaia DR3 provided a massive catalog of red giants with precise motions and distances. The team identified bar-like motions in metal-rich stars but found no such behavior in metal-poor stars. This reinforced the idea that the bulge's metal-rich and metal-poor populations had different origins and dynamics. The researchers also observed that halo stars contaminated the bulge samples, which could inflate velocity dispersion measurements.

The Bulge’s Shape: Boxy or X-Shaped?

The Milky Way's bulge has long been debated as being either boxy or X-shaped. By comparing the density of stars to theoretical models, the team found that a boxy bulge model better matched their observations, though some evidence for the X-shaped structure was present. This aligns with the idea that the bulge's structure evolved from the Galactic disk through bar instability.

Conclusion

This study revealed that the Milky Way's bulge is a dynamic and diverse region, shaped by the Galaxy's history of star formation and gravitational evolution. The distinction between the inner and central bulge populations, as well as the role of the halo interlopers, offers new insights into how galaxies like ours grow and change over billions of years. By combining data from multiple surveys, the researchers provided a clearer picture of our Galaxy's heart.

Source: Han