Exploring the Galactic Halo with RR Lyrae Stars
The Milky Way’s halo is a vast, diffuse region surrounding the galaxy, composed of stars with different origins. Some of these stars formed within the Milky Way, while others were brought in by galaxies that merged with our own in the past. Understanding the chemical makeup and motion of stars in the halo helps astronomers reconstruct the history of our galaxy. In this study, Cabrera Garcia et al. examine more than 135,000 RR Lyrae stars—pulsating variable stars that act as excellent tracers of the Milky Way’s structure—to investigate the halo’s formation history.
RR Lyrae Stars as Galactic Tracers
RR Lyrae stars are old, metal-poor stars that have been used for decades to study the Milky Way. They are useful because their periodic brightness changes allow astronomers to determine their distances with high precision. The authors use a sample of RR Lyrae stars identified by the Gaia satellite, which provides high-quality measurements of their positions, motions, and brightness. By combining this data with metallicity estimates, the researchers can classify stars into different components of the halo and track their movement through space.
Dividing the Halo into Inner and Outer Components
The Milky Way’s halo is often described as having two main parts: an inner halo and an outer halo. The inner halo contains stars with relatively higher metallicity (richer in elements heavier than hydrogen and helium) and more circular orbits. The outer halo, on the other hand, consists of more metal-poor stars on highly eccentric orbits, suggesting that many of these stars were once part of smaller galaxies that were absorbed by the Milky Way.
By analyzing how the metallicity of RR Lyrae stars changes with distance from the Galactic center, the authors confirm this two-part structure. They find that the inner halo is dominant within 25 kpc (about 80,000 light-years) from the Galactic center, whereas the outer halo becomes more prominent at greater distances. This result supports previous studies that suggested a dual-halo model for the Milky Way.
Finding Groups of Stars with Common Origins
Stars that were once part of the same satellite galaxy or cluster tend to move together through space. To identify such groups, the authors apply a machine-learning clustering algorithm called HDBSCAN, which groups stars based on their orbital properties. This analysis reveals 97 dynamically tagged groups (DTGs)—collections of RR Lyrae stars that share similar motions. Some of these groups correspond to known halo substructures, such as the Gaia-Sausage-Enceladus (GSE), a remnant of a major galaxy collision that shaped the Milky Way’s inner halo.
The study also identifies stars associated with other known structures like the Helmi Stream, Sequoia, and the Sagittarius Stream, providing further evidence that the Milky Way’s halo was built up through multiple merger events.
The Bigger Picture
The results of this study reinforce the idea that the Milky Way’s halo is a complex structure with multiple origins. The presence of distinct stellar groups suggests that our galaxy has experienced several major and minor merger events over billions of years. By combining detailed chemical and motion data from RR Lyrae stars, astronomers can continue to piece together the story of how the Milky Way grew over time.
Source: Cabrera Garcia
