Unraveling the Galactic Halo: Identifying Components in the Milky Way’s Stellar Halo

In a recent study, Elliot Y. Davies and collaborators tackle the complex structure of the Milky Way’s stellar halo by applying a technique known as Blind Source Separation (BSS), specifically through a method called Non-negative Matrix Factorization (NMF). This approach allows them to isolate and study distinct chemical and kinematic components of the halo using data from the APOGEE survey. By separating the stellar halo into different chemical components, the team aims to uncover more about the galaxy’s formation and accretion history.

Introduction to the Galactic Halo

The Milky Way’s stellar halo consists of stars with varying chemical compositions, tracing back to both in-situ formation and accretion from other galaxies. Davies and his team treat this halo as a “blind source separation” problem, where NMF helps separate the mixed signals from different stellar populations. This approach is effective because NMF doesn’t rely on prior assumptions about the data, allowing for the identification of unexpected substructures in the stellar halo.

Data and Methods

The researchers utilize data from the APOGEE DR17 catalog, focusing on stars in the Milky Way’s stellar halo. After making quality cuts to filter out unreliable data, they categorize the halo into two samples: a chemically pure halo (based on metallicity) and a kinematically pure halo (based on motion and position). This division allows the team to study both chemical and spatial features of the halo. The NMF method then decomposes the data into distinct, non-overlapping components, identifying trends in chemical markers like [Fe/H], [Mg/Fe], and [Al/Fe] across the halo.

Identifying the Halo’s Key Components

Davies’ team initially applies a two-component model to the halo, uncovering a distinction between low-energy (likely in-situ) and high-energy (likely accreted) stars. The low-energy component, which makes up about 66.5% of the halo, is primarily found in the galaxy's inner regions. It has a relatively high [Fe/H] content, indicating that it may have originated from the Milky Way itself. In contrast, the high-energy component, occupying the outer halo, displays lower [Fe/H] and lower [Al/Fe], suggesting an accreted origin, possibly from dwarf galaxies absorbed by the Milky Way.

Expanding the Model

The team further divides the halo into four components for a detailed examination. Here, they identify components that correlate with known substructures: HE1 and HE2 dominate the outer halo, while LE1 and LE2 populate the inner regions. The HE2 component reveals an intriguing structure named "Eos," which has both in-situ and accreted characteristics. Eos is thought to have formed from material originating within the Milky Way but influenced by an accretion event, possibly the Gaia Sausage/Enceladus (GSE) merger, a significant contributor to the Milky Way’s halo.

Applications of the Halo Components

By analyzing the spatial distribution of these components, the researchers observe that the inner halo (within ~8.7 kpc) is dominated by in-situ material, while accreted stars are more prevalent beyond this distance. This trend aligns with prior studies suggesting that the outer halo primarily consists of accreted stars. When examining the chemical distributions, they find that the in-situ halo is metal-rich with lower [Mg/Fe], while the accreted halo is metal-poor with higher [Mg/Fe].

Investigating Unusual Substructures

In addition to Eos, another unique component named "Aurora" is identified. Aurora spans a range in metallicity like Eos but displays distinct chemical markers, connecting it to both in-situ and accreted origins. The existence of these substructures within the Milky Way’s halo reflects the complex history of mergers and star formation that shaped the galaxy’s current structure. Aurora’s properties provide insights into how accreted material interacts with in-situ star populations.

Conclusion

This study by Davies et al. highlights the power of NMF in disentangling the Milky Way’s stellar halo, revealing intricate details about its formation history. Their findings support the idea that the stellar halo is a composite of stars from both the Milky Way and smaller galaxies it accreted over time. By uncovering these different components, the team contributes valuable insights into the galaxy’s evolutionary past, advancing our understanding of how large galaxies like the Milky Way come together.

Source: Davies