Gaia-Sausage-Enceladus (GSE) is a galaxy that merged with the Milky Way around 10 billion years ago. This event significantly shaped the structure of the Milky Way, especially its halo. To understand the history of the GSE galaxy, researchers have examined the chemical composition of stars that originated from it. Ernandes and collaborators aimed to uncover GSE's star formation history by analyzing specific elemental abundances in these stars, focusing on how they compare to those in other small galaxies.

Key Aims and Methods

The study by Ernandes et al. investigates the GSE galaxy's star formation by analyzing the relative abundances of four elements: Magnesium (Mg), Iron (Fe), Barium (Ba), and Europium (Eu). These elements are associated with different processes in stars, and their ratios can provide clues about the history of a galaxy. Using data from the SAGA database, which contains extensive information about stars in the Milky Way, the team measured how these elements relate to each other in stars from the GSE population. They compared these results to dwarf galaxies such as Sculptor and Fornax to identify patterns that might explain GSE's star formation timeline.

Elemental Abundances and What They Tell Us

Different elements in stars form through various processes. For example, Mg forms quickly during supernovae explosions, while Fe comes from both supernovae and longer-lived processes, such as white dwarf stars exploding in type Ia supernovae. Ba and Eu are produced through processes called the "slow" and "rapid" neutron capture, respectively, with Eu sometimes forming in neutron star mergers. By measuring the ratios of these elements in GSE stars, Ernandes' team could map the timeline of when and how star formation occurred in the GSE galaxy.

Findings: Gradual Formation, Abrubt End

The team found three key characteristics of GSE’s star formation history:

1. A slow beginning: At low metallicity, the GSE galaxy showed higher-than-expected Ba/Mg ratios, similar to the Fornax galaxy. This suggests that star formation began gradually rather than in a single, massive burst.

2. Extended star formation: The abundances of elements indicate that star formation continued for at least 2 billion years, based on the slow increase of Eu/Mg, which tracks the contribution of the r-process.

3. Sudden quenching: The sharp cutoff in [Fe/Mg] and [Ba/Mg] ratios at higher metallicities suggests that star formation in GSE stopped abruptly, likely when it merged with the Milky Way. This prevented further gas accretion and star formation.

Comparison to Other Galaxies

Comparing the GSE to the dwarf galaxies Sculptor and Fornax helped the authors place GSE’s evolution into context. Sculptor’s star formation was rapid and stopped early, as shown by its chemical patterns. On the other hand, Fornax experienced a longer period of star formation, resembling GSE’s slower and more extended history. However, GSE’s star formation ended more abruptly than either galaxy.

Conclusion

By examining the elemental abundances of stars from GSE, Ernandes et al. concluded that GSE formed stars gradually over a long period but experienced a sudden halt in star formation, likely triggered by its merger with the Milky Way. These findings help us understand how such large-scale events shaped the Milky Way we observe today.

Source: Ernandes