Unveiling the Chemical Legacy of the Sagittarius Dwarf Galaxy

The Sagittarius dwarf spheroidal galaxy (Sgr dSph) is a small satellite galaxy of the Milky Way that provides a window into the processes of galactic formation and evolution. Sara Vitali and collaborators conducted an in-depth chemical analysis of 111 giant stars from Sgr’s core to study its chemical enrichment and star formation history. Using high-resolution spectroscopy from the Pristine Inner Galaxy Survey (PIGS), this study spans a broad range of metallicities and explores the evolutionary interplay between the Sgr dSph and the Milky Way.

Introduction

The researchers framed their study in the context of the hierarchical formation of galaxies, where large galaxies like the Milky Way form through mergers and accretion of smaller systems. The Sgr dSph, discovered in 1994, has been heavily shaped by tidal interactions with the Milky Way over billions of years. It hosts a range of stellar populations, from ancient metal-poor stars to younger, more metal-rich stars. Understanding these populations helps uncover the galaxy’s star formation history and its role in enriching the Milky Way's halo.

Methods

The team targeted stars in Sgr using data from the PIGS survey, focusing on metallicity-sensitive photometry and high-resolution spectra. Observations with the FLAMES/GIRAFFE spectrograph provided detailed chemical abundances for elements such as iron, magnesium, calcium, and europium. The study relied on spectroscopic tools and models to derive atmospheric parameters like temperature and surface gravity, while also ensuring precise stellar membership criteria to exclude contamination from foreground Milky Way stars.

Results

• Metallicities and Star Formation: Sgr stars have metallicities ranging from -2.13 to -0.35 dex. This range confirms an extended star formation history, with older, metal-poor stars forming early and subsequent generations experiencing chemical enrichment.

• Chemical Trends: The α-elements (e.g., Mg, Ca) are lower in Sgr stars than in the Milky Way at comparable metallicities, indicating a slower star formation rate. Heavy elements formed through neutron capture (e.g., Ba, Eu) reveal contributions from both slow and rapid nucleosynthesis processes, reflecting Sgr’s unique chemical evolution.

• Age-Metallicity Relation: The data suggest a rapid phase of star formation early in Sgr’s history, followed by a gradual decline and episodic later star formation.

Discussion

The observed chemical patterns indicate that Sgr’s evolution mirrors that of other dwarf galaxies. Its slower star formation rate and contributions from asymptotic giant branch (AGB) stars have led to distinct elemental abundances compared to the Milky Way. These findings highlight the galaxy’s protracted history of chemical enrichment and its role as a contributor to the Milky Way’s stellar halo.

Conclusion

This comprehensive study demonstrates how spectroscopic analysis of Sgr stars reveals its layered star formation history and chemical evolution. By comparing these findings with other dwarf galaxies and Milky Way components, the authors enhance our understanding of galaxy formation in the early universe.

Source: Vitali