Unpacking the Chemical History of a Galaxy in Ruins: A Close Look at the Sagittarius Dwarf

The Sagittarius dwarf spheroidal galaxy (Sgr dSph) is one of the Milky Way’s nearest companions—and one of its most disrupted. As it orbits the Milky Way, Sgr is gradually being torn apart by gravitational forces, forming long streams of stars. In this context, A. Liberatori and collaborators aim to understand how the galaxy evolved chemically by examining the elemental makeup of its stars. Their study presents a detailed analysis of 37 red giant branch (RGB) stars located in Sgr’s main body, offering fresh insights into the galaxy’s star formation and enrichment history.

Choosing the Right Stars

Previous work focused mainly on Sgr’s dense nucleus, where stars are easier to observe but may not represent the entire galaxy. The authors took a different approach: they selected stars based on precise motion and distance data from the Gaia mission, ensuring the sample included a wide range of metallicities and excluded stars from M54, a nearby globular cluster. This strategy allowed them to examine stars across the galaxy’s metallicity spectrum—an important step for tracing how chemical elements were built up over time.

Observing Chemical Fingerprints

The researchers used the FLAMES-UVES spectrograph at the Very Large Telescope to obtain high-resolution spectra of the stars. From these spectra, they measured the abundances of 21 elements ranging from oxygen to europium. These elements originate in different types of stars and stellar explosions. For instance, α-elements like oxygen and magnesium are produced in core-collapse supernovae (CC-SNe), while iron and manganese come from Type Ia supernovae (SNe Ia), which explode later in a galaxy’s life. Europium is made in rare, energetic events like neutron star mergers through what’s known as the r-process.

Tracking Chemical Evolution

One major result is the identification of a feature known as the “α-knee”—the metallicity at which SNe Ia begin to significantly affect chemical enrichment. In Sgr, this knee occurs at a lower metallicity (\[Fe/H] ≈ –1.5 to –1.3) than in the Milky Way. This supports the idea that Sgr had a slower star formation rate. At metallicities below the knee, Sgr’s stars resemble those in the Milky Way. Above it, the chemical signatures start to diverge, reflecting a different balance of supernova types and stellar contributions.

A Reduced Role for Massive Stars

Key elements like manganese, nickel, and zinc reveal that Sgr’s chemical evolution relied less on very massive stars. The lower [Zn/Fe] values suggest that hypernovae—extremely powerful explosions of massive stars—were less common in Sgr than in the Milky Way. Meanwhile, the trend of decreasing [Ni/Fe] points to a higher rate of sub-Chandrasekhar mass SNe Ia in Sgr. These less massive explosions contribute less nickel relative to iron, helping explain the observed chemical patterns.

A Boost in Neutron-Capture Elements

The study also finds unusually high abundances of elements made through neutron-capture processes. Elements like barium, lanthanum, and neodymium, produced in asymptotic giant branch (AGB) stars, are enhanced at high metallicity—suggesting that low- and intermediate-mass stars played a strong role in Sgr’s later chemical evolution. Even more striking is the elevated europium content, hinting at an efficient r-process in Sgr. This implies that events like neutron star mergers occurred frequently enough to leave a lasting chemical signature.

Galactic Comparisons and Final Thoughts

Overall, the study shows that Sgr followed a unique chemical path. While its earliest stars shared similarities with those of the Milky Way, its later evolution involved fewer massive stars, a higher fraction of certain types of supernovae, and strong neutron-capture element production. When compared with other dwarf galaxies like the Large Magellanic Cloud, the chemical similarities suggest that Sgr may have once been much more massive than its current state implies. This detailed chemical snapshot not only deepens our understanding of Sgr’s past but also informs how galaxies like our own are built from the merging of smaller systems.

Source: Liberatori