A Massive Ancient Merger: Tracing the Origins of the Gaia–Enceladus Galaxy

Astronomers have long known that the Milky Way didn’t form in isolation, it grew by merging with smaller galaxies. One of the most important of these ancient mergers involved the Gaia–Enceladus dwarf galaxy, sometimes called the “Gaia Sausage.” In their new study, Olcay Plevne and Furkan Akbaba attempt to answer a difficult question: how massive was Gaia–Enceladus before it was swallowed by the Milky Way? To do this, they combine state-of-the-art observations with chemical evolution models, showing how the galaxy grew and eventually stopped forming stars.

Introduction: Why Gaia–Enceladus Matters

Gaia–Enceladus is thought to be the largest known merger event in the Milky Way’s history. It brought in a huge number of stars that now make up much of the stellar halo, especially those on eccentric, plunging orbits with low metal content. Metals in astronomy mean any element heavier than helium, and metal-poor stars are important because they formed early in the universe. However, distinguishing Gaia–Enceladus stars from other accreted systems, like the Sequoia or Sagittarius galaxies, is tricky. Plevne and Akbaba argue that looking at detailed chemical fingerprints, alongside stellar motions, offers the best path to uncovering its history.

Data and Methods: Combining Surveys with Machine Learning

The authors began with nearly 734,000 stars from the APOGEE and Gaia surveys, narrowing this down to about 137,000 giant stars after applying strict quality cuts. They then used two machine learning tools, t-SNE (a dimensionality reduction algorithm) and HDBSCAN (a clustering method), to separate different stellar populations. This allowed them to identify groups of stars that likely belonged to Gaia–Enceladus, based on both their chemical composition (especially elements like magnesium, manganese, and aluminum) and their orbital properties. After carefully excluding contamination from other known structures, they isolated a clean sample of 884 stars tied to Gaia–Enceladus.

Identifying the Gaia–Enceladus Population

The key evidence came from comparing chemical ratios and orbital energies. Gaia–Enceladus stars stood out for being metal-poor, high in orbital eccentricity, and distinct in abundance diagrams such as [Mg/Fe] versus [Fe/H]. These chemical “maps” show how galaxies enrich themselves with metals over time through processes like supernova explosions. By combining multiple diagnostic planes, the authors confirmed that their selected stars matched the expected patterns of Gaia–Enceladus debris.

Modeling Chemical Evolution

With these stars in hand, the authors turned to OMEGA+, a chemical evolution code. They fitted models using a technique called MCMC, which explores many possible parameter values until the best match is found. Their results indicate that Gaia–Enceladus had an initial gas mass of about 4.9 billion times the mass of the Sun. This is at the higher end of previous estimates and suggests Gaia–Enceladus may have been as massive as the Large Magellanic Cloud, one of the Milky Way’s largest current satellites. The model shows that the galaxy experienced a short, intense burst of star formation, followed by strong outflows of gas driven by supernovae, which cut its star formation off within the first 4 billion years.

Discussion: What the Results Mean

The findings imply that Gaia–Enceladus played a major role in building today’s Milky Way. Its rapid star formation was dominated by core-collapse supernovae, while Type Ia supernovae had less time to contribute. This explains why its stars are relatively low in iron compared to those formed later in the Milky Way. The study also highlights how chemical evolution modeling can independently estimate the masses of ancient galaxies, even when only stellar fossils remain. Interestingly, the authors note that if Gaia–Enceladus had continued evolving without merging, its stars might have become chemically indistinguishable from those formed inside the Milky Way.

Conclusion: A Giant Among Dwarfs

Plevne and Akbaba’s work suggests that Gaia–Enceladus was not just another small dwarf galaxy but one of the most massive the Milky Way ever accreted. By combining machine learning with chemical evolution models, they provide strong evidence that its initial gas mass was nearly five billion solar masses. This reinforces the view that early mergers like Gaia–Enceladus were central in shaping the Milky Way’s halo, and that chemical signatures offer a powerful way to reconstruct the hidden past of our Galaxy.

Source: Plevne