Tracing the Milky Way’s Past: How Globular Clusters Reveal the History of the Gaia-Sausage-Enceladus Merger
In this second paper of the CARMA series, Fernando Aguado-Agelet and collaborators aim to uncover the assembly history of our Milky Way by studying a special group of ancient star clusters called globular clusters (GCs). They focus on those GCs thought to have arrived in the Milky Way during a major merger with a smaller galaxy known as the Gaia-Sausage-Enceladus (GSE) about 9 to 11 billion years ago. The team uses deep observations from the Hubble Space Telescope to precisely measure the ages and metallicities (a measure of chemical content) of 13 GCs, seeking clues about when and where these clusters formed.
Introduction: Why Study Globular Clusters?
The introduction explains that understanding the Milky Way’s history is like solving a giant cosmic puzzle. Previous studies, especially those using data from the Gaia satellite, have shown that our galaxy has grown by merging with smaller galaxies over billions of years. The GSE is one such galaxy, which left behind stars and GCs that now orbit in the Milky Way’s halo. Since GCs are among the oldest objects in the galaxy and their ages can be measured with high precision, they act as “fossils,” recording when and how the Milky Way grew. Earlier work showed that it’s hard to tell which stars and clusters came from which merger based on their motions alone, so the CARMA project combines motion, chemistry, and precise age measurements to untangle the story.
Data and Methods: Measuring Ages of Ancient Clusters
The authors selected 13 GCs that are likely associated with the GSE. They used archival images from the Hubble Space Telescope, focusing on two specific optical filters that work well for distinguishing stars in crowded clusters. They removed stars with uncertain membership, corrected for effects of dust (reddening), and avoided stars in the crowded centers of clusters. Ages were determined by fitting theoretical models, called isochrones, to the clusters’ color–magnitude diagrams, plots of star brightness against color that reveal a population’s age and chemical composition. They also compared their results to previously published measurements to ensure consistency.
Results: An Age-Metallicity Story
The authors found that the majority of the clusters followed a clear pattern in the age–metallicity relation (AMR): clusters that formed later had higher metallicity, consistent with chemical enrichment over time. However, two clusters, NGC 288 and NGC 6205, stood out as being more than 2 billion years older than others at the same metallicity. This suggests they likely formed within the Milky Way itself rather than being brought in by GSE. Two other clusters (NGC 5286 and NGC 7099) did not fit the main trend, possibly indicating that they came from different merger events, such as with the Sequoia galaxy. Notably, the AMR of the remaining nine clusters revealed two distinct episodes of cluster formation, separated by about 2 billion years, hinting at two major bursts of star formation.
Comparing with Field Stars: A Consistent Picture
To strengthen their findings, the team compared their GC results to the ages and metallicities of individual GSE field stars near the Sun. They found that the two bursts of cluster formation corresponded to similar features in the field star population. This suggests that the GSE experienced episodic star formation, likely triggered by interactions with the Milky Way, including its first close passage and eventual merger.
Chemical Clues: Europium and Silicon
The authors also examined the chemical properties of the clusters, focusing on ratios of elements like europium and silicon. They found a tight correlation between these chemical ratios and the clusters’ ages, which aligns with predictions from galaxy formation models. Notably, one cluster (NGC 5286) showed an unusual chemical signature, supporting the idea that it may not belong to GSE.
Summary and Implications
The study concludes that precise age and chemical measurements can reveal not just whether a globular cluster is native to the Milky Way or accreted, but also details about the history of the galaxy that brought it. The two bursts of cluster formation seen in GSE GCs may correspond to distinct phases in the GSE’s interaction with the Milky Way. The findings help pinpoint when key events in our galaxy’s assembly occurred and demonstrate the value of globular clusters as tracers of galactic history.
Source: Aguado-Agelet
