NGC 288: The First "Splashed" Globular Cluster?
The Milky Way didn’t form in one smooth process—it was built over billions of years, partly by merging with smaller galaxies. A major merger occurred about 10 billion years ago when a dwarf galaxy, called Gaia-Sausage-Enceladus (GSE), collided with our galaxy. This event had a major impact: it disturbed the structure of the Milky Way and even changed the orbits of some of its oldest star clusters, known as globular clusters (GCs).
Astronomers use a combination of age, chemical composition, and movement (dynamics) to figure out where these clusters originally formed. Some GCs were born inside the Milky Way (in situ clusters), while others came from galaxies that later merged with it (accreted clusters). However, one cluster, NGC 288, has been particularly puzzling. It moves like an accreted cluster but looks like one that formed inside the Milky Way. This study, led by Edoardo Ceccarelli and colleagues, aims to solve this mystery.
Age: Does NGC 288 Fit with In Situ or Accreted Clusters?
A key method for identifying whether a GC is in situ or accreted is by comparing its age to other clusters. Typically, GCs that formed inside the Milky Way are older than those that came from smaller galaxies, because the Milky Way had a stronger and faster star-forming history.
The authors used a highly precise method developed by the CARMA collaboration to measure the age of NGC 288. They compared it to two other GCs, NGC 6218 and NGC 6362, which are confirmed in situ clusters. The result? NGC 288 is just as old as these in situ clusters, at around 13.7 billion years. This is much older than the GCs that came from GSE, which are about 2.5 billion years younger. Based on this, NGC 288 looks like it was born inside the Milky Way—not in an accreted galaxy.
Chemical Composition: A Fingerprint of Its Origins
If NGC 288 really did form in the Milky Way, then its chemical makeup should match other in situ clusters. The team analyzed high-resolution spectra of 10 stars in the cluster, focusing on elements like magnesium (Mg), silicon (Si), titanium (Ti), zinc (Zn), and europium (Eu). These elements reveal the type of environment where a cluster was formed.
Their results showed that NGC 288 has an elemental composition that strongly resembles in situ clusters. It has higher amounts of α-elements (like Mg and Ti), which form in environments with rapid star formation, such as the Milky Way. In contrast, accreted clusters tend to have lower α-elements, since dwarf galaxies formed stars more slowly. This chemical evidence further supports that NGC 288 was not part of the GSE galaxy.
A New Idea: The "Splash" Effect
If NGC 288 was really born inside the Milky Way, why does it move like an accreted cluster? The answer may lie in the Splash event. When the GSE galaxy merged with the Milky Way, it didn’t just bring new clusters—it also disturbed the ones already there. Many in situ stars were "splashed" into new orbits, making them look accreted even though they weren’t.
The study suggests that NGC 288 was originally on a stable, circular orbit but was violently shaken by the GSE merger, pushing it into a highly elliptical (stretched-out) orbit. This means that NGC 288 might be the first confirmed case of a "Splashed" GC—one that was born in the Milky Way but got kicked around by an ancient collision.
Conclusion: The Importance of Homogeneous Data
This study highlights the need for careful and consistent comparisons when determining the origin of GCs. Previous studies had contradictory results for NGC 288, but by using a uniform approach—measuring age, chemistry, and dynamics in the same way—the researchers were able to paint a clear picture. NGC 288 was born in the Milky Way but later got flung into a new orbit by a galaxy merger.
This discovery opens the door to re-examining other GCs that might have experienced similar events. Future studies may find more "Splashed" clusters, helping us better understand how ancient mergers shaped our galaxy.
Source: Ceccarelli
