Liller 1: A Galactic Mystery, Uncovering the Origins of a Massive Star Cluster in the Milky Way’s Heart
Anna Liptrott and her collaborators set out to answer a long-standing question in Galactic archaeology: could the dense stellar system known as Liller 1 be one of the original building blocks of the Milky Way’s bulge? In the modern model of galaxy formation, large galaxies like our own form through the merging of smaller systems. Some astronomers have proposed that ancient star clusters like Liller 1 might be remnants of these primordial “bulge seeds.” Earlier studies revealed that Liller 1 contains stars of different ages and metallicities (chemical compositions), suggesting a complex and massive history. Using data from the APOGEE survey, the authors compare Liller 1’s detailed chemical fingerprint to stars in other parts of the Galaxy to find its true origins.
Methods: Using APOGEE to Decode Stellar Chemistry
The team used data from SDSS-IV APOGEE, which measures precise chemical abundances and motions of hundreds of thousands of stars using infrared light. They first identified stars belonging to Liller 1 using their positions, velocities, and proper motions, narrowing the list down to fourteen confirmed members. For comparison, they selected stars from the Galactic bulge, inner and outer disk, and the Solar neighbourhood, grouped by distance from the Galactic center. Each comparison was designed to test one of several competing ideas: that Liller 1 might be a leftover from the bulge’s early formation, a rejuvenated star cluster that formed new stars later, or a clump from the early disk that migrated inward over time.
Analysis: Comparing the Chemistry of Liller 1 and the Galaxy
The authors focused on six key elements, carbon, nitrogen, oxygen, magnesium, silicon, and calcium, which can reveal the history of star formation in a system. They also removed one “second generation” star (rich in nitrogen) to avoid bias, as such stars can appear in globular clusters due to internal chemical recycling. Liptrott and colleagues then compared Liller 1’s chemical makeup with that of stars in each Galactic region, using a statistical method that measures how different the abundance ratios are from the average values in the Milky Way’s bulge and disk.
Results: Liller 1 Stands Apart
The results showed that Liller 1 is chemically distinct from the Galactic bulge at a high level of significance, particularly in the α-elements (Mg, Si, and Ca), which are crucial indicators of rapid star formation in early galaxies. These elements in Liller 1 are lower than in the bulge by roughly 2–3 standard deviations, meaning that its stars likely formed under different conditions. The same analysis also showed that Liller 1’s chemistry does not match that of the inner or outer disk, nor of the Solar neighbourhood. Its stars are too metal-rich and chemically inconsistent with these Galactic components, suggesting it did not form as part of the Milky Way’s normal disk evolution.
Discussion: Ruling Out the Building Block Hypothesis
Because Liller 1’s chemical pattern differs from the bulge, Liptrott et al. conclude that its progenitor system could not have been a major contributor to the bulge’s mass. This finding mirrors earlier studies of another complex cluster, Terzan 5, once thought to be a bulge building block. Moreover, the mismatch with inner disk stars rules out the idea that Liller 1 was a rejuvenated cluster, one that later accreted gas and formed new stars. The differences with outer and Solar neighborhood stars also argue against the idea that it was an inward-migrating clump formed from early disk instabilities.
Conclusion: An Isolated Survivor
In summary, the APOGEE data reveal that Liller 1 is a unique and chemically distinct system, likely representing a minor contributor to the inner Galaxy’s stellar population. It may even be of extragalactic origin, perhaps accreted from a smaller galaxy long ago. While it shares some traits with known globular clusters, its multi-age, multi-metallicity populations and mass point to a more complex origin story. As Liptrott and her team show, even deep in the Milky Way’s crowded core, some ancient systems still preserve the chemical fingerprints of events that shaped our Galaxy billions of years ago.
Source: Liptrott
