Decoding the Origins of Globular Clusters with Magnesium and Aluminum Clues
Globular clusters (GCs) are densely packed groups of stars, some of the oldest in our galaxy. These ancient star clusters are thought to be leftovers from the Milky Way’s formation and from smaller galaxies that the Milky Way absorbed over time. In their paper, Lin et al. investigate whether the chemical fingerprints of stars, specifically the relative amounts of magnesium (Mg) and aluminum (Al), can tell us where these globular clusters originally formed. This method presents an alternative to traditional techniques that rely on tracking the orbits of these clusters, which can become unreliable due to the galaxy’s complex merger history.
Mining APOGEE Data for Chemical Clues
To carry out this chemical investigation, the authors used data from APOGEE, a survey that captures the light of stars in near-infrared wavelengths to measure their chemical makeup with high precision. They focused on 2,142 stars from 27 globular clusters. A major challenge is that stars in GCs come in multiple populations, with later generations showing altered chemical compositions due to internal processes. To get a clean signal from the original environment where the cluster formed, the team concentrated on “primordial populations”, the stars least affected by internal enrichment. These stars were identified by having the lowest [Al/Fe] (aluminum relative to iron) ratios in each cluster.
A Clear Chemical Divide
The results revealed a clear distinction between GCs formed in the Milky Way (called “in-situ”) and those that were captured from other galaxies (called “accreted”). For metal-rich clusters (those with [Fe/H] > –1.5), primordial stars from in-situ GCs tended to have higher [Al/Fe] values compared to accreted ones. These differences were especially clear in the [Mg/Fe]-[Al/Fe] plane, a kind of “chemical map” used to understand star formation histories. This chemically driven classification agrees well with older dynamical models, but not always, most notably for NGC 288, M4, and Terzan 9, which the authors reclassify based on their chemical signatures.
Why Chemical Abundances Matter
The paper goes further by exploring the physical reasons behind these chemical differences. In-situ clusters come from environments like the early Milky Way, which had deeper gravitational wells and more vigorous star formation, producing more massive stars that enriched their surroundings with Mg and Al. In contrast, accreted clusters formed in smaller galaxies with slower star formation, resulting in less chemical enrichment. These environmental differences are imprinted in the chemical compositions of the stars we see today.
Testing the Method's Strength
The robustness of this classification was tested in several ways. For example, using silicon (Si) instead of magnesium didn’t change the results, but the difference in Al levels between in-situ and accreted clusters was much more striking than for Si. This reinforces Al as a strong tracer of GC origin. Additionally, the authors expanded their analysis to include newer data from more globular clusters, finding consistent results and even identifying more candidates, like Terzan 9, whose chemical makeup points to an accreted origin despite its location in the bulge of the galaxy.
Conclusion: A Promising Path for Galactic Archaeology
In conclusion, Lin et al. propose a new and reliable method for tracing where globular clusters came from, based not on their orbits but on the chemistry of their oldest stars. This approach offers a powerful new tool for "galactic archaeology", the effort to piece together the history of the Milky Way. As more clusters are analyzed and higher-quality data become available, this method promises to shed even more light on the building blocks of our galaxy.
Source: Lin
