Unraveling the Mystery of the Faintest Galaxies: A Deep Dive into Sagittarius II and Aquarius II
Our Milky Way is surrounded by many small, faint companions, known as dwarf galaxies and star clusters. Some of these are so dim that it is challenging to determine whether they are truly galaxies with dark matter or just clusters of stars held together by their own gravity. In a new study, Daria Zaremba and her collaborators explore two of these puzzling objects—Sagittarius II (Sgr2) and Aquarius II (Aqu2)—using observations from the powerful Gemini/GHOST spectrograph. Their goal? To figure out whether these objects are galaxies or merely large star clusters.
A Challenge in Classification
Recent astronomical surveys have revealed many small star systems, but their true nature is often unclear. Dwarf galaxies are believed to be surrounded by dark matter, an invisible substance that provides extra gravity, while star clusters do not have this extra mass. One way to tell them apart is by measuring how fast their stars move—if the stars are moving very fast in random directions, the system likely has dark matter. Another clue comes from stellar chemistry: galaxies typically show a broader range of chemical elements compared to clusters, since they have undergone more complex histories of star formation and supernova explosions.
The Stars of the Study
The research team used high-resolution spectra from the Gemini/GHOST instrument to study five stars—three from Sagittarius II and two from Aquarius II. By analyzing their velocities, metallicities (the amount of elements heavier than hydrogen and helium), and detailed chemical compositions, they aimed to uncover whether these systems are galaxies or star clusters.
Aquarius II: A Miniature Galaxy?
Aquarius II seems to behave like an ultra-faint dwarf galaxy (UFD)—one of the smallest known types of galaxies dominated by dark matter. The two stars in Aqu2 show significant differences in their chemical makeup, with one star having much more iron than the other. This suggests that Aqu2 underwent an incomplete mixing of heavy elements, which is a hallmark of a dark matter-dominated system that has gone through only a brief period of star formation. The presence of potassium (K) enrichment also hints at rare contributions from massive stars called super-AGB stars, which is unusual for small stellar clusters.
Sagittarius II: A Star Cluster or Something More?
Sagittarius II, on the other hand, remains an enigma. Its stars do not show a clear spread in velocities or metallicities, which would usually suggest it is a star cluster rather than a dwarf galaxy. However, the discovery of one star that is enhanced in europium (Eu)—a rare element produced in neutron star mergers—makes the classification tricky. Such r-process enhanced stars have been found in both galaxies and star clusters, so this alone does not provide a clear answer. The researchers conclude that Sgr2 may be one of the faintest, most ambiguous satellites of the Milky Way, and additional studies, including simulations, are needed to fully understand its nature.
Why This Matters
The smallest galaxies in the universe are key to understanding how dark matter works and how galaxies like the Milky Way formed. By pushing the limits of current technology with GHOST, the researchers show how difficult it is to classify the faintest objects orbiting our galaxy. Their work highlights the need for even more advanced techniques, such as detailed star-by-star analyses and computer simulations, to truly determine whether these systems contain dark matter.
Final Thoughts
Sagittarius II and Aquarius II represent the challenge of identifying the faintest galaxies in the universe. Aquarius II appears to be a dark matter-dominated dwarf galaxy, while Sagittarius II remains uncertain. This study demonstrates how astronomers use chemistry and motion to classify these objects and emphasizes the importance of continuing to observe and model the smallest, most ancient structures in the cosmos.
Source: Zaremba
