Searching for Ghost Clusters in the LMC: Are the Missing Star Clusters Really Missing?

The Large Magellanic Cloud (LMC), a small neighboring galaxy to our Milky Way, has puzzled astronomers for decades with a curious mystery: a noticeable lack of star clusters that are between 4 and 11 billion years old. This period, known as the "LMC cluster age gap," stands in contrast to the thousands of star clusters in the galaxy’s catalogue, and even more starkly against the millions of field stars, individual stars not grouped in clusters, that did form during this time. The study by Andrés E. Piatti sets out to test whether some newly identified candidates might finally fill in this missing chapter of the LMC’s history.

Targeting Three Candidates

To investigate, the research focuses on three star cluster candidates named STEP0004, STEP0012, and YMCA0012, which were originally identified by Gatto et al. (2020) as potentially belonging to the age gap. Using the powerful GEMINI South telescope and its GMOS instrument, Piatti and collaborators collected high-resolution images through special filters to analyze the light and color of stars in these regions. These observations allowed the team to construct detailed color-magnitude diagrams (CMDs), a key tool in astronomy that plots a star’s brightness against its color, helping to determine its age and other properties.

Cleaning the Star Fields

The researchers cleaned these CMDs to remove field stars (unrelated stars in the same line of sight) by comparing them with thousands of control regions. By assigning statistical probabilities to each star, based on how often similar stars appeared in other nearby regions, the team isolated the stars most likely to belong to real clusters. They found that very few stars had a high enough probability of being cluster members. In fact, the CMDs did not reveal the clear and coherent stellar sequences expected from true star clusters.

Testing with Stellar Clustering Algorithms

To confirm these results, the team used a clustering algorithm called HDBSCAN to detect any actual physical groupings of stars. This method doesn’t assume a cluster exists but instead searches for clumps of stars based on their positions. While the tool did detect several stellar overdensities, including the candidate locations from Gatto et al., their CMDs looked very similar to those of random fluctuations in the background star field. In other words, these "clusters" might just be illusions caused by uneven distributions of stars across the LMC.

Revisiting Cluster Properties

Further analysis added to the skepticism. The distances and metallicities (chemical compositions) assigned to these candidates didn’t match expectations for star clusters formed during the LMC’s age gap. Instead, their properties resembled much younger stellar populations, inconsistent with their supposed ancient origins. Moreover, these three candidates and several others from the same study are all located in a small southwestern section of the LMC, an odd pattern not seen in confirmed age gap clusters, which are spread across the galaxy.

What These Results Mean

Taken together, the evidence points toward a disappointing but important conclusion: these candidates are likely not real star clusters. Instead, they may simply be small areas where stars in the LMC’s disk appear slightly more densely packed. However, this result is still valuable. It suggests that rather than being hidden, most of the LMC’s missing clusters may never have formed, or were later stripped away by gravitational interactions with the Milky Way. This idea fits with recent simulations showing that close encounters between galaxies could have disrupted cluster formation or even ejected clusters into intergalactic space.

Looking Forward

Piatti’s detailed and methodical study reinforces that finding real LMC age gap clusters is extremely challenging. But it also nudges astronomers to refine their models of how galaxies like the LMC form, evolve, and interact. The mystery of the age gap persists, but each new analysis brings us a step closer to understanding the cosmic forces that shape our universe.

Source: Piatti