Astronomers from the University of California Santa Cruz (UCSC), along with their collaborators, have used the Hubble Space Telescope (HST) to conduct an in-depth study of an intriguing ultra-diffuse galaxy known as FCC 224, located in the Fornax galaxy cluster. This galaxy, approximately 65 million light years away from Earth, has a rich and peculiar globular cluster system, providing valuable clues about the evolution of galaxies and their star populations. The findings, published on January 18 on the arXiv pre-print server, offer new insights into the properties of FCC 224 and its globular clusters (GCs), which are crucial for understanding the broader picture of galaxy formation and evolution.
The Mystery of Ultra-Diffuse Galaxies
FCC 224 is classified as an ultra-diffuse galaxy (UDG), a term used to describe galaxies that are characterized by their extremely low surface brightness, despite having relatively large sizes. These galaxies are distinct from the more common, compact galaxies in that they are spread out over much larger areas with a diffuse collection of stars, making them difficult to detect and study.
FCC 224 is estimated to be around 10 billion years old, which places it among the older galaxies in the universe. It is a quiescent galaxy, meaning it is not undergoing significant star formation at present, but instead has evolved to a point where star formation has ceased. This characteristic is important because it can provide a snapshot of the later stages of galaxy evolution. With a major axis effective radius of approximately 6,160 light years, FCC 224 is relatively large, though its low surface brightness makes it appear more diffuse compared to other galaxies of similar size. The galaxy’s mass has been estimated to be around 200 million solar masses, placing it in the category of relatively low-mass galaxies.
The Globular Clusters of FCC 224
One of the most striking features of FCC 224 is its globular cluster system. Globular clusters (GCs) are dense collections of tightly bound stars that orbit galaxies, serving as useful laboratories for studying the evolution of stars and galaxies. These clusters are often some of the oldest and most massive star systems in the universe, and their properties can reveal important information about the early stages of galaxy formation. In particular, the formation of globular clusters is believed to be closely linked to intense periods of star formation in the early history of galaxies.
FCC 224 is particularly interesting due to the presence of 12 unusually luminous globular clusters. Some of these clusters exhibit absolute magnitudes of around -9.0, which is significantly brighter than typical globular clusters in other galaxies. These highly luminous clusters have drawn the attention of astronomers because they suggest that FCC 224 might have experienced a unique or unusual star formation history.
Investigating the Overluminous Globular Cluster System
A team of astronomers, led by Yimeng Tang from UCSC, sought to investigate the reasons behind the unusually luminous globular clusters in FCC 224. To carry out this research, they utilized the powerful imaging capabilities of Hubble’s Wide Field Camera 3 (WFC3), which enabled them to capture detailed images of the galaxy and its globular clusters. In addition to Hubble’s observations, the team complemented their study with spectroscopic data obtained from the Keck Cosmic Web Imager (KCWI), an instrument mounted on the Keck Observatory in Hawaii.
Their findings provided several key insights into the nature of the galaxy and its globular clusters. One of the most notable observations is that the colors of the globular clusters in FCC 224 closely match the diffuse starlight of the galaxy itself, with a very narrow color spread. This uniformity in color indicates that the globular clusters likely formed during an intense, single-burst star formation event, which would have occurred early in the galaxy’s history. This suggests that FCC 224 experienced a rapid and short-lived period of star formation in its youth, which is typical of early-type galaxies.
Mass and Size of the Globular Cluster System
The total mass of the globular clusters in FCC 224 was calculated to be around 3.8 million solar masses, representing approximately 2% of the galaxy’s total stellar mass. This fraction is higher than what is typically seen in other galaxies, suggesting that FCC 224 has an unusually massive globular cluster system relative to its overall mass. The globular clusters in this galaxy are not just more numerous but also appear to be overmassive on average. This finding challenges the typical scaling relations between galaxy mass and the mass of its globular cluster system, offering new clues into the processes that govern globular cluster formation.
In addition to their overmassive nature, the globular clusters in FCC 224 were found to have relatively small sizes, with half-light radii ranging from 7.8 to 15.6 light years. This size range places the clusters on the smaller end of the spectrum compared to typical globular clusters observed in other galaxies. Despite their small sizes, the clusters are notably bright, which further supports the idea that they are unusually massive.
Radial Mass Segregation and the Top-Heavy Luminosity Function
Another intriguing feature of the globular clusters in FCC 224 is the presence of radial mass segregation. This phenomenon occurs when the more massive globular clusters are located closer to the center of the galaxy, while the less massive ones are found further out. The researchers observed that the globular clusters closer to the galaxy’s core were brighter and more massive, a trend that mirrors the behavior seen in globular cluster systems of other galaxies. This suggests that the formation and evolution of the globular clusters in FCC 224 may have followed a pattern similar to that of other galaxies, with more massive clusters forming at the galaxy’s center.
The researchers also noted that the luminosity function of the globular clusters in FCC 224 was top-heavy, meaning that there are more bright, massive clusters than would be expected based on a typical distribution. This is another unusual characteristic of the galaxy’s globular cluster system, as most galaxies tend to have more faint clusters than bright ones. The top-heavy luminosity function suggests that the conditions in FCC 224 may have been conducive to the formation of particularly luminous globular clusters, potentially driven by a high-density star-forming environment.
Insights into Galaxy Evolution
The findings of this study provide important insights into the formation history and evolution of FCC 224. The observed narrow color spread of the globular clusters, along with the lack of significant color gradients in the galaxy, points to a history of intense, burst-like star formation in the galaxy’s past. The fact that the galaxy is now quiescent suggests that this burst of star formation occurred early in the galaxy’s evolution and has since ceased.
The overmassive globular cluster system in FCC 224 further suggests that the galaxy may have experienced an intense period of star formation, leading to the formation of exceptionally massive clusters. These clusters could have formed in a highly dense environment, where gas and dust were concentrated, promoting the creation of larger, brighter stars.
Conclusion and Future Research
The results from this study are significant in that they provide new information about the nature of ultra-diffuse galaxies and their globular cluster systems. FCC 224’s peculiar characteristics challenge our current understanding of galaxy formation, and the study authors conclude that further theoretical research is required to explain the origin of these unusual properties. While the study reveals much about the properties of FCC 224 and its globular clusters, it also raises new questions about the processes that govern the formation and evolution of galaxies.
By continuing to investigate galaxies like FCC 224, astronomers can gain deeper insights into the early stages of galaxy evolution, the formation of star systems, and the dynamics of globular clusters. As new tools, like the James Webb Space Telescope (JWST), come online, it is likely that our understanding of galaxies such as FCC 224 will continue to evolve, revealing even more about the mysteries of the universe.
Reference: Yimeng Tang et al, An Unexplained Origin for the Unusual Globular Cluster System in the Ultra-diffuse Galaxy FCC 224, arXiv (2025). DOI: 10.48550/arxiv.2501.10665