Finding Galaxies in the Dark: Using Globular Clusters to Identify Ultra-Diffuse Galaxies

While classic spiral and elliptical galaxies shine brightly, another class of galaxy flies under the radar. A recent study uses a new technique to search for galaxies so faint they are nearly completely dark.

Ultra-Diffuse Galaxies

Hard to see and harder to understand, ultra-diffuse galaxies are a galaxy class with extremely low surface brightness. With low luminosities but large physical extents, these galaxies appear as barely visible smudges, making them difficult to detect. Historically, astronomers have searched for ultra-diffuse galaxies using methods that search for diffuse stellar light; however, this method only catches those bright enough to spot — if even darker galaxies exist, they will not be found this way. Is there another way to discover these elusive galaxies?

Recent studies have shown that ultra-diffuse galaxies tend to have more globular clusters on average than typical galaxies. While this raises questions surrounding their formation and evolution, the abundance of globular clusters provides another way to search for ultra-diffuse galaxies. In general, globular clusters do not clump together in space unless there is additional mass, like dark matter, pulling them together. Thus, searching for overdensities of globular clusters could reveal a hidden population of ultra-diffuse galaxies.

Candidate Confirmation

Discovered as an overdensity of three globular clusters, Candidate Dark Galaxy-2 (CDG-2) was found in data from a Hubble Space Telescope (HST) program aimed at imaging the Perseus galaxy cluster. With an updated globular cluster catalog and improved statistical method for detecting overdensities, Dayi (David) Li (University of Toronto) and collaborators perform a follow-up search and analysis to confirm CDG-2 as a galaxy. Their method searches for clustering of globular clusters that do not appear to belong to any bright galaxy and are unlikely to be randomly grouped so closely in the intergalactic medium.

From their analysis, the authors find an additional globular cluster that belongs to CDG-2, making the object even more statistically likely to be a galaxy as opposed to four randomly clumped globular clusters. Further pointing to CDG-2’s galactic nature, two stacked HST images reveal a very faint light surrounding these globular clusters. To confirm that this diffuse emission is truly associated with CDG-2, the authors use data from Euclid, a space telescope that is optimized for detecting diffuse structures. The Euclid data confirm a low-surface-brightness fuzzy emission that coincides with the emission seen with HST and corresponds to the location of the four globular clusters — strong evidence that CDG-2 is indeed a galaxy.

Globular Cluster and Dark Matter Dominated

Further analysis of CDG-2 reveals that at least ~16.6% of its total light is contained within its globular clusters. However, if CDG-2 follows the typical distribution of globular clusters in galaxies and some are still undetected, this fraction could be as high as 33%, which would make CDG-2 have the highest globular cluster-to-stellar mass ratio of any galaxy to date. With additional analysis of the mass contained within the observed globular clusters, the authors estimate that CDG-2 is highly dark-matter dominated with 99.94%–99.98% of its total mass contained within dark matter. The dark-matter fraction could be even higher if CDG-2 contains more undetected globular clusters. Future observations of CDG-2 with deeper imaging and spectroscopy will more confidently determine the galaxy’s extreme and unique structure.

Through this study, the authors have confirmed the first galaxy ever detected solely through its globular cluster population. This technique opens a promising avenue to discovering more galaxies hiding out in the dark.

Citation

“Candidate Dark Galaxy-2: Validation and Analysis of an Almost Dark Galaxy in the Perseus Cluster,” Dayi (David) Li et al 2025 ApJL 986 L18. doi:10.3847/2041-8213/adddab