Astronomers have identified a galaxy, dubbed CDG-2, in the Perseus galaxy cluster composed of 99.99% dark matter, a record proportion for such a structure. The discovery, made using data from the Hubble Space Telescope and the Euclid mission, challenges conventional understanding of galaxy formation.
The galaxy was initially detected through an unusual concentration of globular clusters – dense, ancient collections of stars that typically orbit galaxies. Francine Marleau of the University of Innsbruck’s Institute for Astro- and Particle Physics, a co-author of the study, confirmed the presence of extremely faint, diffuse light surrounding the clusters using Euclid data. “They clearly confirm the presence of extremely faint, diffuse light and thus reveal the galaxy behind these globular clusters for the first time,” Marleau stated.
Analysis revealed that CDG-2 contains minimal gas and stars outside of its four identified globular clusters. The astronomers estimate that the globular clusters account for more than 16% of the galaxy’s brightness and 15% of its stellar mass. While ultradiffuse galaxies with dominant globular clusters have been observed previously, they were significantly brighter and richer in stars than CDG-2.
The extraordinarily high dark matter content – estimated at 99.99%, though conservative estimates place it between 99.94% and 99.98% – makes CDG-2 a unique object for studying the nature of dark matter. “CDG-2 could be the most dark-matter-dominated galaxy ever discovered,” the research team wrote in their published findings.
Researchers have also identified a second candidate, CDG-1, within the Perseus cluster. CDG-1 possesses larger and brighter globular clusters but exhibits almost no diffuse light around them. The team suggests that CDG-1 may represent an earlier stage in the evolution of galaxies like CDG-2, where all the stars are still concentrated within the globular clusters, with the remainder of the galaxy consisting solely of dark matter. “Such galaxies would be ideal test objects to check models of dark matter,” according to the research team.