Astronomers have uncovered an incredibly faint galaxy known as Candidate Dark Galaxy-2 (CDG-2). This discovery could shed light on the mysterious substance called dark matter, which makes up about 27% of the universe.
Using the Hubble Space Telescope, researchers believe CDG-2 consists of at least 99.9% dark matter. If this is confirmed, it will be among the most dark matter-dense galaxies ever identified.
Dark matter is five times more prevalent than the ordinary matter that forms stars and planets. While we can’t see dark matter directly, we know it exists due to its gravitational pull on visible matter, acting like “the glue that holds the universe together.”
Many galaxies, including our Milky Way, have a large amount of dark matter. However, some galaxies, like CDG-2, have such a high ratio of dark matter that they contain very few stars, making them faint or “low surface brightness galaxies.” These were first discovered in the 1980s, and astronomers have since identified thousands of them.
CDG-2, located about 300 million light-years away, seems to fit a category of galaxies known as “dark galaxies,” which may have few to no stars. Dayi Li, a post-doctoral fellow at the University of Toronto, explains that these galaxies are on the extreme end of the spectrum. Unlike low surface brightness galaxies that still emit some light, dark galaxies like CDG-2 are virtually invisible.
Li mentions that dark galaxies are defined more by what they lack—like stars—than by any hard rules. CDG-2 nudges us closer to understanding this elusive class of galaxies because we previously thought something this faint couldn’t exist.
The discovery process took advantage of three telescopes: Hubble, the European Space Agency’s Euclid, and the Subaru Telescope in Hawaii. Researchers focused on globular clusters—tight groups of ancient stars. These clusters can shine brightly, even when surrounded by dark matter. There appears to be a link between globular clusters and dark matter, indicating that if CDG-2 has almost no stars, it must be predominantly composed of dark matter.
Further observations found a glow around four globular clusters in the Perseus Cluster, hinting at a galaxy’s presence. How does a galaxy end up with so little star formation? Li suggests that larger galaxies may have stripped CDG-2 of its gas needed to form stars. With that gas gone, it was left mostly as a celestial “skeleton,” primarily composed of dark matter.
In terms of brightness, CDG-2 has only 0.005% of our galaxy’s brightness, translating to about 6 million times the brightness of our sun. By focusing on globular clusters, scientists might discover more dark galaxies, although confirmation requires advanced observations, possibly with the James Webb Space Telescope.
Neal Dalal from the Perimeter Institute emphasizes the significance of studying these galaxies. Observing dark galaxies provides clear insights into dark matter’s behavior, as there is minimal interference from visible matter.
Robert Minchin, an astronomer, adds that identifying dark galaxies using globular clusters could bypass the limitations of finding them through traditional methods that rely on hydrogen gas detection, which might miss galaxies like CDG-2.
To fully validate CDG-2’s status as a dark galaxy, scientists need to measure its dark matter content. Yao-Yuan Mao from the University of Utah notes that while this is challenging due to CDG-2’s distance, the faint light observed strengthens the case that it is indeed a cohesive object rather than just random star alignments.
This exciting discovery reinforces that there is much more to explore in our universe, especially regarding the enigmatic realm of dark matter.
