NASA's Hubble Telescope Detects Possible 'Dark Galaxy'
Astronomers have discovered a galaxy so dim, it's almost invisible, shedding light on one of the universe's most enigmatic components: dark matter. The Hubble Space Telescope has identified Candidate Dark Galaxy-2, or CDG-2, which is believed to be composed of at least 99.9% dark matter. If further observations confirm this, CDG-2 would be among the most dark matter-rich galaxies ever found.
Dark matter, invisible and undetectable, constitutes five times more of the universe than regular matter, which forms stars, planets, and visible objects. Its gravitational influence on regular matter is how we infer its existence. Most galaxies, including our Milky Way, are dominated by dark matter. However, some galaxies have an extreme ratio of dark to normal matter, resulting in sparse stars and a faint appearance, known as 'low surface brightness galaxies.'
CDG-2, located 300 million light-years away, appears to be exceptionally rich in dark matter, potentially belonging to a hypothetical category of low surface brightness galaxies called 'dark galaxies,' which are thought to have few or no stars. Dayi Li, a post-doctoral fellow at the University of Toronto, explains that dark galaxies are at the extreme end of faint galaxies, lacking any faint light or structure typically associated with galaxies.
The definition of dark galaxies is not strict, but their existence is predicted by dark matter theories and cosmological simulations. CDG-2, while almost dark, is significant because it brings us closer to understanding the truly dark regime, previously thought impossible for such faint galaxies.
To observe CDG-2, researchers utilized data from Hubble, the European Space Agency's Euclid space observatory, and the Subaru Telescope in Hawaii, along with a novel approach involving globular clusters. These clusters, consisting of old stars, are bright even in faint galaxies and have been linked to dark matter in previous observations.
The Perseus Cluster, a massive collection of galaxies, was studied, revealing a set of four globular clusters and a glow or halo around them, suggesting a galaxy's presence. Astronomers believe that larger surrounding galaxies stripped the hydrogen gas required for star formation early in CDG-2's existence, leaving it with a dark matter halo and globular clusters.
CDG-2's brightness is 0.005% of our galaxy's, making it about 6 million times dimmer than the sun. This discovery method, using globular clusters, could be a new way to find dark galaxies, which are expected to be abundant. However, more observations are needed to confirm CDG-2's dark matter content, which could be achieved with the James Webb Space Telescope.
Studying dark galaxies is crucial as they offer a nearly pristine view of dark matter behavior. Neal Dalal, a researcher at the Perimeter Institute for Theoretical Physics, notes that in large galaxies with many stars, the stars and gas can influence dark matter distribution, making it challenging to separate ordinary matter from dark matter. In faint galaxies, this separation is more straightforward, providing a cleaner probe of dark matter physics.
The discovery of CDG-2, found using globular clusters, is intriguing. Robert Minchin, an astronomer at the National Radio Astronomy Observatory, finds it odd to search for light from dark galaxies, but emphasizes the importance of understanding the difference between mostly dark and all dark galaxies. Most dark galaxy candidates have been identified using radio telescopes and hydrogen gas, but this method misses galaxies like CDG-2, where gas has been removed.
Measuring CDG-2's dark matter content is essential to confirm its status as a dark galaxy, a challenging task due to its distance. Yao-Yuan Mao, an assistant professor at the University of Utah, calls it an exciting find, and further observations with the Hubble images suggest a cohesive object rather than a random alignment of globular clusters.
