LONDON — Astronomers have discovered a galaxy so faint it’s nearly undetectable. This find could shed light on dark matter, a mysterious substance that makes up most of the universe but can’t be directly seen.
They named this galaxy Candidate Dark Galaxy-2, or CDG-2. Researchers believe it consists of at least 99.9% dark matter. If confirmed, CDG-2 would be one of the densest dark matter galaxies ever identified.
Dark matter is crucial in the universe, being five times more prevalent than visible matter—the stuff that forms stars and planets. We can’t see dark matter, but we see its effects, like its grip on galaxies and clusters.
Most galaxies, including our Milky Way, are rich in dark matter. Sometimes, the dark matter far outweighs regular matter. This causes the galaxy to emit very little light, making it hard to notice. These are known as low surface brightness galaxies, and since the first one was found in the 1980s, thousands have been identified.
At about 300 million light-years away, CDG-2’s extreme dark matter content might categorize it as a “dark galaxy.” These galaxies have few or no stars at all. Dayi Li, a post-doctoral fellow at the University of Toronto, mentions, “Low surface brightness galaxies still shine faintly, but dark galaxies are at the extreme, where one may see no light.”
There’s ongoing debate about what exactly defines a dark galaxy. Li notes that while CDG-2 might not fit the strictest definitions, it brings us closer to understanding genuine dark galaxies.
To discover CDG-2, researchers utilized data from Hubble, the European Space Agency’s Euclid mission, and the Subaru Telescope. They looked for globular clusters—distant groups of old stars. Globular clusters shine brightly, even when their host galaxies don’t, hinting at the presence of dark matter.
The study revealed four globular clusters in the Perseus Cluster, a colossal cosmic formation. The glow around these clusters suggested a hidden galaxy, potentially supported by dark matter.
But how does a galaxy end up so sparse? Li believes that larger galaxies nearby stripped CDG-2 of the hydrogen it needed to create new stars, leaving just dark matter behind. “It’s like a ghost galaxy with a failed star formation process,” he states.
Current estimations suggest CDG-2 has only 0.005% of our own galaxy’s brightness—around 6 million times brighter than our sun, compared to the Milky Way’s 20 billion times.
Li argues that searching for globular clusters could be a novel way to find more dark galaxies. However, additional observations are needed to confirm CDG-2’s dark matter content, possibly using the James Webb Space Telescope.
Neal Dalal, a researcher at the Perimeter Institute, emphasizes the significance of these faint galaxies. In typical galaxies, understanding dark matter is complicated due to the interference from stars and gas. In almost-empty galaxies like CDG-2, the effects of dark matter can be more clear-cut, leading to better insights about it.
Robert Minchin, an astronomer at the National Radio Astronomy Observatory, finds the method intriguing. “It’s surprising to look for light in dark galaxies, but it highlights an important difference: there’s a big gap between what’s mostly dark and completely dark,” he observes.
Traditionally, most searches for dark galaxies used radio telescopes focusing on hydrogen gas. Minchin suggests that searching for globular clusters may lead to the discovery of many more dark galaxies.
Yao-Yuan Mao, an assistant professor at the University of Utah, expresses excitement about this finding. He notes the faint light captured in Hubble’s images, suggesting we might be observing a real cosmic object rather than just a random collection of stars.
The discovery of CDG-2 opens new doors in our quest to understand dark matter. As we gather more data, we may unlock more secrets of our universe’s hidden layers.
For further reading on dark matter, check out NASA’s exploration of these cosmic mysteries here.
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