Faint Light Could Be a ‘Dark Galaxy’

Astronomers have spotted a galaxy so faint, it’s almost invisible — a discovery that could help illuminate one of the most elusive substances in the universe.

The researchers found Candidate Dark Galaxy-2, or CDG-2, using the Hubble Space Telescope, and they believe it’s made of at least 99.9% dark matter. If their finding is backed by further observations, CDG-2 would be one of the most dark matter-heavy galaxies ever found.

Dark matter dominates the universe. It is five times more abundant than regular matter — which is what all stars, planets and everything else that we can see is made of — but it is invisible and has never been directly observed.

However, its presence can be inferred due to its gravitational effects on regular matter because dark matter is the glue that holds the universe together.

Most galaxies, including our own, are dominated by dark matter. But in some cases, the ratio of dark matter to normal matter becomes so extreme that a galaxy is left with only sparse stars, making it appear very faint. Astronomers call these bodies “low surface brightness galaxies,” and have observed thousands of them since the first one was discovered in the 1980s.

CDG-2, which is about 300 million light-years from Earth, appears to be so rich in dark matter that it could belong to a hypothesized subset of low surface brightness galaxies called “dark galaxies,” which are believed to contain few or no stars. “Low surface brightness galaxies are very faint, but there’s still some light coming from them,” said Dayi Li, a post-doctoral fellow in statistics and astrophysics at the University of Toronto and lead author of a study on the discovery, which was published in The Astrophysical Journal Letters. “But a dark galaxy is just on the extreme end of that, where you basically will not have any kind of faint light or structure that you would expect from a typical galaxy.”

There is no strict definition of dark galaxies, Li explained, but their existence is predicted by dark matter theories and cosmological simulations. “Where exactly do we draw the line in terms of how many stars they should have is still ambiguous, because not everything in astronomy is as clear-cut as we like,” he said. “To be technically correct, CDG-2 is an almost-dark galaxy. But the importance of CDG-2 is that it nudges us much closer to getting to that truly dark regime, while previously we did not think a galaxy this faint could exist.”

- Author: Jacopo Prisco, CNN