A spinning dark ‘ghost’ galaxy discovered in our galactic neighbourhood

Astronomers believe they have found a ghost galaxy that is nothing but a fast-moving cloud of gas that is small and has no stars. It is a “dark galaxy”, and researchers say it was hiding in plain sight. Scientists say that if the galaxy really exists, then it could help solve the mysterious “missing satellite” problem.

The study was published in Science Advances on Friday (April 18). An international team of researchers stumbled upon the galaxy while using China’s huge FAST radio telescope. What they have found is AC G185.0–11.5, a compact hydrogen cloud sitting inside a larger high-velocity cloud (HVC) known as AC-I. HVCs typically move around at great speeds. However, AC-I is a gas cloud that spins.

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The gas in the cloud is arranged in a disk shape and has a clear rotational pattern. While it looks like a galaxy, AC G185.0–11.5 only comprises hydrogen gas, with no lights, no stars. This is why it is called a “dark galaxy”. The team believes a large dark matter halo holds the cloud together. 

Are dark galaxies real? 

“Dark galaxies” have existed only in theory till now. But the scientists say that AC G185.0–11.5 fits the bill perfectly, since it has a lot of dark matter and no stars. Although such galaxies can have stars, they are not visible.

They concluded that it is a galaxy, even though it has no stars, based on the parameters it fulfils. Using the cosmic yardstick called the Tully-Fisher relation, the cloud is estimated to be at a distance of about 278,000 light-years from Earth. This means it is our galactic neighbourhood. Its mass also fits the definition of a galaxy, as the cloud sits between 30 million and nearly 500 million Solar masses. 

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The team says that AC G185.0–11.5 is the best evidence yet for a galaxy that is all gas and cloud but no stars. Previously, some high-velocity clouds have been suspected to be galaxies, but either they didn’t rotate or were too tough to be distinguished from the Milky Way’s halo.

If the discovery proves to be true, it might be a starting point from which to learn more about “missing galaxies.” They might just be there, it’s just that we can’t see them because they are all dark matter and no stars.