NASA’s Hubble telescope detects possible ‘dark galaxy’

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

Searching for light in darkness

To observe CDG-2, the researchers used data from three telescopes — Hubble, the European Space Agency’s Euclid space observatory and the Subaru Telescope in Hawaii — along with a novel approach that involved looking for objects called globular clusters. “These are very tight, spherical groupings of very olds stars, basically the relics of the first generation of star formation,” Li said.

Globular clusters are bright even if the surrounding galaxy is not, and previous observations have shown a relationship between them and the presence of dark matter in a galaxy, Li added. Because CDG-2 appears to have very few stars, there must be something else providing the mass that the clusters need to hold themselves together. Li and his colleagues assume that the source of the mass is dark matter.

The researchers found a set of four globular clusters in the Perseus Cluster, a group of thousands of galaxies immersed in a cloud of gas and one of the most massive objects in the universe. Further observations revealed a glow or halo around the globular clusters, suggesting the presence of a galaxy.

But how does a galaxy end up with few or no stars and mostly dark matter?

Astronomers believe, Li explained, that after the formation of the clusters early in the galaxy’s existence, larger surrounding galaxies stripped it of the hydrogen gas required to make more individual stars like our sun. “The material that this galaxy needed to continue to form stars was no longer there, so it was left with basically just a dark matter halo and the four globular clusters.” The process, he added, would leave behind a skeleton or ghost of “a galaxy that pretty much just failed.”

As a result of this formation mechanism, the galaxy only has 0.005% of the brightness of our own galaxy, Li said. “In terms of starlight, it is about 6 million times as bright as our sun. Our own galaxy’s brightness is about 20 billion times the brightness of the sun,” he noted.

Looking for globular clusters could be “an entirely new method to find these potentially dark galaxies,” Li argued, adding that they should exist in abundance. However, more observations are needed to detail the physical properties of CDG-2 and confirm how much dark matter it contains, which Li said could be achieved by using the James Webb Space Telescope.

‘Dark is slightly bright’

Studying potential dark galaxies is important because they provide nearly pristine views of the behavior of dark matter, according to Neal Dalal, a researcher at the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, Canada, who was not involved with the study.

“In big galaxies with lots of stars, like our Milky Way galaxy, the stars and gas can have a significant impact on the distribution of dark matter, making it difficult to disentangle the effects of ordinary matter from the effects of dark matter,” he wrote in an email. But in these extremely faint galaxies, there are so few stars and so little gas that the behavior of their dark matter must be nearly unaffected by the ordinary matter, he concluded. “Therefore we get a much cleaner probe of dark matter physics.”

One of the most interesting things about this galaxy is the way it was found, using globular clusters, said Robert Minchin, an astronomer at the National Radio Astronomy Observatory in Socorro, New Mexico, via email. “It seems odd, at first sight, to search for light from dark galaxies but, to misquote the Princess Bride, ‘There’s a big difference between mostly dark and all dark. Mostly dark is slightly bright,’” he noted. Minchin also did not participate in the work.

Most dark and almost-dark galaxy candidates, Minchin said, have been identified using radio telescopes and searching for hydrogen gas, but those efforts will miss galaxies like CDG-2, where the gas has been removed. “Looking for their globular clusters avoids that problem, and it seems likely that other very dark galaxies will be found by this method in the future.”

To truly confirm CDG-2 as a dark galaxy, we need to measure its dark matter content, which remains incredibly challenging given its distance, according to Yao-Yuan Mao, an assistant professor of physics and astronomy at the University of Utah in Salt Lake City.

“This is a very exciting find,” Mao, who was also not part of the study, added in an email. “The faint, diffuse light seen in the Hubble images of CDG-2 makes a strong case that we are looking at a cohesive object, rather than a random alignment of four bright globular clusters.”

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