Astronomers identified a galaxy so faint it’s almost invisible — a discovery that could help shed light on one of the most elusive substances in the Universe.
Researchers have found Candidate Dark Galaxy-2, or CDG-2, using the Hubble Space Telescope, and believe it to be composed of at least 99.9% dark matter.
If the discovery is confirmed by new observations, CDG-2 may be one of the galaxies with the highest concentration of dark matter ever found.
A dominates the Universe. It is five times more abundant than ordinary matter — which makes up all the stars, planets and everything else we can see — but it is invisible and has never been directly observed.
However, its presence can be inferred because of its gravitational effects on ordinary matter, as 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 the galaxy is left with just a few 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 may belong to a hypothetical subset of low-surface-brightness galaxies called “dark galaxies.”which is believed to contain few or no stars.
“Low surface brightness galaxies are very faint, but there is still some light coming from them,” said Dayi Li, a postdoctoral researcher in statistics and astrophysics at the University of Toronto and lead author of the study on the discovery, published in The Astrophysical Journal Letters. “But a dark galaxy is at the extreme end of that, where there’s basically no kind of faint light or structure that you would expect from a typical galaxy.”
There is no rigid definition for dark galaxies, Li explained, but their existence is predicted by dark matter theories and cosmological simulations. “Where exactly we draw the line in terms of how many stars they should have is still somewhat ambiguous, because not everything in astronomy is as clear as we would like,” he said. “Technically, CDG-2 is a near-dark galaxy. But the importance of CDG-2 is that it brings us much closer to reaching this truly dark regime, whereas before we didn’t believe such a faint galaxy could exist.”
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Searching for light in the darkness
To observe CDG-2, researchers used data from three telescopes — Hubble, the European Space Agency’s Euclid space observatory, and the Subaru Telescope in Hawaii — as well as an innovative approach that involved searching for objects called globular clusters. “They are very dense and spherical groupings of very old stars, basically relics of the first generation of star formation,” explained Li.
Globular clusters are bright even when the surrounding galaxy is not, and previous observations have shown a link between them and the presence of dark matter in a galaxy, Li added. Since CDG-2 appears to have very few stars, there must be something else providing the mass necessary for the clusters to hold together. Li and his colleagues assume that the source of this mass is dark matter.
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. Additional observations revealed a glow or halo around the clusters, suggesting the presence of a galaxy.
But how does a galaxy end up having few or no stars and being mostly composed of dark matter?
Astronomers believe, Li explained, that after the clusters formed early in the galaxy’s existence, larger surrounding galaxies stripped it of the hydrogen gas needed to form more individual stars like our Sun. “The material that this galaxy needed to continue forming stars was no longer there, so there was basically just a halo of dark matter and the four globular clusters.” The process, he added, would leave behind the skeleton or ghost of “”.
As a result of this formation mechanism, the galaxy is only 0.005% as bright as our own galaxy, Li said. “In terms of starlight, it is about 6 million times brighter than our Sun. Our galaxy’s brightness is about 20 billion times the Sun’s brightness,” he noted.
Searching for globular clusters could be “an entirely new method for finding these potentially dark galaxies,” Li argued, adding that they likely exist in abundance. However, more observations are needed to detail the physical properties of CDG-2 and it contains, something that, according to Li, could be achieved using the James Webb Space Telescope.
‘Dark is slightly bright’
Studying possible dark galaxies is important because they offer a nearly untouched view 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 in the study.
“In large galaxies with many stars, like our Milky Way, stars and gas can have a significant impact on the distribution of dark matter, making it difficult to separate 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 dark matter should be virtually unaffected by ordinary matter. “So we get a much cleaner analysis of dark matter physics.”
One of the most interesting things about this galaxy is the way it was discovered, using globular clusters, said Robert Minchin, an astronomer at the National Radio Astronomy Observatory in Socorro, New Mexico, also in an email. “It seems strange, at first glance, to look for light in dark galaxies, but, to quote freely from the movie ‘The Princess Bride,’ ‘there is a big difference between almost completely dark and totally dark. Almost totally dark is slightly bright,'” he noted. Minchin also did not participate in the work.
Most dark or near-dark galaxy candidates, Minchin explained, have been identified using radio telescopes and searching for hydrogen gas, but these efforts would not detect galaxies like CDG-2, where the gas has been removed. “Searching for their globular clusters avoids this problem, and it seems likely that other very dim galaxies will be found by this method in the future.”
To truly confirm CDG-2 as a dark galaxy, it is necessary to measure its dark matter content, which remains extremely challenging due to its distance, according to Yao-Yuan Mao, assistant professor of physics and astronomy at the University of Utah in Salt Lake City.
“This is a very exciting discovery,” added Mao, who was also not part of the study, in an email. “The faint, diffuse light seen in Hubble images of CDG-2 strongly supports the idea that we are observing a cohesive object, rather than a random alignment of four bright globular clusters.”
