Astronomers have spotted a galaxy they believe is made of 99.9% dark matter, reports CNN — and it's so faint, it's almost invisible:
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.... [Post-doctoral astrophysics/statistics fellow Dayi Li at the University of Toronto was lead author on a study about the discovery, and tells CNN] There is no strict definition of dark galaxies... 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."
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... 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...
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.
Robert Minchin, an astronomer at New Mexico's National Radio Astronomy Observatory, told CNN that "it seems likely that other very dark galaxies will be found by this method in the future."
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