Astronomers using NASA's Hubble Space Telescope have identified a faint galaxy called CDG-2, located 300 million light-years away in the Perseus cluster, that consists almost entirely of dark matter. The discovery relied on detecting four globular clusters rather than the galaxy's dim stars. This finding highlights the role of dark matter in low-surface-brightness galaxies.
Most galaxies are visible due to their billions of stars, but low-surface-brightness galaxies like CDG-2 are exceptionally faint, with dark matter comprising the majority of their mass. Reported in The Astrophysical Journal Letters, the detection of CDG-2 marks a milestone in astronomical observation techniques.
David Li from the University of Toronto and his team employed advanced statistical methods to scan for globular clusters—dense groups of stars that orbit galaxies. These clusters served as indicators of hidden galaxies. Their search confirmed 10 known low-surface-brightness galaxies and pinpointed two new candidates, including CDG-2.
To confirm the find, researchers integrated data from NASA's Hubble Space Telescope, the European Space Agency's Euclid observatory, and Japan's Subaru Telescope in Hawaii. Hubble's high-resolution images showed four tightly grouped globular clusters within the Perseus cluster. A subtle halo of light around them suggested an underlying galaxy.
"This is the first galaxy detected solely through its globular cluster population," Li stated. He added, "Under conservative assumptions, the four clusters represent the entire globular cluster population of CDG-2."
The galaxy emits light equivalent to about 6 million Sun-like stars, with the clusters contributing 16% of that visible output. Estimates indicate that 99% of CDG-2's total mass is dark matter, an invisible substance that neither emits nor absorbs light. The scarcity of stars likely results from hydrogen gas being stripped by gravitational interactions with neighboring galaxies in the dense Perseus cluster. Globular clusters, bound tightly by gravity, resisted such disruption and thus revealed the galaxy's presence.
Ongoing sky surveys, including those from Euclid, NASA's Nancy Grace Roman Space Telescope, and the Vera C. Rubin Observatory, will leverage machine learning to uncover more such elusive objects.