A potential dark galaxy — one made primarily of dark matter — may have been spotted in the local universe.
Dark galaxies are theoretical, starless systems whose discovery could help astronomers better understand galaxy formation. The new candidate was found within a large, fast-moving cloud of gas first seen in the 1960s. High-resolution observations of the cloud, reported April 18 in Science Advances, revealed a compact clump of gas that might be a dark galaxy.
“This is the first discovery of a potential dark galaxy in the nearby universe,” says astronomer Jin-Long Xu of the Chinese Academy of Sciences in Beijing.
Not all astronomers agree. The clump’s classification as a dark galaxy is plausible, says astronomer Tobias Westmeier, but he finds the evidence lacking. Instead of a relatively distant dark galaxy, Westmeier thinks the object is more likely a regular gas cloud at the edge of the Milky Way.
“I’m very skeptical about the claims that they’re making in the paper for a number of reasons,” says Westmeier, of the University of Western Australia in Crawley. “There’s no convincing evidence.”
Since the early 2000s, a handful of candidate dark galaxies have been discovered around the Milky Way. But in most cases, further observations revealed the alleged dark galaxies to be misclassified. Many turned out to have a small population of faint stars that were initially overlooked. So far, none of the candidates have proved to be truly dark.
“People have been trying to find these starless galaxies for many years and so far without much success,” Westmeier says.
The new results were compiled from observations using three radio telescopes, including high-resolution images from the Five-hundred-meter Aperture Spherical Telescope in southern China. In addition to dark matter, dark galaxies contain hydrogen gas, which can be observed at radio wavelengths. Additional observations at visible wavelengths from two stellar surveys suggest that the clump is starless.
Analysis of the radio telescope data allowed the researchers to determine the speed and direction of hydrogen gas in different parts of the clump. The data were also used to indirectly measure the distance to the clump, which the team estimated to be 900,000 light-years from Earth. Rotational movement seen in the clump’s gas suggests it could be a disk galaxy, rather than a clump of ordinary gas.
“The most [exciting] part is the finding of the rotating disk structure,” says study coauthor Ming Zhu, an astronomer also at the Chinese Academy of Sciences.
But Westmeier notes that if the distance measurement is wrong, this rotational signature could indicate the clump is just a gas cloud falling into the Milky Way. Zhu agrees that a more accurate distance estimate and higher resolution data of the disk’s rotation are necessary to confirm the candidate as a dark galaxy.
Many astronomers theorize that dark galaxies are building blocks of larger galaxies. As dark galaxies collide with star-studded ones, their extra gas promotes more star formation. Dark galaxies are also thought to form from regular galaxies stripped of their stars following a collision or interaction. The new study’s team thinks that the new potential dark galaxy formed in this way after colliding with intergalactic gas in our galactic neighborhood.
Ultimately, finding dark galaxies could help astronomers refine computer simulations to better understand how galaxies form and evolve. Studying the shape of these galaxies could also refine astronomers’ understanding about what dark matter is made of.
“Dark galaxies are the most primitive state of a general galaxy formation,” Xu says. Their discovery, he says, can help confirm whether galaxy formation starts with dark galaxies.