Some galaxies appear to contain all the ingredients needed to make stars, yet have failed to produce almost any.
Scientists have puzzled over why for years. Recent findings suggest that these “dark galaxies”—that is, galaxies that contain gas and dark matter but few or no stars—may be lurking throughout the cosmos. Finding them could help astronomers understand why some galaxies form stars while others remain dark, and test the best models of galaxy formation.
Researchers’ most successful model of the cosmos, known as Lambda-CDM, assumes the existence of two invisible ingredients: dark energy, the mysterious property thought to be driving the accelerating expansion of the universe, as well as cold dark matter, a hypothetical substance that physicists think makes up about 85 percent of the universe’s matter.
Clumps of dark matter, called haloes, provide the gravitational scaffolding for galaxies to form and grow, according to the model. Gas collects inside these haloes, eventually cooling and collapsing to form stars. But astronomers are still trying to understand why this process appears to happen in some haloes but not others.
One possibility is that some contain vast quantities of hydrogen gas, yet form very few stars. That would make them extremely difficult to spot.
Now, researchers have found a promising candidate for such a dark galaxy and published their findings in the Monthly Notices of the Royal Astronomical Society. The object, known as AGC 322753, contains roughly 2 billion times the sun’s mass in hydrogen, yet appears to have no stars, though the researchers note that it is too big to be totally starless. However, if confirmed, it would be an unusually massive example of a starless galaxy.
Researchers at Swinburne University of Technology in Australia made the discovery by comparing observations from two existing radio surveys, one conducted by the Arecibo Observatory in Puerto Rico and the other by China’s Five-Hundred-Meter Aperture Spherical Radio Telescope. Both telescopes had picked up a massive cloud of hydrogen.
Although other researchers have identified several dark galaxy candidates over the past decade, this one stands out for another reason: It may be rotating.
Hydrogen gas emits radio waves at a signature wavelength, which shortens when the gas moves toward Earth and stretches when it moves away. The signal from AGC 322753 shows two distinct peaks, suggesting that one side of the gas cloud is moving towards us while the other is receding.
This could mean that the gas has settled into an organized, rotating structure, much like the discs in ordinary galaxies, despite apparently having formed no stars.
“The theoretical models and the simulations show us that dark galaxies shouldn’t rotate, so this is extremely interesting,” says Marco Monaci, an author of the study at Swinburne University of Technology.
If AGC 322753 really is a rotating disc with no stars, astronomers will have to explain how the gas organized into such a structure without producing them.
That isn’t the only puzzle. Most models of galaxy formation predict that a cloud holding this much hydrogen should have formed at least a few stars, as its densest patches collapse under their own gravity and ignite.
The paper’s coauthors suggest it may be very unlikely that AGC 322753 contains absolutely no stars. So, instead, they think it may contain stars too faint to see–an “almost dark” galaxy. If so, they must be very faint indeed, because even deep optical images from the DESI Legacy Imaging Surveys, which search for starlight rather than the hydrogen gas detected by radio telescopes, reveal none.
From this, the team estimates that AGC322753 contains at least 40 times more mass in hydrogen than in stars, raising questions about what exactly suppressed stars from forming.
But further observations, such as ones made by the Very Large Array, a network of radio antennas in New Mexico, will be needed to confirm whether AGC 322573 is truly a dark—or almost dark—galaxy.
“Finding out more about it really is an essential next step at the moment. It’s intriguing, and I want to know more,” says Robert Minchin at the National Radio Astronomy Observatory.
Minchin knows how misleading such signals can be. He was involved in the discovery of VIRGOHI21, a dark galaxy candidate found in the early 2000s that was later reinterpreted as a long stream of hydrogen gas pulled from a nearby galaxy by gravity, known as a tidal tail.
Gas torn away like this can keep some of its original motion and mimic the signature of rotation, so Minchin says more observations are needed to rule out the same explanation for AGC 322753. Yet there is no obvious nearby galaxy it could have come from.
Rhys Taylor, an astronomer at the Astronomical Institute of the Czech Academy of Sciences in Prague, says tidal debris is unlikely to explain the observations. Reproducing the candidate dark galaxy’s distinctive double-peaked profile through such a process “is technically possible, but it’s fantastically unlikely,” Taylor says.
Taylor and Minchin both flag another concern, though. The two radio surveys that the researchers used placed the hydrogen at noticeably different positions in the sky. That suggests AGC 322753 may be an extended cloud of gas rather than a compact galaxy, another possibility that follow-up observations could help untangle.
Whether AGC 322753 turns out to be completely dark or merely exceptionally faint, understanding objects like it could still help astronomers work out why some galaxies form stars while others apparently do not, says Elisabeth Adams at the Planetary Science Institute.
One possible reason is a galaxy that spins so fast that its gas stays too spread out to collapse. Another is that ultraviolet light from the universe’s earliest stars heated the gas, keeping it too warm to condense. Studying such almost-dark galaxies could help tell these phenomena apart, revealing how the conditions of the early universe shaped the galaxies we see today.
“Once it’s no longer a dark galaxy, people seem to stop caring, and I actually think that’s not the right approach,” says Adams.
But if AGC 322753 truly is starless, it would be one of the most massive dark galaxies ever identified, potentially helping explain where the missing galaxies predicted by Lambda-CDM have been hiding. It could even help astronomers understand why some gas-rich galaxies form stars while others don’t, providing a valuable test for models of galaxy formation—and perhaps even offering new clues about the nature of dark matter itself.