Starless Galaxy Candidate Holds 2 Billion Suns’ Mass in Hydrogen
Researchers at Swinburne University of Technology in Australia have identified AGC 322753, a gas cloud holding about 2 billion times the sun's mass in hydrogen but no visible stars. It may also be rotating, which dark galaxy models say should not happen, according to a new study.
Scientists discovered massive starless galaxy candidate AGC 322753, a cloud holding roughly 2 billion times the sun's mass in hydrogen but no visible stars, according to a study published in the Monthly Notices of the Royal Astronomical Society.
If confirmed, it would be an unusually massive example of a so-called dark galaxy: one made of gas and dark matter with few or no stars.
How the object was found
Researchers at Swinburne University of Technology in Australia compared observations from two existing radio surveys. One came from the Arecibo Observatory in Puerto Rico, the other from China's Five-Hundred-Meter Aperture Spherical Radio Telescope. Both had picked up the same massive hydrogen cloud.
Other teams have identified several dark galaxy candidates over the past decade. This one stands out because it may be rotating.
Why rotation matters
Hydrogen gas emits radio waves at a signature wavelength. That wavelength shortens when the gas moves toward Earth and stretches when it moves away.
The signal from AGC 322753 shows two distinct peaks. That suggests one side of the cloud is approaching while the other recedes, and the gas may have settled into an organized, rotating disc like those in ordinary galaxies.
"The theoretical models and the simulations show us that dark galaxies shouldn't rotate, so this is extremely interesting," said Marco Monaci, an author of the study at Swinburne University of Technology.
If the object really is a rotating disc with no stars, astronomers will have to explain how the gas organized itself without producing any.
Not quite starless
The researchers note the object is too big to be totally starless. Most galaxy formation models predict that a cloud with this much hydrogen should have formed at least a few stars, as its densest patches collapse under their own gravity and ignite.
The coauthors suggest it may instead be an "almost dark" galaxy with stars too faint to see. Even deep optical images from the DESI Legacy Imaging Surveys, which search for starlight rather than hydrogen, reveal none.
From this, the team estimates AGC322753 holds at least 40 times more mass in hydrogen than in stars. That raises the question of what suppressed star formation.
Testing the leading model
Astronomers' most successful model of the cosmos, known as Lambda-CDM, assumes two invisible ingredients: dark energy and cold dark matter. Dark matter is a hypothetical substance that physicists think makes up about 85 percent of the universe's matter.
Under the model, clumps of dark matter called haloes provide the gravitational scaffolding for galaxies. Gas gathers inside them, cools and collapses into stars, yet this appears to happen in some haloes but not others. Finding dark galaxies could help explain why.
Further observations, such as ones from the Very Large Array, a network of radio antennas in New Mexico, will be needed to confirm the finding.
