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.