Astronomers using China's FAST radio telescope discovered a compact hydrogen cloud that may be a dark galaxy. The team posted their paper on Sept. 24. They said, "This isolated system is an ideal candidate for either a RELHIC or an ultra-faint galaxy." It contains no visible stars at all.
Astronomers using China's FAST radio telescope have discovered a compact cloud of hydrogen with no visible stars or galaxy around it. This could be one of the strongest candidates yet for an isolated "dark galaxy," containing nearly 500,000 times the sun's mass in neutral hydrogen. The discovery paper was posted to the arXiv preprint server on Sept. 24.
Ghost of a galaxy
In the lambda cold dark matter (ΛCDM) model, structure forms hierarchically: small dark matter halos form first and then merge to build larger halos and galaxies. The model also predicts that some low-mass halos never form stars at all.
Several processes can suppress star formation at low mass. One is that, after cosmic reionization, the halo's gas is heated by the ultraviolet (UV) background (the diffuse radiation from quasars and galaxies) and so never becomes cold and dense enough to form stars. Such starless, gas-bearing halos are predicted to exist and are called reionization-limited H I clouds (RELHICs) or "dark galaxies."
Since they have few to no stars, they can only be found through the light emitted by the neutral hydrogen gas—called the 21-cm hydrogen line. There is a catch: a faint dwarf galaxy can look dark just because of surface-brightness limits, so astronomers have to distinguish those from genuinely starless halos. So far, only one RELHIC has been observationally confirmed and two promising candidates have been found.
A lone wolf
In this new study, Jin-Long Xu of the National Astronomical Observatories in China and the team study an isolated neutral hydrogen cloud, J1351+0039, found in a blind H I survey with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). "It is classified as isolated because no large galaxies lie within a projected angular separation of 1.5∘ (<100 kpc at its distance) and system velocity of less than 250 km/s," the team writes in the paper.
The researchers observed it three times between July and August 2026 to get high-quality data. Astronomers did not find any cometary tails, gaseous bridges or nearby companions associated with J1351+0039—therefore ruling out a gravitational "tidal" origin. The study of its dynamics revealed a total mass of about 130 million suns.
Still, deep optical observations from the DESI Legacy Survey found no stellar counterpart. So most of its mass is attributed to dark matter, "making this isolated system an ideal candidate for either a RELHIC or an ultra-faint galaxy," the researchers say.
It has a round shape, a positive velocity and a temperature of around 10,000 Kelvin (17,540°F) in equilibrium with the UV background. All of these properties are consistent with a dark galaxy that has never cooled enough to form stars.
Random motion
J1351+0039 shows weak rotation, unlike rotationally supported disk galaxies. The gas rotates at only about 4.2 km/s, while its random motions are about 11.6 km/s. Because random motion outweighs ordered rotation, the cloud is held up mainly by the outward pressure of its own randomly moving gas.
"The pressure-supported nature therefore suggests that J1351+0039 may be a RELHIC that has gradually acquired angular momentum," the team explains.
The team notes that an optical counterpart with very low surface brightness may go undetected in DESI images. Therefore, deep space-based photographs captured with the James Webb Space Telescope or the Hubble Space Telescope may be required to confirm whether J1351+0039 is truly starless.
Previous examples of dark galaxies were not completely isolated in space. They were found near massive galaxies. The complete isolation of J1351+0039 makes it the cleanest and most compelling "isolated" RELHIC candidate identified to date, researchers conclude.