Phys.org reports astronomers found the cloud using China's FAST radio telescope, with no visible stars around it.
Phys.org reports astronomers using China's FAST radio telescope discovered a compact cloud holding nearly 500,000 times the sun's mass in hydrogen. With no visible stars or surrounding galaxy, it may be an isolated 'dark galaxy'.
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Astronomers using China’s Five-hundred-meter Aperture Spherical radio Telescope, or FAST, have identified an isolated cloud of neutral hydrogen that may be a rare galaxy with almost no stars. The object, J1351+0039, is described in a Sept. 24 preprint and in a Phys.org report.
A cloud holding nearly 500,000 times the sun’s mass in hydrogen may be an isolated “dark galaxy,” with no visible stars or surrounding galaxy. @arxiv
The researchers estimate that the cloud contains about 480,000 times the sun’s mass in neutral hydrogen and has a total dynamical mass of roughly 130 million suns. Deep images from the DESI Legacy Survey did not reveal a visible stellar counterpart, making the object a candidate for a reionization-limited H I cloud, or RELHIC.
Dark-matter halos are thought to form before ordinary matter collects inside them. In some low-mass halos, gas may never become cold and dense enough to make stars, partly because ultraviolet radiation heated it after the universe’s reionization. These gas-bearing but apparently starless halos are often called dark galaxies.
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Without stars, astronomers cannot find them through ordinary optical surveys. FAST instead detected J1351+0039 through the 21-centimeter radio signal emitted by neutral hydrogen. Follow-up observations in July and August 2026 confirmed its position and helped the team rule out radio interference.
The study estimates a dark-to-baryonic mass ratio of about 200. That figure is inferred from the cloud’s gas motion and size, not measured directly. The gas shows only weak rotation, about 4.2 kilometers per second, while its random internal motion is about 11.6 kilometers per second. The authors argue that pressure from that motion supports the cloud inside a dark-matter halo.
Other dark-galaxy candidates have been found near large galaxies, where gravity can pull gas into tails or debris that mimic an independent object. The researchers classify J1351+0039 as isolated because they found no large galaxy within their specified distance and velocity limits. They also saw no gaseous bridge, comet-like tail or nearby companion.
Those observations led the team to reject a tidal origin and describe J1351+0039 as one of the strongest isolated RELHIC candidates yet. Its nearly round shape and estimated temperature of about 10,000 Kelvin also match theoretical expectations for gas held in equilibrium by a dark-matter halo and the cosmic ultraviolet background.
The “starless” label still needs a stricter test. The DESI images reached a g-band surface-brightness limit of 28.1 magnitudes per square arcsecond, but an extremely faint population of stars could remain below that threshold. The researchers say deeper images from Hubble or the James Webb Space Telescope are needed to determine whether J1351+0039 is truly starless or an ultra-faint galaxy.
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