Astronomers have taken one of the deepest looks yet at Cloud-9, a candidate “dark galaxy,” and found no trace of stars. The paper outlining this discovery was posted to the arXiv preprint server on Aug. 21.
No stars at all?
The widely accepted model of the universe—the Lambda Cold Dark Matter model—predicts that the universe should contain a large number of small, low-mass dark matter clumps that contain gas but never managed to form stars. This is because the balance between gravity trying to collapse the gas, gas cooling and heating from the ultraviolet background—diffuse radiation from quasars and stars throughout the universe—is thought to prevent gas in these small halos from condensing into stars. These galaxies that can’t produce stars are “dark” or “starless” galaxies, and finding one would confirm a key prediction of standard cosmology.
Cloud-9 has been a promising candidate. It is a gas cloud near the spiral galaxy M94, sitting about 4.66 megaparsecs (around 15 million light-years) away. It is estimated to contain about 1 million solar masses of hydrogen gas but shows no obvious stars in previous surveys. But previous attempts to completely rule out stars in Cloud-9 were unsuccessful.
“Alternative interpretations have been proposed for starless HI clouds, including tidal debris, high-velocity clouds, and transient gaseous structures,” the team writes in the paper. Some of these alternatives may host very faint stellar components, which is why deep imaging is needed to test whether it truly lacks stars.
Five-filter imaging
In this study, a team of researchers led by Ignacio Trujillo of the Institute of Astrophysics of the Canary Islands used a high-speed camera called HiPERCAM on the Gran Telescopio Canarias (GTC)—one of the world’s largest optical telescopes—to image Cloud-9 simultaneously in five filters. They observed over two nights in June 2026.
To maximize sensitivity to any faint, diffuse stellar signal, they systematically shifted the camera pointing between exposures and masked the region centered on Cloud-9 before subtracting the background. This ensured that any real stellar emission was not accidentally removed in processing. The observations reached a limiting surface brightness of 31.4 magnitudes per square arcsecond in the g-band—deep enough to detect extremely faint stellar populations invisible to previous surveys.
Using these ultra-deep observations, the team detected no starlight across the object’s central region. This non-detection placed an upper limit of just 16,000 solar masses of stars within a region about 4,200 light-years across. For context, Cloud-9 contains around 1 million solar masses of hydrogen gas, meaning there is at least 60 times as much gas as stars, if any stars exist at all. That’s remarkably little for an object that appears to have the properties of a galaxy.
The team concludes the result “supports the idea that Cloud-9 is an excellent candidate for a starless galaxy.”
This result sits between two prior estimates: an earlier survey-based limit that was “not designed to preserve the low surface brightness signal,” and a deeper HST star-counting estimate that heavily relied on a comparison galaxy that may be too compact. The new observations independently strengthen earlier evidence that Cloud-9 could be a rare starless galaxy, a system dominated by gas and dark matter that has somehow failed to form stars.
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Publication details
Ignacio Trujillo et al, Ultra-Deep imaging of the starless galaxy candidate Cloud-9, arXiv (2026). DOI: 10.48550/arxiv.2608.20911
Journal information:
arXiv
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A galaxy with almost no stars? New Cloud-9 observations strengthen the case (2026, September 10)
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