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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallA radio survey has found an isolated cloud of neutral hydrogen with no visible galaxy at its position. The object, designated J1351+0039, could be a rare reionization-limited H I cloud (RELHIC)—sometimes described as a “dark galaxy”—but the available images do not yet prove that it contains no stars.
What astronomers found
Jin-Long Xu and colleagues identified J1351+0039 in a blind neutral-hydrogen survey carried out with China’s Five-hundred-meter Aperture Spherical radio Telescope (FAST). A blind survey searches the sky for the radio signature of atomic hydrogen rather than starting with a known optical galaxy.
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The team’s preprint, “Discovery of an isolated RELHIC Candidate: J1351+0039,” was submitted to arXiv on 24 September 2026 (arXiv:2609.29852). Follow-up inspection of deep images from the DESI Legacy Survey found no optical counterpart. That combination—detectable gas but no visible stellar system—is why J1351+0039 has attracted attention as a possible dark galaxy.
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Why a radio telescope can find an invisible galaxy
Neutral hydrogen emits radio waves at a wavelength of 21 centimeters. Those emissions can reveal a cloud’s mass, velocity and internal motions even when starlight is too faint for an optical survey. FAST’s large collecting area makes it sensitive to extremely weak H I signals.
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J1351+0039 has a measured systemic velocity of 429.0 km/s, allowing astronomers to distinguish it from unrelated foreground and background emission. The detected H I mass is about 4.8×105 solar masses. The gas therefore provides a direct, radio-based detection of a structure that does not stand out in visible-light images.
The measurements behind the “dark galaxy” idea
| Quantity | Reported value | Interpretation |
|---|---|---|
| Neutral-hydrogen mass | 4.8×105 solar masses | Mass of the detected atomic gas |
| Estimated dynamical mass | 1.3×108 solar masses | Total mass inferred from the object’s motions, including dark matter |
| Dark-to-baryonic mass ratio | About 200 | Indicates a system overwhelmingly dominated by dark matter under the team’s modeling |
| Systemic velocity | 429.0 km/s | Line-of-sight velocity assigned to the cloud |
| DESI g-band surface-brightness limit | 28.1 mag arcsec−2 | Approximate faintness threshold reached by the optical search |
| DESI stellar-mass limit | 1.5×105 solar masses | Estimated upper sensitivity for an underlying stellar population |
| Gas rotation | About 4.2 km/s | Weak ordered motion |
| Random velocity dispersion | About 11.6 km/s | Larger disordered motion that can provide pressure support |
| Gas temperature | About 10,000 K | Warm, diffuse conditions consistent with a reionization-affected cloud |
The kinematic contrast is especially important: random motions are roughly three times larger than the measured rotation. In a normal, rotation-dominated disk, ordered motion would be expected to provide most of the support. Here, the gas appears primarily pressure-supported, while the team suggests it may have gradually acquired some angular momentum.
What a RELHIC is
In the standard ΛCDM cosmological model, some small dark-matter halos may never convert their gas into stars. After cosmic reionization, ultraviolet radiation filled intergalactic space with an energy background capable of heating and ionizing gas in low-mass halos. Gas that remains warm and diffuse can resist collapse, leaving a dark-matter halo containing neutral hydrogen but little or no star formation.
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These proposed objects are called reionization-limited H I clouds, or RELHICs. They should be easier to detect through 21-cm emission than through visible light. J1351+0039 fits that general picture because its gas is warm, its rotation is weak, and its inferred total mass is far larger than the detected baryonic component.
Why isolation makes J1351+0039 unusual
Gas without stars can also arise through interactions. A tidal tail pulled from a larger galaxy, a gaseous bridge between companions or debris from a recent encounter may produce an apparently dark H I source. Those explanations are harder to rule out when a cloud sits close to a massive galaxy.
The team reports no nearby large galaxy, tidal tail, gaseous bridge or obvious companion around J1351+0039. Its apparent isolation removes several common alternatives and makes it a cleaner RELHIC candidate than examples embedded in visibly disturbed environments. Isolation does not establish the object’s nature by itself, but it strengthens the case that the cloud belongs to its own low-mass dark-matter halo.
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Why “starless” is not yet proven
An absent optical counterpart means that DESI did not detect a galaxy above the stated limits; it does not demonstrate that the cloud contains zero stars. A very faint, diffuse dwarf could fall below a surface brightness of 28.1 mag arcsec−2, and the estimated stellar-mass sensitivity is about 1.5×105 solar masses.
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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteFor that reason, the authors describe J1351+0039 as a candidate rather than a confirmed dark galaxy. They note that deeper imaging with the Hubble Space Telescope or James Webb Space Telescope may be needed to reveal—or more tightly constrain—an extremely faint stellar population.
What happens next
Deeper optical imaging
Longer exposures and higher-resolution observations could test whether a low-surface-brightness dwarf is hidden beneath the DESI limits. A true non-detection at substantially deeper thresholds would make the starless interpretation more compelling.
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More detailed H I mapping
Resolved 21-cm observations can measure the cloud’s size, density profile and velocity field. They can also search for faint extensions or asymmetric structures that would betray an interaction.
Independent dynamical tests
Better measurements of rotation and velocity dispersion will test whether the reported pressure support is stable and whether the inferred 1.3×108-solar-mass dynamical estimate is robust.
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Searches for hidden companions
Additional radio and optical surveys can look for nearby low-mass galaxies or diffuse debris that might provide a tidal explanation. The absence of such features is significant, but every deeper survey can narrow the remaining alternatives.
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What the discovery would mean if confirmed
A genuinely isolated, starless H I cloud would give astronomers a rare laboratory for studying dark matter and gas physics without the complications introduced by stellar feedback, supernovae and an established galactic disk. It would directly test how reionization suppresses star formation in small halos and how much neutral gas those halos can retain.
For now, J1351+0039 is best described as one of the most promising isolated RELHIC candidates. FAST has shown that a potentially galaxy-sized dark-matter structure can be found through its hydrogen even when conventional optical images show nothing, while deeper observations must decide whether “starless” is an accurate description or simply the current observational limit.
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