Astronomers using China’s FAST telescope find starless hydrogen cloud

The researchers say that in July-August 2026, astronomers using China’s FAST radio telescope discovered a compact hydrogen cloud lacking visible stars in the region designated J1351+0039. The researchers say that the cloud, dubbed J1351+0039, holds about 500,000 solar masses of neutral hydrogen (hydrogen atoms without an electric charge).

The researchers say that they observed J1351+0039 three times between July and August 2026 to obtain high‑quality data. They note that they uploaded the discovery paper to arXiv on September 24, 2026.

The researchers say that they classify J1351+0039 as isolated, noting no large galaxies lie within 1.5 degrees projected separation. The researchers say that at its distance this corresponds to under 100 kiloparsecs (a unit of distance used in astronomy), and that its system velocity is below 250 kilometers per second.

The researchers say that they see no cometary tails, no gaseous bridges, and no nearby companions near J1351+0039, which excludes a gravitational tidal origin. The researchers say that dynamical analysis gave the cloud a total mass of roughly 130 million solar masses.

The researchers say that they describe J1351+0039 as round, moving with a positive velocity, and having a temperature near 10,000 Kelvin (17,540°F) in balance with the ultraviolet background (the UV radiation that fills space after reionization). The researchers say that these properties match expectations for a dark galaxy that has never cooled sufficiently to form stars.

The researchers say that they measure the gas in J1351+0039 rotating at roughly 4.2 km/s, with random motions around 11.6 km/s. Since random motion exceeds ordered rotation, the cloud is mainly supported by the pressure of gas moving randomly within it.

The researchers say that the pressure‑supported state suggests J1351+0039 may be a RELHIC (reionization‑limited H I clouds) that has gradually acquired angular momentum. They note that an optical counterpart of low surface brightness might not show up in DESI images.

The researchers say that they might need deep space‑based photos from JWST (James Webb Space Telescope) or Hubble (Hubble Space Telescope) to confirm true starlessness. They note that earlier dark galaxy candidates appeared near massive galaxies, whereas J1351+0039 is isolated in space.

They say that J1351+0039’s total isolation renders it the most compelling and cleanest isolated RELHIC candidate known to date. The researchers say that in the lambda cold dark matter (ΛCDM) model, structure forms hierarchically, with small dark matter halos assembling first and later merging to build larger halos and galaxies (dark matter halos are regions of concentrated dark matter).

They note that according to the ΛCDM model, some low‑mass halos never form stars. The researchers say that after cosmic reionization, heating of halo gas by the ultraviolet background prevents the gas from becoming cold and dense enough to form stars.

They explain that they expect such starless gas‑bearing halos to exist and they term such starless gas‑bearing halos reionization‑limited H I clouds (RELHICs) or dark galaxies. They say that because they contain few or no stars, they can detect such halos only via the 21‑cm line emitted by neutral hydrogen (the 21‑cm line is a radio wavelength emitted by neutral hydrogen).

The researchers say that a faint dwarf galaxy may appear dark because of surface‑brightness limits (surface‑brightness limits refer to the faintest detectable glow). The researchers say that to date, only one RELHIC has been observationally confirmed while researchers have reported two promising candidates.