This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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. 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. 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∘ (
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