Astronomers Find Two Starless Hydrogen Clouds Near the Whirlpool Galaxy

Astronomers using China's Five-hundred-meter Aperture Spherical Telescope, or FAST, have found two clouds of hydrogen gas near the Whirlpool galaxy that appear to contain almost no stars. Each cloud holds roughly 3 million times the mass of the Sun in hydrogen but has no detectable stars. The findings, led by Qingze Chen of the National Astronomical Observatories, Chinese Academy of Sciences, were published in the journal Astronomy & Astrophysics on August 4, 2026.

The standard Lambda Cold Dark Matter, or LCDM, model of the universe holds that structure forms in stages: small clumps of dark matter form first, then merge to build larger clumps and eventually galaxies. But not every clump of dark matter goes on to form a galaxy. If its gravity is too weak, its gas never becomes cold and dense enough, or the gas is heated by ultraviolet light from quasars and stars, stars may never form at all.

The researchers call the result a reionization-limited H I cloud, or RELHIC: gas that survived the universe's early reionization period but was kept too warm by ultraviolet radiation to collapse into stars. To qualify as one, a candidate must be compact and roughly spherical, show narrow spectral lines with gas moving around 20 kilometers per second, and show no stars at optical wavelengths.

The team searched for the radio signal given off by neutral hydrogen gas, using FAST's FEASTS survey data on 55 galaxies. Only two objects, named Cloud N and Cloud S, met every criterion. Each has a hydrogen mass of about 3 million times the Sun's mass and a total halo mass, hydrogen plus surrounding dark matter, of about 3.7 billion solar masses. That ratio closely matches theoretical predictions for RELHICs. Both lie between roughly 70 and 90 kiloparsecs, or 228,000 to 293,000 light-years, from the center of M51, known as the Whirlpool galaxy.

Because M51 is known to be interacting with a companion galaxy, the researchers cannot yet rule out that Cloud N and Cloud S are instead tidal debris, gas pulled loose by that interaction, rather than genuinely starless dark matter halos. Higher-resolution radio observations will be needed to determine which explanation is correct.