Astronomers Spot Massive Starless Hydrogen Clouds Near Whirlpool Galaxy Using FAST

Chinese astronomers using the Five-hundred-meter Aperture Spherical Telescope (FAST) have identified two enormous clouds of neutral hydrogen floating in the vicinity of the Whirlpool galaxy (M51). Each cloud holds roughly three million times the Sun’s mass in hydrogen, yet neither shows any sign of starlight in optical surveys, making them among the most massive star‑less structures ever recorded.

The detection was made by scanning the 21‑centimetre radio emission that hydrogen atoms emit, a method that allows radio telescopes to map gas independent of visible light. Follow‑up imaging with ground‑based optical instruments failed to reveal any associated galaxies or star clusters, confirming that the clouds are essentially invisible in the wavelengths where stars would normally dominate.

Such objects are of particular interest to astronomers because they may represent “dark” galaxies—hypothetical systems that contain the raw material for star formation but have never ignited a stellar population. The existence of massive, star‑free hydrogen reservoirs challenges conventional models that predict gas of this density should collapse under gravity to form stars within a relatively short cosmic timescale.

FAST, with its 500‑metre dish and unprecedented sensitivity, is uniquely suited to uncovering faint, diffuse structures that elude smaller instruments. Since becoming fully operational, the telescope has been used to probe the Milky Way’s halo, search for fast radio bursts, and now to extend the census of extragalactic gas beyond the reach of traditional surveys.

The Whirlpool galaxy, a well‑studied spiral interacting with a smaller companion, sits about 23 million light‑years from Earth. Its gravitational tides are known to stir up gas in the surrounding intergalactic medium, and the newly found clouds may be remnants of such interactions, or they could be primordial clumps that have survived untouched for billions of years.

Researchers plan to target the clouds with higher‑resolution radio interferometers and deep infrared observations to search for any hidden, low‑mass star formation. Confirming whether these structures are truly starless or merely host a population of faint, undetectable stars will help refine estimates of the universe’s missing baryonic matter and improve our understanding of how galaxies acquire and retain their gas reservoirs.