A FAST survey found two clouds near M51, each carrying about 3 million solar masses of hydrogen and no optically detectable stars. The team interprets them as candidate dark-matter haloes that never formed stars. The telescope's resolution, however, cannot yet distinguish that explanation from gas pulled out during M51's interaction with NGC 5195.
China's FAST radio telescope detected two hydrogen clouds near M51, named Cloud N and Cloud S. Each contains about 3 million solar masses of hydrogen, yet neither has a single optically detectable star. The clouds lie 70 to 90 kiloparsecs from M51's centre; that position and the absence of stars leave room for two different readings of their origin.[1]
The team treats the two clouds as candidates for dark-matter haloes in which reionization limited star formation. Its inference that each sits inside a halo of about 3.7 billion solar masses suggests how starless hydrogen could remain in such a structure. Their location around M51 keeps another possibility alive: the gas may have been pulled from the galaxy during its interaction with nearby NGC 5195.[1]
Current radio images do not settle the clouds' shapes. A compact structure in equilibrium could support the dark-halo reading, while gas stretched towards M51 could strengthen the tidal-debris alternative. Both reports identify sharper radio maps as necessary for that distinction. Until those images arrive, Cloud N and Cloud S remain dark-halo candidates with another origin still open.[1], [2]
