FAST finds two mysterious hydrogen clouds with no visible stars

Astronomers using China's FAST radio telescope have discovered two hydrogen clouds near the Whirlpool galaxy (M51) that appear to contain almost no stars. The clouds, called RELHIC candidates, each have roughly 3 million times the mass of the sun in hydrogen but lack detectable optical counterparts. The study, published in Astronomy & Astrophysics, outlines the properties of these clouds, which are compact, spherical, and have narrow spectral lines. The research suggests these clouds are remnants of dark-matter halos that were heated by UV radiation during cosmic reionization, preventing star formation.

The study found two objects—Cloud N and Cloud S—that met the criteria for RELHICs. Both have a hydrogen mass of roughly 3 million solar masses and a total halo mass of 3.7 billion solar masses. This ratio closely matches theoretical predictions for such systems. The clouds are located around 70–90 kpc from the center of M51, a well-known interacting system. The researchers note that the possibility of these clouds being tidal debris from galaxy interactions cannot be ruled out, and higher-resolution observations are needed to confirm their nature.

The findings highlight the existence of starless dark-matter halos, which are predicted to be numerous but have not yet been conclusively identified. The study provides a new framework for understanding the formation of such structures and their role in the universe. The research is led by Qingze Chen and published in the journal Astronomy & Astrophysics.

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