FAST and Arecibo point to possible dark galaxy with vast hydrogen reservoir

Astronomers have uncovered a massive reservoir of hydrogen gas in an object called AGC 322753, which may represent a dark galaxy—a system with very few stars or none at all. The discovery comes from comparing data collected by two radio telescopes, Arecibo and FAST, which both detect neutral hydrogen in the universe. The object is located roughly 310 million light-years away and contains an estimated 2 billion times the sun's mass in neutral hydrogen.

While radio signals suggest the gas may be rotating in a disk, deep images of the area reveal no corresponding starlight, leaving its stellar content uncertain. The team compared AGC 322753 against other dark galaxy candidates and identified it as a strong candidate after confirming its radio signal's velocity and signal shape, as well as ruling out nearby galaxies and potential radio interference as sources of the gas.

The researchers also used simulations to estimate the upper limit of stars within the object, finding that it could contain at most 50 million solar masses of stars, with a corresponding 40 times more neutral hydrogen than stars. This discrepancy between the vast amount of hydrogen and the lack of detected starlight leaves open the possibility of an exceptionally faint galaxy or gas pulled from neighboring galaxies by gravitational interactions.

Further observations using sharper radio and deeper optical imaging could help determine whether the gas forms a rotating disk and if any hidden stars exist, ultimately clarifying the nature of this intriguing dark galaxy candidate.