A team of astronomers led by Marco Monaci of Swinburne University of Technology in Australia has described the object AGC 322753 - a possible dark galaxy, that is, a system with almost no stars. The work was published in the journal Monthly Notices of the Royal Astronomical Society.
The object is about 310 million light-years away. It contains about two billion solar masses of neutral hydrogen - gas from which stars can form under certain conditions. The radio signal hints that the gas may be rotating in a disk, but deep optical images show no corresponding glow.
To find candidates, the researchers compared data from the ALFALFA survey, carried out with the Arecibo radio telescope (now decommissioned), with results from a survey on China's FAST telescope. Both instruments detect radio emission from neutral hydrogen, so they can spot gas even where stars are too faint. Of 13 matching sources, the scientists singled out AGC 322753: two independent detections matched in velocity and signal shape, and the presence of two peaks is consistent with gas rotating in a disk.
The absence of stars in the image does not yet mean there are none. To estimate how much light could have gone unnoticed, the authors added artificial galaxies of various sizes and masses to the images. According to these tests, the upper limit on stellar mass is about 50 million solar masses. Under such assumptions, the object contains at least 40 times more neutral hydrogen than stars. The limit depends on how widely the stars are distributed and whether bright foreground glow interferes with their light.
Curiously, AGC 322753 has more hydrogen than current models predict for a completely starless system. The authors write that the object's stellar mass "may be too low to be detected in Legacy Surveys."
There is also an alternative explanation: the gas could have been pulled out of neighboring galaxies by gravitational interaction. The great distance to the nearest bright neighbor, the enormous gas mass and signs of rotation argue against such a scenario, although they do not completely rule it out.
Clearer radio observations will help determine whether the gas really forms a rotating disk, and deeper optical images will help find hidden stars. However, even if they are found, an intriguing question will remain: how the galaxy accumulated so much gas while hardly revealing itself through starlight.