Starless ‘Cloud-9’ Is an Entirely New Astrophysical Object

Starless ‘failed galaxy’ is first of its kind ever seen

By Jenna Ahart edited by Lee Billings

A potential new type of celestial object has all the elements of a normal small galaxy. It’s rich in the same hydrogen gas that gives rise to suns and planets, and it sits in a halo of dark matter, the same invisible substance that holds galaxies together. Yet it lacks a key element of twinkling galaxies like our own Milky Way: stars.

Dubbed Cloud-9, the gas cloud is technically the best example of a RELHIC, or Reionization-Limited HI Cloud. The “HI” represents Cloud-9’s wealth of neutral hydrogen, and “RELHIC” refers to what astronomers think the object is: a primordial fossil – or relic – from the earliest epochs of the universe that, for whatever reason, never managed to form stars or become a galaxy in its own right. That makes Cloud-9 a “failed galaxy,” Rachael Beaton, an astronomer at the Space Telescope Science Institute, said during a Jan. 5 news conference at the 247th meeting of the American Astronomical Society in Phoenix, Arizona.

Based on their understanding of the behavior of dark matter and the hierarchical process of galaxy formation, astronomers have long predicted that such starless objects should exist throughout the cosmos. But until recently, RELHICs were notoriously difficult to spot.

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The results, presented by Beaton at the meeting and published in the Astrophysical journal letters last November, strengthen the hypothesis that we have finally found one of these elusive ghost galaxies. Cloud-9 burst onto the astronomy scene in 2023, when the five-hundred-meter-aperture spherical radio telescope in China’s Guizhou province discovered a nearly 5,000 light-year-wide spherical cloud of hydrogen gas, about 14 million light-years from Earth, that appeared to be a faint dwarf galaxy, albeit one lacking visible stars. Further studies of the cloud showed that it contained about a million solar masses of hydrogen and some five billion solar masses of dark matter, but researchers could not confirm that it was truly starless. Perhaps it was instead a strange sort of dwarf galaxy sparsely populated with very old and dark stars.

Beaton and his colleagues therefore once again scrutinized the object through the piercing gaze of the Hubble Space Telescope. And in all of Hubble’s observations, she said, it found evidence of a single star within Cloud-9. It could be that other stars had simply gone unnoticed, but based on further simulations, the team found that the cloud probably couldn’t host more than 3,000 solar masses of stars – a measly handful that would rule out the object being a dwarf galaxy. This new result not only makes Cloud-9 the leading REHLIC candidate in astronomers’ catalogs, but is also an important step in verifying the common prediction that “not every dark matter halo will contain a galaxy,” Beaton said.

While the new information from Hubble “certainly eliminates the possibility that [Cloud-9] is a dwarf galaxy,” there is still much to learn about this particular object, says Kristine Spekkens, an astronomer at Queen’s University in Ontario, who was not involved in the work. For example, she says, Cloud-9 doesn’t have as smooth a shape as astronomers might hope. Better mapping its gas distribution could provide more information about exactly how it formed and evolved over cosmic time.

Still, it will be difficult to definitively confirm that Cloud-9 is indeed a RELHIC as long as it remains in a category of its own, says Ethan Nadler, an astronomer at the University of California, San Diego, who was not involved in the Hubble observations. Although it will be difficult to officially call the cloud “starless,” the discovery of similar objects could help researchers shed light on this dark area of ​​astronomy.

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