Starless ‘failed galaxy’ is the first of its kind seen
Scientists find the best evidence yet of long-conjectured “failed galaxies”
“Failed galaxy” Cloud-9, a dark matter-dominated blob of hydrogen gas about 14 million light-years from Earth. The diffuse magenta represents radio data from the ground-based Very Large Array (VLA) that shows the presence of gas. The dashed circle marks the peak of radio emission, where researchers have focused their exploration of the stars. Follow-up observations by the Hubble Space Telescope found no stars within the cloud. The few objects visible within its boundaries are background galaxies.
NASA, ESA, VLA, Gagandeep Anand (STSCI), Alejandro BenÃtez-Lambé (University of Milan-Bicocca) (ScienceJoseph DePasquale (STScI) (image processing,
A possible new type of astronomical object has all the properties of a typical small galaxy. It is rich in the same hydrogen gas that gives rise to the Sun and the planets, and it lies within a halo of dark matter, the same invisible material that holds galaxies together. Yet it is missing a key component of bright galaxies like our own Milky Way: stars.
Nicknamed Cloud-9, the gas cloud is technically the best example of a RELHIC, or reionization-limited HI cloud. “HI” stands for Cloud-9’s abundance of neutral hydrogen, and “RELHIC” stands for what astronomers consider the object to be: a primordial fossil – or relic – from the earliest ages of the universe that, for some reason, never managed to form stars or become a full-fledged galaxy. This makes Cloud-9 a “failed galaxy,” Rachel Beaton, an astronomer at the Space Telescope Science Institute, said during a press conference at the 247th meeting of the American Astronomical Society in Phoenix, Ariz., on Jan. 5.
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 universe. But until recently, RELHICs were extremely difficult to identify.
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The results—presented by Beaton at the meeting and published in Astrophysical Journal Letters Last November it was confirmed that we had finally found one of these elusive ghost galaxies. Cloud-9 first burst onto the astronomy scene 2023When the Five Hundred Meter Aperture Spherical Radio Telescope in Guizhou Province, China, discovered an approximately 5,000 light-year-wide spherical cloud of hydrogen gas about 14 million light-years away from Earth, what appeared to be a faint dwarf galaxy, although it had no visible stars. More intensive studies of the cloud revealed that it contains about one million solar masses of hydrogen and about five billion solar masses of dark matter, but researchers could not confirm that it was actually starless. Perhaps, instead, it was actually a strange type of dwarf galaxy sparsely populated by very old and dim stars.
So Beaton and his colleagues took another look at the object with the deep eyes of the Hubble Space Telescope. And in all of Hubble’s observations, he said, it found signs of only one star within Cloud-9. It may be that other stars are not easily visible, but based on further simulations, the team found that the cloud probably cannot host stars with more than about 3,000 solar masses – a modest sliver that would prevent the object from becoming a dwarf galaxy. This new result not only makes Cloud-9 the most prominent REHLIC candidate in astronomers’ catalog, Beaton said, but also is a milestone in verifying the common prediction that “not every dark matter halo will contain a galaxy.”
While the latest information from Hubble “definitely eliminates the possibility that (Cloud-9) is a dwarf galaxy,” says Christine Specksens, an astronomer at Queen’s University in Ontario, who was not involved in the work. For example, she says, the shape of Cloud-9 is not as smooth as astronomers would expect. Better mapping of its gas distribution could provide more information about how it actually formed and evolved over cosmic time.
Still, it will be difficult to definitively confirm that Cloud-9 is indeed a RELHIC until it remains completely in a league of its own, says Ethan Nadler, an astronomer at the University of California, San Diego, who did not participate in the Hubble observations. Although it would be challenging to officially dub the cloud “starless,” finding similar objects could help researchers shed some light on this dark area of ​​astronomy.
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