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Explaining Cloud-9: A Celestial Object Like No Other
Some three years ago, the Five-Hundred Meter Aperture Spherical Telescope (FAST) in Guizhou, China discovered a gas agglomeration that was later dubbed Cloud-9. It’s a cute name, though unintentionally so, as this particular cloud is simply the ninth thus far identified near the spiral galaxy Messier 94 (M94). And while gas clouds don’t particularly call attention to themselves, this one is a bit of a stunner, as later research is now showing. It’s thought to be a gas-rich though starless cloud of dark matter, a holdover from early galaxy formation.
Scientists are referring to Cloud-9 as a new type of astronomical object. FAST’s detection at radio wavelengths has been confirmed by the Green Bank Telescope and the Very Large Array in the United States. The cloud has now been studied by the Hubble telescope’s Advanced Camera for Surveys, which revealed its complete lack of stars. That makes this an unusual object indeed.
Alejandro Benitez-Llambay (Milano-Bicocca University, Milan) is principal investigator of the Hubble work and lead author of the paper just published in The Astrophysical Journal Letters. The results were presented at the ongoing meeting of the American Astronomical Society in Phoenix. Says Benitez-Llambay:
“This is a tale of a failed galaxy. In science, we usually learn more from the failures than from the successes. In this case, seeing no stars is what proves the theory right. It tells us that we have found in the local Universe a primordial building block of a galaxy that hasn’t formed.”
Here there’s a bit of a parallel with our recent discoveries of interstellar objects moving through our Solar System. In both cases, we are discovering a new type of object, and in both cases we are bringing equipment online that will, in relatively short order, almost certainly find more. We get Cloud-9 through the combination of radio detection via FAST and analysis by the Hubble space telescope, which was able to demonstrate that the object does lack stars.
Image: This image shows the location of Cloud-9, which is 14 million light-years from Earth. The diffuse magenta is radio data from the ground-based Very Large Array (VLA) showing the presence of the cloud. The dashed circle marks the peak of radio emission, which is where researchers focused their search for stars. Follow-up observations by the Hubble Space Telescope’s Advanced Camera for Surveys found no stars within the cloud. The few objects that appear within its boundaries are background galaxies. Before the Hubble observations, scientists could argue that Cloud-9 is a faint dwarf galaxy whose stars could not be seen with ground-based telescopes due to the lack of sensitivity. Hubble’s Advanced Camera for Surveys shows that, in reality, the failed galaxy contains no stars. Credit: Science: NASA, ESA, VLA, Gagandeep IAnand (STScI), Alejandro Benitez-Llambay (University of Milano-Bicocca); Image Processing: Joseph DePasquale (STScI).
We can refer to Cloud-9 as a Reionization-Limited H Ι Cloud, or RELHIC (that one ranks rather high on my acronym cleverness scale). H I is neutral atomic hydrogen, the most abundant form of matter in the universe. The paper formally defines RELHIC as “a starless dark matter halo filled with hydrostatic gas in thermal equilibrium with the cosmic ultraviolet background.” This would be primordial hydrogen from the earliest days of the universe, the kind of cloud we would normally expect to have become a ‘conventional’ spiral galaxy.
The lack of stars here leads co-author Rachael Beaton to refer to the object as an ‘abandoned house,” one which likely has others of its kind still awaiting discovery. In comparison with the kind of hydrogen clouds we’ve identified near our own galaxy, Cloud-9 is smaller, certainly more compact, and unusually spherical. Its core of neutral hydrogen is measured at roughly 4900 light years in diameter, with the hydrogen gas itself about one million times the mass of the Sun. The amount of dark matter needed to create the gravity to balance the pressure of the gas is about five billion solar masses. While the researchers do expect to find more such objects, they point out that ram pressure stripping can deplete gas as any cloud moves through the space between galaxies. In other words, the population of objects like RELHIC is likely quite small.
The paper places the finding of Cloud-9 in context within the framework now referred to as Lambda Cold Dark Matter (ACDM), which incorporates dark energy via a cosmological constant into a schemata that includes dark matter and ordinary matter. Quoting the paper’s conclusion:
In the ΛCDM framework, the existence of a critical halo mass scale for galaxy formation naturally predicts galaxies spanning orders of magnitude in stellar mass at roughly fixed halo mass. This threshold marks a sharp transition at which galaxy formation becomes increasingly inefficient (A. Benitez-Llambay & C. Frenk 2020), yielding outcomes that range from halos entirely devoid of stars to those able to form faint dwarfs, depending sensitively on their mass assembly histories. Even if Cloud-9 were to host an undetected, extremely faint stellar component, our HST observations, together with FAST and VLA data, remain fully consistent with these theoretical expectations. Cloud-9 thus appears to be the first known system that clearly signals this predicted transition, likely placing it among the rare RELHICs that inhabit the boundary between failed and successful galaxy formation. Regardless of its ultimate nature, Cloud-9 is unlike any dark, gas-rich source detected to date.
The paper is Gagandeep et al., “The First RELHIC? Cloud-9 is a Starless Gas Cloud,” The Astrophysical Journal Letters, Volume 993, Issue 2 (November 2025), id.L55, 7 pp. Full text.
Scientists are referring to Cloud-9 as a new type of astronomical object. FAST’s detection at radio wavelengths has been confirmed by the Green Bank Telescope and the Very Large Array in the United States. The cloud has now been studied by the Hubble telescope’s Advanced Camera for Surveys, which revealed its complete lack of stars. That makes this an unusual object indeed.
Alejandro Benitez-Llambay (Milano-Bicocca University, Milan) is principal investigator of the Hubble work and lead author of the paper just published in The Astrophysical Journal Letters. The results were presented at the ongoing meeting of the American Astronomical Society in Phoenix. Says Benitez-Llambay:
“This is a tale of a failed galaxy. In science, we usually learn more from the failures than from the successes. In this case, seeing no stars is what proves the theory right. It tells us that we have found in the local Universe a primordial building block of a galaxy that hasn’t formed.”
Here there’s a bit of a parallel with our recent discoveries of interstellar objects moving through our Solar System. In both cases, we are discovering a new type of object, and in both cases we are bringing equipment online that will, in relatively short order, almost certainly find more. We get Cloud-9 through the combination of radio detection via FAST and analysis by the Hubble space telescope, which was able to demonstrate that the object does lack stars.
Image: This image shows the location of Cloud-9, which is 14 million light-years from Earth. The diffuse magenta is radio data from the ground-based Very Large Array (VLA) showing the presence of the cloud. The dashed circle marks the peak of radio emission, which is where researchers focused their search for stars. Follow-up observations by the Hubble Space Telescope’s Advanced Camera for Surveys found no stars within the cloud. The few objects that appear within its boundaries are background galaxies. Before the Hubble observations, scientists could argue that Cloud-9 is a faint dwarf galaxy whose stars could not be seen with ground-based telescopes due to the lack of sensitivity. Hubble’s Advanced Camera for Surveys shows that, in reality, the failed galaxy contains no stars. Credit: Science: NASA, ESA, VLA, Gagandeep IAnand (STScI), Alejandro Benitez-Llambay (University of Milano-Bicocca); Image Processing: Joseph DePasquale (STScI).
We can refer to Cloud-9 as a Reionization-Limited H Ι Cloud, or RELHIC (that one ranks rather high on my acronym cleverness scale). H I is neutral atomic hydrogen, the most abundant form of matter in the universe. The paper formally defines RELHIC as “a starless dark matter halo filled with hydrostatic gas in thermal equilibrium with the cosmic ultraviolet background.” This would be primordial hydrogen from the earliest days of the universe, the kind of cloud we would normally expect to have become a ‘conventional’ spiral galaxy.
The lack of stars here leads co-author Rachael Beaton to refer to the object as an ‘abandoned house,” one which likely has others of its kind still awaiting discovery. In comparison with the kind of hydrogen clouds we’ve identified near our own galaxy, Cloud-9 is smaller, certainly more compact, and unusually spherical. Its core of neutral hydrogen is measured at roughly 4900 light years in diameter, with the hydrogen gas itself about one million times the mass of the Sun. The amount of dark matter needed to create the gravity to balance the pressure of the gas is about five billion solar masses. While the researchers do expect to find more such objects, they point out that ram pressure stripping can deplete gas as any cloud moves through the space between galaxies. In other words, the population of objects like RELHIC is likely quite small.
The paper places the finding of Cloud-9 in context within the framework now referred to as Lambda Cold Dark Matter (ACDM), which incorporates dark energy via a cosmological constant into a schemata that includes dark matter and ordinary matter. Quoting the paper’s conclusion:
In the ΛCDM framework, the existence of a critical halo mass scale for galaxy formation naturally predicts galaxies spanning orders of magnitude in stellar mass at roughly fixed halo mass. This threshold marks a sharp transition at which galaxy formation becomes increasingly inefficient (A. Benitez-Llambay & C. Frenk 2020), yielding outcomes that range from halos entirely devoid of stars to those able to form faint dwarfs, depending sensitively on their mass assembly histories. Even if Cloud-9 were to host an undetected, extremely faint stellar component, our HST observations, together with FAST and VLA data, remain fully consistent with these theoretical expectations. Cloud-9 thus appears to be the first known system that clearly signals this predicted transition, likely placing it among the rare RELHICs that inhabit the boundary between failed and successful galaxy formation. Regardless of its ultimate nature, Cloud-9 is unlike any dark, gas-rich source detected to date.
The paper is Gagandeep et al., “The First RELHIC? Cloud-9 is a Starless Gas Cloud,” The Astrophysical Journal Letters, Volume 993, Issue 2 (November 2025), id.L55, 7 pp. Full text.
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