Cloud-9: A Starless Relic Shedding Light on Dark Matter and Galaxy Formation

In recent astronomical observations, the Hubble Space Telescope has confirmed the existence of Cloud-9, a compact cloud of neutral hydrogen gas devoid of stars. This object, classified as a Reionization-Limited H I Cloud (RELHIC), represents a type of structure predicted by theories of galaxy formation but rarely observed. Located approximately 14 million light-years from Earth near the spiral galaxy Messier 94, Cloud-9 provides evidence for the role of dark matter in the early universe.

Cloud-9 lies about 14 million light-years from Earth. The pink glow in the image comes from radio observations made with the ground-based Very Large Array, revealing a cloud of gas. The dashed circle highlights the strongest radio signal, where astronomers expected to find stars. However, follow-up observations with NASA’s Hubble Space Telescope found no stars inside the cloud. The few visible objects in the area are actually distant background galaxies. While scientists once thought Cloud-9 might be a faint dwarf galaxy whose stars were too dim to detect from the ground, Hubble’s sharper vision shows that this “failed galaxy” never formed stars at all.  Credit: NASA

Discovery and Observations Of Cloud-9

Cloud-9 was first detected three years ago during a radio survey conducted by the Five-hundred-meter Aperture Spherical Telescope (FAST) in China. Subsequent confirmations came from the Green Bank Telescope and the Very Large Array in the United States. These radio observations identified the cloud through the emission of radio waves from its neutral hydrogen content. However, it was the Hubble Space Telescope's Advanced Camera for Surveys that provided definitive proof of its starless nature by imaging the region in visible light and finding no stellar objects within its boundaries—only distant background galaxies.

500m Aperture Spherical Radio Telescope located in Guizhou Province, China. . FAST is the world’s largest single-dish radio telescope and is designed to detect extremely faint radio signals from space.

The cloud's detection highlights the complementary nature of different observational techniques. Radio telescopes detect the gas, while optical instruments like Hubble rule out the presence of stars, which would otherwise indicate a more developed galactic structure.

Physical Properties of Cloud-9

Cloud-9 is a highly spherical structure with a diameter of about 4,900 light-years, making it smaller and more compact than typical hydrogen clouds associated with galaxies. Its core is composed primarily of neutral hydrogen gas, with a mass equivalent to roughly one million times that of the Sun. Despite its gaseous content, the cloud shows no signs of rotation, and slight distortions in its gas suggest possible interactions with the nearby Messier 94 galaxy.

Estimates of its dark matter content, derived from balancing the gravitational pull against the gas pressure, place it at around five billion solar masses. This dominance of dark matter—non-luminous material that interacts primarily through gravity—explains why Cloud-9 has remained in a primordial state without forming stars.

Significance in Astrophysics

Objects like Cloud-9 are considered "failed galaxies," remnants from the universe's early epochs that did not accumulate sufficient material or conditions to ignite star formation. In standard models of cosmology, the universe began with a hot, dense state and cooled to form hydrogen and helium atoms. Dark matter halos then gravitationally attracted this gas, leading to the formation of the first stars and galaxies. However, in cases like Cloud-9, the process stalled, leaving behind a gas cloud embedded in a dark matter halo.

This discovery supports theories predicting the existence of numerous small, dark matter-dominated structures. As Andrew Fox, a team member from the Space Telescope Science Institute, noted, "This cloud is a window into the dark universe. We know from theory that most of the mass in the universe is expected to be dark matter, but it’s difficult to detect this dark material because it doesn’t emit light. Cloud-9 gives us a rare look at a dark-matter-dominated cloud."

Principal investigator Alejandro Benitez-Llambay added, "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."

Broader Implications

The study of Cloud-9 underscores the challenges in detecting faint, diffuse objects in the universe. Such structures are vulnerable to environmental factors, including ram-pressure stripping from interactions with larger galaxies, which may disrupt them over time. Future astronomical surveys, potentially using advanced observatories like the James Webb Space Telescope, may reveal more RELHICs, offering deeper insights into the distribution of dark matter and the processes that govern galaxy evolution.

By examining these “failed” systems, astronomers can refine models of the cosmos, particularly those related to the reionization era when the first stars illuminated the universe. Based on publicly available NASA and ESA observations and science communications, the case of Cloud-9 serves as a key piece in understanding why some cosmic building blocks evolve into vibrant galaxies while others remain inert.

-Sayyed Danish