Cloud 9, the galaxy candidate with no stars

Cloud 9, located 14 million light-years away, could be the first galaxy without stars observed by humanity. Its gas reserves and dark matter halo contrast with the almost total absence of starlight, although researchers still need new observations to confirm the finding.

• Cloud 9 contains hydrogen gas equivalent to 1 million solar masses and a dark matter halo estimated at around 5 billion solar masses.
• HiPERCAM images did not detect starlight and set an upper limit of just 16,000 solar masses in stars.
• Post-reionization ultraviolet radiation may have prevented the gas from cooling and forming stars.

Cloud 9, the galaxy candidate with no stars

Cloud 9 has become one of the most compelling candidates for being a starless galaxy discovered so far. The object, located 14 million light-years away and near the spiral galaxy Messier 94, contains a reserve of hydrogen gas and a dark matter halo, but emits virtually no starlight.

This possibility is particularly striking because galaxies are usually identified by the light of their stars, while Cloud 9 seems to reveal its presence through the material that failed to become a stellar population. A study led by Ignacio Trujillo from the Institute of Astrophysics of the Canary Islands in Spain argues that current observations bring scientists closer to confirmation, although they do not yet allow for a definitive conclusion.

A candidate dominated by gas and dark matter

The available data describes Cloud 9 as a structure with an estimated hydrogen gas mass of 1 million times that of the Sun. This reserve is complemented by a dark matter halo calculated at around 5 billion solar masses, a combination that would normally provide the basic ingredients for forming a small galaxy.

Dark matter does not appreciably interact with light, so astronomers cannot directly observe it using optical telescopes. However, its gravity helps keep the gas together, and this link makes Cloud 9 an opportunity to study how galactic structures can exist without the brightness that usually reveals them.

The contrast between its components and its appearance is central to the finding: there is enough gas to attract the instruments' attention, but no visible stellar population to illuminate the cloud. The absence of a signal recorded at the position of Cloud 9 constitutes one of the most compelling results of the research, although it does not alone serve as absolute proof that the object lacks stars.

Previous observations had already raised doubts about the nature of the object, but the new images took the search to a much greater depth. The Hubble Space Telescope examined Cloud 9 in January and also found no significant starlight emission. The team then decided to study it with instruments capable of ruling out even extremely faint brightness.

Deep images to search for a signal that does not appear

Trujillo and his colleagues used the HiPERCAM camera installed on the Gran Telescopio Canarias. In 2.36 hours of integration, the team obtained images approximately ten times deeper than those previously available for the region where Cloud 9 is located.

The result was an absence of starlight emission within the area corresponding to the object, according to the research description. This lack of signal does not alone equate to absolute proof, as a very old, faint, and metal-poor stellar population could be difficult to observe, but it drastically reduces conventional explanations.

When considering the most challenging scenario for detection, scientists established an upper limit of only 16,000 solar masses for the total stellar mass of Cloud 9. This figure is minuscule compared to its estimated gas content and dark matter halo, reinforcing the idea that the object may never have formed a significant stellar population.

The finding also illustrates an important methodological difficulty: a galaxy without stars cannot be found using the same strategies employed to locate luminous galaxies. Astronomers must combine gas emission, typically neutral hydrogen detected at radio wavelengths, with deep optical imaging that allows for the exclusion of any hidden stellar components.

The Cosmic Explanation for a Failed Galaxy

The main theoretical explanation points to the ultraviolet background radiation permeating the universe after the reionization epoch. This field of radiation would have heated the gas within low-mass dark matter halos, preventing it from cooling effectively enough to collapse and form stars.

Simulations from that period predict that some halos with masses of several billion solar masses could remain essentially starless. The estimated mass for the Cloud 9 halo aligns with this scenario, meaning its existence would not necessarily require modifications to standard cosmological models.

From this perspective, starless galaxies would not be isolated anomalies but rather a natural and potentially abundant consequence of cosmic evolution. The historical problem would have been finding them, as the absence of visible light precisely eliminates the signal that allows most galaxies to be identified.

Still, researchers have not issued a final verdict on Cloud 9 and prefer to describe it as an exceptionally strong candidate. Deeper observations from space could resolve some of the uncertainties, especially if they manage to distinguish individual stars and avoid the issues associated with measuring diffuse light.

What Observations Could Confirm the Finding

The James Webb Space Telescope could take the search further, although its utility will depend on carefully selecting the appropriate filters and stellar indicators. Detecting a small number of stars would not necessarily invalidate the entire relevance of the object, but it would force a reconsideration of whether Cloud 9 is completely barren or if it harbors a tiny population that previous instruments could not see.

Resolving individual stars would have a decisive advantage over simply measuring the integrated brightness of the galaxy. Instead of asking how much glow the structure produces as a whole, astronomers could directly check if stars exist in the region and determine, more accurately, the real limit of its population.

Confirmation would also have implications for understanding dark matter and galactic formation, although it would not turn Cloud 9 into a direct test of the nature of that invisible substance. The object would allow for the study of how a gravitational halo can retain gas while simultaneously remaining devoid of the stellar activity that transforms that material into light, metals, and new generations of stars.

For now, the evidence offers an extraordinary picture: a cloud with gas equivalent to 1 million solar masses, a dark matter halo of approximately 5 billion solar masses, and a stellar content that would not exceed 16,000 solar masses under the most challenging detection assumption. Cloud 9 still awaits confirmation, but its luminous silence has already made it one of the most intriguing targets in contemporary astronomy.

The research team’s findings are available as a scientific paper authored by collaborators led by Ignacio Trujillo, and their results complement previous observations from Hubble and images obtained with HiPERCAM. The next step will be to gather independent evidence to determine whether astronomers are facing a truly starless galaxy or a structure with a stellar population too faint to be detected in current observations.

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