Hubble’s view into a “failed galaxy”: Cloud-9 without a single star
Deep imaging by the Hubble Space Telescope aimed at one seemingly ordinary clump of gas on the edge of the nearby spiral galaxy Messier 94 (M94) ended with results that in astronomy rarely sound this “negative” – the team found nothing. Precisely that emptiness, the absence of stars where they would otherwise be expected, became the key evidence for the idea that Cloud-9 is not a faintly visible dwarf galaxy, but a relic cloud of neutral hydrogen that never ignited a stellar “spark”.
The authors of a paper published in
The Astrophysical Journal Letters in November 2025 describe Cloud-9 as the most compelling candidate for a so-called reionization-limited H I cloud (RELHIC) – a hypothetical type of object that should be almost completely invisible in the optical part of the spectrum, yet massive in terms of dark matter. In translation: it is a possible “failed galaxy”, a building block from the early phases of the universe that remained without a stellar population, and thus without the light that usually reveals galaxies.
What Cloud-9 is and why it matters
Cloud-9 is a compact cloud of neutral hydrogen (H I), detected through radio-astronomical observations of the 21-centimeter line. That very line allows astronomers to “see” cooler atomic hydrogen even where there are no stars. According to the research team’s description, Cloud-9 is dynamically “cold”, without clear signs of rotation, and its signal in the radio domain corresponds to an object that, by its recession velocity, lies at approximately the same distance as the galaxy M94 – about 4.4 megaparsecs (roughly 14 million light-years), although for M94 popular catalogs often also list a value near 16 million light-years.
It is precisely that proximity (in cosmological terms) that makes Cloud-9 an exceptionally attractive target: if there truly exists in the local universe a dark-matter halo filled with gas but without stars, then we gain a rare laboratory to test fundamental predictions of the standard cosmological model (ΛCDM), and to understand the threshold at which a “galaxy forms”. In other words, where is the boundary between a structure that can trigger star formation and one that, despite gas, remains dark?
The program’s principal investigator Alejandro Benítez-Llambay (University of Milano-Bicocca) described Cloud-9 in team statements as a story of a “failed galaxy” from which science often learns more than from success: the nonexistence of stars is not seen as a disappointment, but as support for the theory.
From a radio discovery to Hubble’s confirmation of “nothing there”
The story began in China, in Guizhou province, where the Five-hundred-meter Aperture Spherical Telescope (FAST) is located – one of the most sensitive radio telescopes in the world. FAST’s survey of the environment of the galaxy M94, described in a paper published in 2023, revealed multiple structures of neutral hydrogen, including a separate cloud without an obvious optical counterpart. In later analyses, that cloud received the nickname Cloud-9 – simply because it was the ninth identified cloud in the series, without any symbolism that the phrase “cloud nine” has in Western culture.
Independent confirmations then arrived from the USA: radio observations by the Robert C. Byrd Green Bank Telescope and the Very Large Array (VLA) interferometer further established that the cloud exists and that it is a compact, relatively orderly system. However, the crucial question remained: is there a very faint dwarf galaxy hidden within it that ground-based telescopes failed to detect, or is it truly a “starless” object?
To resolve that dilemma, the team requested time on Hubble and aimed its Advanced Camera for Surveys (ACS) at the position of Cloud-9. According to the paper, the observations were carried out from February 17 to 19, 2025, through multiple visits, precisely in order to reach sufficiently deep magnitudes and reveal at least the brightest individual stars – if any exist at all.
Lead author Gagandeep Anand (Space Telescope Science Institute, Baltimore) explains that before Hubble there was always the possibility that this was a dwarf galaxy so faint that it “escaped” the sensitivity of ground-based instruments. But with Hubble’s ACS, he emphasizes, the team could set firm limits: no stellar component is seen in the images, and the analysis of the color–magnitude diagram further reinforces the conclusion that there is not even a very modest stellar mass there.
RELHIC: a fossil from the era of reionization
The acronym RELHIC comes from “Reionization-Limited H I Cloud”. The concept is rooted in the period of the early universe when, after the formation of the first stars and galaxies, ultraviolet radiation ionized most of the intergalactic hydrogen. In such an environment, the smallest dark-matter halos – too small to retain and cool gas strongly enough – could be left without the stellar “fuel” that would condense into stars. In theory, those structures could still exist today as compact clouds of neutral hydrogen in equilibrium with the surrounding ultraviolet background radiation, but without a stellar population that would “flag” them in the optical spectrum.
Cloud-9 fits that picture precisely because of its combination of properties: it is compact, with a relatively narrow radio profile (interpreted as a sign of low velocity dispersion), it has no clear rotation, and yet it is sufficiently massive in gas to be measurable. Earlier, based on radio data and modeling, the team indicated that the total dark-matter mass could be on the order of a few billion solar masses, while the mass of neutral hydrogen is about a million solar masses.
Andrew Fox (AURA/STScI, in collaboration with the European Space Agency) described this type of object as a “window into the dark universe”: most of the mass of the universe is, according to theory, not visible, and systems like this – without stars dominating the light – provide a rare opportunity to study the dark component indirectly, through its influence on the gas.
Numbers that change the picture: gas, dimensions, and dark matter
Compared with many other hydrogen clouds in the broader environment of the Milky Way, which often look irregular and stretched out, Cloud-9 stands out for its compactness. It is described as an object with a radius of about 1.4 kiloparsecs (roughly 4,600 light-years), and its H I mass is estimated at approximately 10^6 solar masses. Taking into account the balance of gas pressure and gravity, the authors conclude that the dark-matter halo holding it together could have a mass of about 5×10^9 solar masses.
Such a ratio – relatively little visible gas versus an enormous total mass – is a common hallmark of a scenario in which dark matter dominates. That is precisely why the term “dark galaxy” is increasingly used in the literature for objects that resemble galaxies in mass and gravitational potential, but have no stars, and thus no classic optical “address” in the sky.
The article also emphasizes the methodological importance: astronomy has for decades been “star-centric”, relying on starlight and the glow of galaxies. Cloud-9 is a reminder that part of cosmological structure can exist in a regime where gas and dark matter are the main actors – and stars only an optional outcome.
Is Cloud-9 really spherical: environmental traces and the “ram-pressure” effect
Although the first radio observations suggested very regular contours of hydrogen density, more detailed VLA data in later papers pointed to finer asymmetries. Signs of gas compression on one side and a tail-like structure on the other were described, which the authors interpret as a possible consequence of ram-pressure stripping – the “scraping” of gas as the cloud moves through the more rarefied intergalactic medium or through the extended gas halo around M94.
Such environmental effects are important for two reasons. First, they can explain why such objects are rare: even if they form, the proximity of larger galaxies and motion through surrounding gas can, over time, strip them, disperse them, or ionize them. Second, asymmetries can be traces of the physics of gas and dark matter on small scales – a domain where standard models and observations often “argue”, for example in debates about the number of satellite dwarf galaxies and the internal structure of dark-matter halos.
In the context of M94, Cloud-9 appears to be an object in a real physical connection with the galaxy: it has a similar velocity, lies in its neighborhood, and shows signs of possible interactions. How tight that connection is – whether the cloud is bound, passing through, or merely part of a broader gas environment – the authors describe as a question that further observations will answer more clearly.
Could a “failed galaxy” one day become a galaxy
One of the intriguing conclusions in interpretations of Cloud-9 is that the object sits in a kind of “sweet spot”. According to these considerations, if it had been significantly more massive, gravity would likely have overcome the effects of reionization and the gas would have cooled and formed stars – and Cloud-9 would have been just another small galaxy. If it had been noticeably less massive, the gas would have dispersed and ionized more easily, leaving only faint traces that would be difficult to catch even with radio telescopes.
As it is, at least according to the available analyses, Cloud-9 is large enough to retain neutral hydrogen, but not “heavy” enough or favorable enough in conditions to trigger stable star formation. Whether in the distant future – through the accretion of additional gas or a change in environmental conditions – it will cross the threshold and begin to form stars is, at this moment, an open question.
Broader significance: testing ΛCDM and the search for “invisible” structures
For cosmologists, Cloud-9 is not merely an interesting nearby object. If the RELHIC interpretation is confirmed in other examples, it would mean that the local universe truly contains an entire population of small dark-matter halos filled with gas but without stars – a population that would help bridge the gap between simulation predictions and the observed abundance of dwarf galaxies.
The authors emphasize that detecting such systems is observationally demanding: nearby bright objects can easily overwhelm weak signals, and environmental processes can alter them quickly. Therefore, combined approaches are needed – wide radio surveys of high sensitivity, interferometry that reveals gas structure, and deep optical imaging that sets limits on the presence of stars.
In that sense, Cloud-9 is also a preview of what might follow with new generations of sky surveys. As radio instruments become more sensitive and deep imaging becomes more routine, the likelihood grows that more “abandoned houses” will begin to appear near already known galaxies – as team member Rachael Beaton vividly described the possibility that among our galactic neighbors there are more such dark structures.
For now, the most concrete message is simple: sometimes the most important result is precisely the one in which, after all expectations, not a single star is seen. In the case of Cloud-9, that silence in the optical spectrum became a strong signal that in our cosmic neighborhood lies one of the purest examples of a structure in which dark matter has the leading role, and stars – at least for now – have not taken the stage.
Sources:- The Astrophysical Journal Letters (Anand et al., 2025) – paper on Hubble ACS observations of Cloud-9 and limits on stellar mass: link- arXiv (Anand et al., 2025) – preprint of the paper “The First RELHIC? Cloud-9 is a Starless Gas Cloud”: link- The Astrophysical Journal (Zhou et al., 2023) – FAST observations of the M94 environment and identification of a cloud without an optical counterpart: link- The Astrophysical Journal (Benítez-Llambay and Navarro, 2023) – modeling Cloud-9 as a possible dark-matter halo without stars: link- The Astrophysical Journal (Benítez-Llambay et al., 2024) – VLA observations of Cloud-9 and interpretation of environmental effects: link- arXiv (Benítez-Llambay et al., 2024) – preprint “Not So Round: VLA Observations of the Starless Dark Matter Halo Candidate Cloud-9”: link- NASA Science – basic data about the galaxy Messier 94 (distance and context): link
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