A galaxy of gas and dark matter has puzzled scientists

Astronomers conducted one of the deepest observations of the Cloud-9 object, which is considered a candidate for "dark galaxies", and did not detect starlight emanating from it. The data obtained reinforces the assumption that the system may consist mainly of gas and dark matter. This was announced on September 10. Phys.org .

Cloud-9 is a cloud of neutral hydrogen located near the spiral galaxy M94 at a distance of about 4.66 megaparsecs, or 15 million light-years from Earth. The mass of hydrogen contained in it is estimated at about 1 million times the mass of the Sun.

At the same time, during previous observations, scientists were unable to detect a noticeable stellar component in Cloud-9. However, the sensitivity of the instruments used was not enough to completely exclude the presence of very dim and scattered stars.

The existence of such objects is predicted by the standard cosmological model lambda-CDM. According to her, there should be a large number of small clumps of dark matter in the universe, which contain gas, but for various reasons could not form stars.

The formation of stars in low-mass halos can be hindered by the balance between the gravitational compression of gas, its cooling and heating under the influence of background ultraviolet radiation. The discovery of such a galaxy would be an important confirmation of one of the predictions of modern cosmology.

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However, there may be other explanations for the nature of Cloud-9. The object may turn out to be a fragment of matter ejected during the gravitational interaction of galaxies, a high-speed gas cloud, or a temporary structure that is not an independent galaxy.

To find possible stellar radiation, a group led by Ignacio Trujillo, an employee of the Institute of Astrophysics of the Canary Islands, used a high-speed HiPERCAM camera mounted on the Grand Canary Telescope. The observations were carried out on June 11 and 12, 2026, simultaneously in five spectral bands.

During the shooting, the scientists sequentially shifted the direction of the camera, and when processing the data, they separately masked the Cloud-9 area. This reduced the risk that the extremely weak radiation of the object would be mistakenly removed along with the background light.

The resulting images turned out to be about 10 times deeper than previous observations of this area of the sky. The maximum surface brightness in the green region of the spectrum reached 31.4 magnitude per square angular second. Despite the high sensitivity of the equipment, the researchers did not detect starlight in the central region of Cloud-9, measuring approximately 1.3 by 1.3 kiloparsecs, which corresponds to approximately 4.2 thousand light-years.

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Assuming that an old stellar population with a low content of heavy elements is hidden inside the object, its total mass should not exceed 16 thousand times the mass of the Sun. Thus, there may be at least 60 times more hydrogen in Cloud-9 than there are stars.

The new data complement the results of previous studies. One group of scientists estimated the number of stars using survey images that were not designed to preserve extremely weak scattered light. Another used counting individual stars in Hubble Space Telescope images, but the result depended on comparison with a more compact galaxy.

The authors of the new work applied an independent method based on the search for a common star glow. The resulting limitation on the mass of stars turned out to be between the results of two previous studies and further strengthened the arguments in favor of the unusual nature of Cloud-9.

However, the absence of detected light does not prove that there are no stars in the object at all. There may be very few of them, or they may turn out to be too dim even for observations. In addition, scientists have yet to confirm that Cloud-9 is indeed located inside the halo of dark matter and is an independent galaxy, and not an ordinary gas cloud.

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