The detection of RELHICs provides direct evidence for the existence of these dark matter halos that are starless. This discovery tests cosmological theories and enhances our understanding of the galaxy formation thresholds that dark matter…
Discovery of Cloud-9, a RELHIC in a Starless Dark Matter Halo
Scientists recently discovered a compact hydrogen gas cloud called Cloud-9, near the spiral galaxy M94. It lies about 4.4 million parsecs from Earth. The cloud emits radio waves at 21 cm, indicating neutral hydrogen. What makes Cloud-9 fascinating is that it appears to lack stars, making it a prime candidate for a celestial object known as a Reionization-Limited H i Cloud (RELHIC).
What Is a RELHIC?
RELHICs are starless clouds made of gas confined by dark matter. They form in low-mass dark matter halos below the limit where stars can form today. After cosmic reionization, only halos more massive than a critical threshold could hold onto enough gas to form stars.
Those halos below this threshold can keep some hydrostatic gas, but that gas never forms stars due to heating by cosmic UV radiation. Scientists call these objects RELHICs because their visible hydrogen signals mark the limits set by reionization.
The Importance of RELHICs
Lambda Cold Dark Matter (ΛCDM) is a model that makes predictions about a large number of small dark matter halos that do not include galaxies. On the other hand, locating these dark objects has proven to be difficult due to the fact that they transmit very little or no visible light.
The detection of RELHICs provides direct evidence for the existence of these dark matter halos that are starless. This discovery tests cosmological theories and enhances our understanding of the galaxy formation thresholds that dark matter mass and photoheating control.
How Scientists Found Cloud-9
The discovery began with observations from the Five-hundred-meter Aperture Spherical Telescope (FAST). FAST detected narrow ionized hydrogen emissions with characteristics typical of RELHICs compact size, cold dynamics, and low rotation velocity. Follow-up observations with the Very Large Array (VLA) and Green Bank Telescope confirmed these signals at high resolution.
The question of whether or not Cloud 9 is home to any stars remained an important one. Astronomers used the Advanced Camera for Surveys (HST/ACS) on the Hubble Space Telescope to conduct in-depth research of the region surrounding Cloud-9 in order to address this question. Their objective was to locate even the most insignificant star equivalent, if one in fact existed.
No Bright Stars Detected Yet Strong Limits Set
With the help of the HST photos, astronomers were able to eliminate the possibility of any dwarf galaxy or star population within Cloud-9 that was more massive than around 10^4 solar masses. This discovery lends credence to the hypothesis that Cloud-9 is indeed starless, matching the original predictions for RELHICs.
This provides support for the hypothesis that such objects should exist but have stayed hidden up until this point due to the fact that they are so faint and the difficulties associated with their physical proximity to larger galaxies like M94.
The Role of Dark Matter in Holding Gas Together
As a result of its location within a large halo of dark matter that weighs around 5 billion solar masses. Throughout its existence, the cloud’s neutral hydrogen gas remains gravitationally bound without producing stars. This mass exerts a force against the heat pressure that surrounding cosmic radiation causes, helping the gas maintain hydrostatic equilibrium.
In spite of the absence of stellar light, this one-of-a-kind state enables detection by narrow radio emissions. So providing support for the predictions of the ΛCDM model on subgalactic scales, where the concept of normal galaxy formation fails.
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Reference
Anand, G. S., Benítez-Llambay, A., Beaton, R., Fox, A. J., Navarro, J. F., & D’Onghia, E. (2025). The First RELHIC? Cloud-9 is a Starless Gas Cloud . The Astrophysical Journal Letters, 993(2), L55. https://doi.org/10.3847/2041-8213/ae1584
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