Astronomers using the NASA/ESA Hubble Space Telescope have confirmed the existence of starless, hydrogen-rich objects dominated by dark matter. These objects, called reionization-limited HI clouds (RELHIC), are relics of the early Universe that challenge traditional ideas about galaxy formation and may indicate a hidden population of such structures.
This image shows the location of the CMB-9 cloud. Image credit: NASA / ESA / VLA / Gagandeep Anand, SSC / Alejandro Benitez-Lambay, University of Milano-Bicocca / Joseph DePasquale, STSCI.
This RELHIC object, designated Cloud 9, was identified through observations with the Five Hundred-meter Aperture Spherical Telescope (FAST) and independently confirmed with the Very Large Array (VLA) and the Green Bank Telescope (GBT).
“This is the story of a galaxy gone wrong,” said Dr. Alejandro Benitez-Lambay, an astronomer at the University of Milan-Bicocca.
“In science, we usually learn more from failures than from successes. In this case, the absence of stars is what proves the theory correct.”
“This tells us that we have found in the local Universe the primordial building block of a galaxy that has not yet formed.”
The core of Cloud 9 is made of neutral hydrogen and has a diameter of about 4,900 light years.
Located in the vicinity of a spiral galaxy. Messier 94the object has the same receding velocity as the galaxy, so it is approximately 14.3 million light-years from Earth.
“This cloud is a window into the dark universe,” said Dr Andrew Fox, an astronomer at ESA's Association of Universities for Research in Astronomy/Space Telescope Institute (AURA/STScI).
“We know from theory that most of the mass of the universe is expected to be dark matter, but this dark material is difficult to detect because it does not emit light.”
“Cloud 9 gives us a rare glimpse of a cloud dominated by dark matter.”
Astronomers used Hubble's Advanced Camera for Filming (ACS) to search for any luminous stellar component in Cloud 9.
Their analysis rules out the presence of any dwarf galaxy with detectable stellar mass.
“The lack of detectable stars supports the interpretation that this system is a RELHIC; that is, a starless dark matter halo filled with hydrostatic gas in thermal equilibrium with the cosmic ultraviolet background,” they said.
Cloud 9 would be a leading candidate for any known compact HI cloud and provide empirical support for a cornerstone prediction of the prevailing cosmological model, the ΛCDM (Lambda Cold Dark Matter) model.
This model assumes the existence of gas-filled halos of starless dark matter of subgalactic mass scales that never formed stars.
“The discovery of Cloud 9 also helps constrain the current halo mass threshold for galaxy formation, bringing us closer to understanding why some dark matter halos contain galaxies while others remain devoid of stars,” the researchers said.
detailed conclusions will appear this week in Letters in an astrophysical journal.
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Gagandeep S. Anand etc.. 2025. First RELIC? Cloud 9 is a starless gas cloud. ApJL 993, L55; two: 10.3847/2041-8213/ae1584
