Hubble Confirms Existence of RELHIC: A Dark Matter-Dominated Relic Cloud

Astronomers have confirmed the existence of an unprecedented cosmic structure: a dense, hydrogen-rich gas cloud entirely devoid of stars, yet overwhelmingly governed by the influence of dark matter. This object, informally dubbed Cloud-9, marks the first verified example of what scientists term a Reionization-Limited H I Cloud (RELHIC) in the universe. The breakthrough discovery, detailed in The Astrophysical Journal Letters, was made possible through meticulous analysis of data captured by the NASA/ESA Hubble Space Telescope.

Cloud-9 resides in the immediate vicinity of the spiral galaxy Messier 94 (M94), situated approximately 14.3 million light-years from Earth. The initial detection of this anomaly occurred in 2023 during a radio survey conducted using China’s Five-hundred-meter Aperture Spherical Telescope (FAST). Subsequent observations carried out with the Green Bank Telescope and the Very Large Array (VLA) in the United States served to verify its distinct radio signature. The definitive confirmation that the cloud contained no stars came from the Hubble Space Telescope's Advanced Camera for Surveys (ACS) instrument, solidifying its status as a relic that failed to initiate star formation.

The quantitative characteristics of Cloud-9 are quite striking. Its core of neutral hydrogen spans roughly 4,900 light-years in diameter, and the mass of the hydrogen gas alone is estimated to be about one million times the mass of our Sun. However, calculations based on the equilibrium between gas pressure and the gravitational pull of dark matter suggest that the total mass of Cloud-9 rockets up to a colossal 5 billion solar masses. This massive discrepancy underscores the profound dominance of dark matter within the structure. Key members of the research team involved in this study include Gagandeep Anand and Rachel Beaton, both from the Space Telescope Science Institute (STScI), alongside Alejandro Benítez-Llambay of the University of Milano-Bicocca.

The central scientific puzzle surrounding Cloud-9 is understanding why it possesses sufficient gas to theoretically trigger star formation yet remains completely dark. Researchers posit that Cloud-9 offers a rare glimpse into the "dark universe," serving as a tangible illustration of a "failed galaxy story." As Alejandro Benítez-Llambay noted, the absence of stars provides empirical support for theories demonstrating a "pristine galactic building block that never quite came together." This finding offers crucial empirical validation for the Lambda-CDM cosmological model, which predicts the presence of such starless dark matter halos on subgalactic scales.

The significance of this discovery for astrophysics cannot be overstated, as it grants researchers a unique opportunity to study a structure entirely dominated by dark matter without the confounding effects of stellar feedback. Cloud-9 stands apart from other known hydrogen clouds due to its compact, spherical shape, suggesting it is dynamically stable rather than being a mere fragment of a larger structure. The ability to probe a dark matter-dominated environment via its neutral hydrogen component opens up entirely new avenues for dark matter physics, as highlighted by Andrew Fox of AURA/STScI.