Astronomers have identified "Cloud 9," a unique gas cloud located near the galaxy Messier 94, characterized by the absence of stars and a significant dark matter component. Classified as a Reionization-Limited H I Cloud (RELHIC), this object is considered by researchers to be a primordial building block of a galaxy that has not undergone star formation, offering insights into early galaxy evolution and the nature of dark matter. The discovery involved initial radio surveys followed by definitive observations from the Hubble Space Telescope.
Introduction: A Unique Cosmic Object
Cloud 9 is a low-mass gas cloud exhibiting characteristics consistent with a Reionization-Limited H I Cloud (RELHIC). An RELHIC is theorized to be a cloud of neutral hydrogen within a dark matter halo that maintains thermal equilibrium with the cosmic ultraviolet background, thereby inhibiting star formation. Researchers have characterized Cloud 9 as a "failed galaxy" or a primordial building block that has not yet formed stars, distinguishing it from previously observed starless gas clouds that typically show rotation or higher stellar mass limits.
The object is situated near Messier 94 (M94), a spiral galaxy estimated to be approximately 14 to 16 million light-years from Earth. Cloud 9 is located around 250,000 light-years from M94 and recedes at a speed of approximately 300 km/s, consistent with M94's recessional velocity. Its naming follows its sequential detection as the ninth gas cloud found on the outskirts of M94.
Discovery and Observation
Cloud 9 was initially detected during a radio survey conducted by the Five-hundred-meter Aperture Spherical Telescope (FAST) in China. Reports vary on the timing of this initial detection, with some sources indicating it occurred approximately three years prior to recent confirmations, and others stating the survey was conducted in 2023. This initial detection was subsequently corroborated by observations from the Green Bank Telescope and the Very Large Array (VLA) facilities in the United States.
Definitive confirmation of Cloud 9's starless nature was achieved through follow-up deep imaging using the Hubble Space Telescope's Advanced Camera for Surveys. Researchers noted that previous ground-based telescopes lacked the sensitivity to definitively rule out the presence of faint stars. Hubble's observations indicated no detectable stars within the highlighted area of interest, with any visible objects identified as background galaxies.
Analysis of the Hubble data allowed researchers to place constraints on the potential stellar mass within the cloud. Simulations indicated that stellar masses exceeding 10,000 solar masses are ruled out with greater than 5-sigma significance, or at the 99.5% confidence level. Stellar masses around 3,000 solar masses are ruled out at approximately the 95% confidence level. These findings suggest Cloud 9 contains no more than about 3,000 solar masses worth of stars, given its estimated neutral hydrogen content.
Characteristics of Cloud 9
Cloud 9 exhibits several key characteristics:
- Composition and Mass: It consists of neutral hydrogen and a substantial dark matter component. Its neutral hydrogen mass is estimated to be approximately 1 to 1.4 million solar masses. The dark matter content is estimated to range from over a billion to approximately five billion solar masses, based on assumptions that gas pressure balances the cloud's gravitational force. This results in a gas-to-star ratio exceeding 400:1.
- Size and Shape: Its neutral hydrogen core measures approximately 4,000 to 4,900 light-years in diameter. It is described as smaller, more compact, and spherical compared to other observed hydrogen clouds, such as those near the Milky Way.
- Internal Dynamics: Observations indicate no evidence of rotation within Cloud 9, which disfavors the formation of a collapsed disk or ongoing star-formation episodes.
- Interaction with M94: High-resolution radio data show slight gas distortions that may signify interaction between Cloud 9 and Messier 94.
Interpretations and Implications
The RELHIC interpretation for Cloud 9 suggests it is a dark matter cloud that did not accumulate sufficient gas density to initiate star formation. This aligns with theories predicting a minimum dark matter threshold required for star formation, with Cloud 9 appearing to be just below this threshold. The object's properties provide data relevant to the early stages of galaxy formation and contribute to the understanding of the nature of dark matter, which is theorized to constitute approximately 85% of the universe's total matter.
The research team considered several alternative explanations for Cloud 9 but found them inconsistent with observations:
- Foreground high-velocity gas cloud in the Milky Way: Disfavored due to the precise match in recessional velocity between Cloud 9 and Messier 94.
- Part of the Magellanic streams: Considered unlikely due to the difference in celestial direction.
- Neutral gas cloud in thermal equilibrium with Messier 94's circumgalactic medium: Inconsistent with the observed long-term stability of Cloud 9, as ram pressure interactions would destabilize the cloud on shorter timescales.
Researchers propose that Cloud 9 is currently in a state of equilibrium, possessing sufficient mass to retain its gas but insufficient mass to initiate star formation. This balance may contribute to the rarity of such objects in the local universe, as they are susceptible to environmental factors like ram-pressure stripping, which can remove gas. The discovery suggests the potential existence of numerous other small, dark matter-dominated structures in the universe.
Alternative Viewpoints and Future Research
While the RELHIC interpretation is the leading explanation, some external experts have expressed caution. Dr. Jacco van Loon of Keele University suggested that alternative explanations cannot be excluded, citing another hydrogen cloud (FAST J0139+4328) that was later identified as a very faint galaxy. He noted that Cloud 9 might also host an even fainter, currently undetected galaxy, emphasizing the need for stronger, unambiguous evidence to claim an optically dark gas cloud as a dark matter relic. Dr. Kristine Spekkens of Queen’s University stated that further study of the starless gas and its unusual shape will help determine its origin as either a starless galaxy or a free-floating cloud.
Future observations, including longer exposures with the Hubble Space Telescope or new data from the James Webb Space Telescope (JWST), are anticipated to refine the upper limits on stellar content or potentially confirm the complete absence of stars. Astronomers plan to monitor future astronomical surveys for additional RELHICs, which could expand understanding of the early universe and dark matter physics. Researchers also suggest that high-resolution observations could provide a clearer view of Cloud 9’s core, yielding information on the central dark matter distribution and potentially helping to rule out certain dark matter candidates.
Publication Information
The findings were published in The Astrophysical Journal Letters and presented at the 247th meeting of the American Astronomical Society in Phoenix. Key researchers involved in the program include Alejandro Benitez-Llambay of Milano-Bicocca University as principal investigator, Gagandeep Anand of STScI as lead author, and Andrew Fox of AURA/STScI for ESA as a co-author.