Astronomers using the Hubble Space Telescope have made an exciting discovery: Cloud-9. This unique celestial object is a gas-rich cloud without stars, located over 14 million light-years from Earth, near the spiral galaxy Messier 94. Researchers believe Cloud-9 is an ancient cosmic relic, a building block of galaxies that proves key concepts about how galaxies form.
An important finding about Cloud-9 supports the Lambda Cold Dark Matter model (LCDM), which explains the structure of our universe. This model suggests that dark matter exists in halos, which can form galaxies if they accumulate enough mass. According to lead astronomer Deep Anand, Cloud-9 is exceptional because it retains gas, making it visible to us through radio observations. This visibility hints at a universe filled with similar low-mass dark matter halos, which we have yet to discover.
Cloud-9 was initially spotted three years ago by the Five-hundred-meter Aperture Spherical Telescope in China. Despite early observations with another telescope that did not clarify its nature, further studies using Hubble revealed that Cloud-9 is a Reionization-Limited H I Cloud, or RELHIC. This means it consists of neutral hydrogen, a remnant from the early universe, providing a rare glimpse into cosmic history.
Research indicates that the gas within Cloud-9 holds about a million times the mass of our Sun. However, this isn’t enough for it to collapse into a full galaxy. Its dark matter component likely weighs around five billion solar masses, keeping it together but not heavy enough to form new stars. This mass is intriguingly close to a threshold theorized by astronomers, suggesting that while Cloud-9 is on the brink, it may not be destined to become a galaxy—at least not yet.
Cloud-9 also interacts with cosmic ultraviolet light, which helps keep its gas hot and ionized. This energy can suppress the formation of stars, adding to the object’s mystery. Despite appearing to be in eternal darkness, researchers believe Cloud-9 could gather enough mass to eventually become a galaxy, though the mechanisms behind this are still uncertain.
Experts are keen to search for similar gas clouds, although they present unique challenges. These objects can be overshadowed by brighter celestial bodies and are fleeting, often stripped of gas as they move through space. Alejandro Benitez-Llambay, another key researcher, notes that for a cloud like Cloud-9 to have persisted this long, it must meet specific conditions: a slow growth in its dark matter halo and a degree of isolation in the cosmos. Fewer than 10% of such pristine gas clouds have likely survived to the present.
In essence, Cloud-9 serves as a reminder of how much of the universe remains unseen. The dazzling stars we often admire are just a fraction of a much larger, more complex cosmic story. Understanding these rare cloud formations not only sheds light on galaxy formation but also enhances our knowledge of the darker aspects of the universe.
For those interested in further reading about the implications of this discovery and the cosmic mysteries surrounding dark matter, you can check out the full study in The Astrophysical Journal Letters.
