The elusive, starless Cloud-9

Compiled by Daniela Mata

A starless, gas-rich cloud of dark matter has been found in the quiet outskirts of the spiral galaxy M94, located some 14 million light years from Earth. This new class of object is a celestial ghost that has spent billions of years defying the traditional definition of a galaxy. Terming it Cloud-9 when announcing its discovery, the international team of astronomers said that it represented a monumental “success of failure” in modern astrophysics.

This ambiguous phrase refers to the fact that for decades, the currently accepted model of the Universe, the so-called Lambda Cold Dark Matter ( Λ CDM) model, has predicted that the cosmos should be filled with small, dark-matter-dominated clouds (known as halos) that never managed to trigger star formation.

Astronomers refer to these objects as Reionization-Limited H I Clouds or RELHIC for short. Cloud-9 is the first confirmed discovery of one of them. It is a primordial building block that only got so far and then stalled. Unlike the billions of successful galaxies that illuminate the cosmos, this failed galaxy consists of a massive dark matter halo filled with hydrogen gas. However, that hydrogen has failed to collapse further into stars and so Cloud-9 has remained starless since close to the dawn of time.

As such, it offers a rare, unobstructed window into the ‘dark universe’, where the gravitational scaffolding of dark matter exists without the distracting glare of starlight. “We think that this indirect evidence for dark matter is what makes Cloud-9 so fascinating,” says team member Andrew Fox at the Space Telescope Science Institute (STScI), Baltimore.

A relic testing time and limits

The journey to identifying Cloud-9 began three years ago during a radio survey by China’s Five-hundred-meter Aperture Spherical Telescope (FAST). As the name suggests, it was the ninth gas cloud identified near M94 in the 2023 study. Subsequent observations by the Green Bank Telescope and the Very Large Array revealed a compact, highly spherical core of hydrogen gas about 4,900 light years in diameter.

However, a critical question remained: was Cloud-9 truly starless, or was it merely a dwarf galaxy whose stars were too dim for ground-based telescopes to see? To resolve the issue, researchers turned Hubble’s Advanced Camera for Surveys (ACS) toward the cloud. If Cloud-9 were a typical dwarf galaxy, Hubble would have easily resolved dozens of individual stars. Instead, the images returned were a hauntingly blank field.

“We expected – this whole time – to find an “ultra-faint” galaxy and prepared an analysis plan for this! Finding nothing was super exciting and we’re happy much of the world also finds it exciting,” team member Rachael Beaton told Astronomy Now.

The anatomy of a phantom

They found that Cloud-9’s hydrogen content amounts to about one million times the mass of the Sun. This may sound like a lot but it is insufficient to hold the cloud together. According to the Λ CDM model, to retain its nearly spherical shape, the cloud must be embedded in a dark matter halo with an estimated mass of about five billion solar masses. This figure is crucial because, while completely overwhelming the mass of the gas, it is just below the threshold required to trigger star formation.

This precarious balance is the key to its identity. It highlights the critical role of a cloud’s mass in determining its evolution across cosmic time. If the halo had been more massive at its inception, the timeline of its evolution would have been much more active; it would likely have collapsed under its own gravity billions of years ago to form the first generation of stars and become a conventional galaxy.

Conversely, if it had a lesser mass, it would have been a victim of the early Universe’s era of reionisation epoch, in which the intense ultraviolet radiation produced by the first celestial objects would have dispersed its gas, leaving the halo empty and almost impossible to see.

Instead, Cloud-9 exists in a Goldilocks zone of mass, assembly history, and environment, according to the study’s team members.

The team members are convinced that Cloud-9’s long-term survival is evidence of dark matter. Its discovery has opened a new frontier for observers. Future studies using the James Webb Space Telescope could push the stellar mass detection limits even lower, or reveal faint emission from the hydrogen gas.

Cloud-9 serves as a reminder that in the vastness of the cosmos, sometimes the most profound discoveries are the ones where we find absolutely nothing at all.

Read More

FAST Reveals New Evidence for M94 as a Merger

The First RELHIC? Cloud-9 is a Starless Gas Cloud