Cloud-9 was first discovered about three years ago by a telescope located in Guizhou, China. However, the Five-hundred-meter Aperture Spherical Telescope (FAST) wasn’t able to exactly identify it for what it really was. The Chinese astronomers classified it as yet another hydrogen gas cloud near the spiral galaxy Messier 94, roughly 14 million light-years from Earth.
It was the follow-up observation with NASA’s Hubble Space Telescope that provided a clearer picture. Cloud-9 isn’t a typical hydrogen gas cloud, and it isn’t a galaxy in its own right either. It contains large amounts of hydrogen gas, but it’s dominated by dark matter, meaning, among other things, that somehow, this dark galaxy emits no visible light. This combination places Cloud-9 in a rare and long theorized category sometimes described as a “failed galaxy.” Its nature was described in detail in the recent study published in The Astrophysical Journal Letters.
One could say the astronomers were on cloud 9 when they made the discovery, but that’s not really how this new celestial object got its name. Cloud refers to its diffuse, gas-rich nature, while “9” marks its position in a catalog of similar hydrogen clouds surrounding the Messier 94. A truly fitting nickname for such a discovery.
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The relic of the universe
A representation of the formation of the universe since the Big Bang – NASA/WMAP Science Team/Wikimedia Commons
Cloud-9 isn’t an ordinary cloud, and it isn’t a galaxy either, at least not in the traditional sense. It’s a completely new space object that astronomers have never encountered before: It’s a starless cloud of gas held together by dark matter. Cloud-9 is described as a Reionization-Limited H I Cloud, or RELHIC. It means it’s a blob of neutral hydrogen gas that never managed to ignite stars inside it. But it’s also a relic because it’s considered to have formed during the early days of the universe.
Typically, galaxies are defined by the stars they contain. Stars make galaxies easily detectable in the vastness of the universe because they are easily visible. But Cloud-9 has no stars — not even the very faint ones. That would imply that Cloud-9 is not really a galaxy. Scientists instead call it a failed galaxy. It likely represents the early stage of galaxy formation that never managed to progress beyond a certain point. Cloud-9 simply never reached the density to spark the birth of stars. However, there’s still a possibility Cloud-9 will eventually manage to reach the needed density, collapse on itself, and become a true galaxy.
For now, Cloud-9 consists of a core made mostly of neutral hydrogen gas, spanning a region of about 4,900 light-years across. Its total gas mass is about a million times the mass of the sun. But what truly dominates this cloud is dark matter. Dark matter is the invisible, “missing” substance that accounts for most of the universe’s mass, and in this case, scientists estimate the amount of dark matter in Cloud-9 is billions of times the sun’s mass. And it’s the gravity from this dark matter that holds the cloud together.
Window into the dark universe
galaxy cluster Abell 1689, with the mass distribution of the dark matter in the gravitational lens shown in purple – NASA, ESA, E. Jullo (JPL/LAM), P. Natarajan (Yale) and J-P. Kneib (LAM)/Wikimedia Commons
Cloud-9 is an important discovery not because of what it contains, but because of what it lacks. With no stars, dust, or active star formations, all that remains is a clean system dominated by dark matter. It’s a valuable playground for all the scientists who are trying to understand dark matter – the substance that might have existed before the Big Bang.
Dark matter doesn’t emit, absorb, or reflect light. Its presence can only be noticed through gravity. Most galaxies mix the gravitational signals of dark matter with the ones of stars, gas, and dust. Cloud-9 strips that complexity away. Nearly all of its gravitational influence comes from dark matter alone. That makes Cloud-9 a rare and almost perfect astronomical laboratory. That’s precisely why Andrew Fox, the astronomer at the Space Telescope Science Institute for the European Space Agency, described Cloud-9 as a “window into the dark universe.” Scientists can use it to study how hydrogen gas moves and how tightly it is bound within the cloud. This will give them insight into how dark matter behaves on a small scale. These measurements will further help refine theories about how dark matter clumps, how it interacts with ordinary matter, and how it shapes the cosmic structures.
With the discovery of Cloud-9, astronomers are encouraged to keep searching for similar starless systems. They already theorized about the possible existence of small dark matter halos throughout the universe, but Cloud-9 is the only known evidence that such objects exist. Finding more such objects would let us know how common dark v matter structures are, and how much of the universe remains hidden from our view.
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