Astronomers have found a giant gas cloud, likely stripped from its parent galaxy long ago, in a cluster 330 million light-years away.

Ten billion Suns’ worth of gas are hanging in space in the form of a cloud almost 6 million light-years across. Bigger than the Milky Way, this “orphan cloud” was probably torn long ago from the galaxy it once called home.

Orphan cloud seen in X-rays and visible light
The orphan cloud is the blue umbrella-shaped part of this image, which is color-coded to show the X-ray part of the cloud in blue and the excited hydrogen gas in red. Visible light is shown in white.
ESA / XMM-Newton

Astronomers discovered this cloud in 2017 by the deep-red emission of its excited hydrogen, hanging in the Leo Cluster (Abell 1367) roughly 330 million light-years away. But it wasn’t until follow-up observations revealed an X-ray-emitting cloud at roughly the same location that it attracted the attention of Chong Ge (University of Alabama in Huntsville) and colleagues. They report their findings on this unique cloud in the August Monthly Notices of the Royal Astronomical Society.

When a galaxy falls into a cluster, it doesn’t pass through an empty vacuum. Hot gas fills the space between galaxies in a cluster, and it pushes back against the galaxy much like the wind you feel when you go for a bike ride. And just like that wind may push back your hair, the hot intracluster medium rams into the galaxy’s cooler, denser gas. Astronomers have seen gas streaming behind galaxies in this fashion before, nicknaming them “jellyfish” for their appearance.

Even bereft of its star-forming reservoir, the galaxy will go on sailing through the cluster, its stars and dark matter sticking with it. The stars will grow older (and redder), and no new stars will replace them.

In this case, though, Ge and colleagues couldn’t find an obvious parent galaxy, just the cloud that it had left behind. Its orphaned gas ought to mix into the hotter and sparser surrounding medium over time, evaporating completely within 30 million years. (The intracluster gas is so spread out, it can’t cool efficiently, so it just stays hot. In mixing with it, the orphan cloud ought to heat up, too.)

Yet, it hasn’t — the cloud seems to have warmed up some, but it’s still intact. Based on spectroscopic observations of the motions within the cloud, and the lack of a parent galaxy, the researchers estimate that it’s half a billion years old. To aid its survival, the team suggests a magnetic field might thread the gas. A field of 6 microgauss would be enough to hold the cloud together — that’s about 100,000 times weaker than Earth’s magnetic field but about the same strength as the field in the interstellar gas that surrounds the Sun.

An Unusual Orphan

While astronomers have seen other lonely clouds hanging around in the nearer Virgo Cluster, none of them emit X-rays the way this one does. The X-rays indicate that there’s really hot gas too, in addition to the merely warm excited hydrogen. “So whatever the similarities in the formation mechanism (if any), clearly there are differences too,” says Rhys Taylor (Astronomical Institute of the Czech Academy of Sciences), who was not involved in the study.

“The fun thing to me about the Orphan is how unusual it is,” Taylor says. “Clearly, whatever process formed it can't be all that common, or we'd find such features everywhere.”

So, why don’t we see more clouds like this one? Or rephrasing the question, why did this cloud survive when others like it presumably did not? “The origin of the cloud via stripping seems the best explanation to me,” Taylor says, “but it also raises a lot of interesting questions.”

Team lead Ming Sun (also at University of Alabama in Huntsville) says obtaining more information about the cooler gas in the cloud will be key to unraveling its mysteries, observations the team is now working on acquiring.


Image of Yaron Sheffer

Yaron Sheffer

July 26, 2021 at 5:03 pm

The optical red color comes from H-alpha of neutral hydrogen. I believe it would much harder to detect the presence of ionized hydrogen...

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Image of Monica Young

Monica Young

July 27, 2021 at 9:38 am

You are right, of course — all instances should read "excited" rather than "ionized." I have fixed the text.

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