Lost & Found: Milky Way-size “Orphan Cloud”

Ten billion Suns’ value of gas are hanging in space within the type of a cloud virtually 6 million light-years throughout. Bigger than the Milky Way, this “orphan cloud” was in all probability torn way back from the galaxy it as soon as known as house.

Orphan cloud seen in X-rays and visible light
The orphan cloud is the blue umbrella-shaped a part of this picture, which is color-coded to indicate the X-ray a part of the cloud in blue and the ionized hydrogen gas in red. Visible gentle is proven in white.
ESA / XMM-Newton

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

When a galaxy falls right into a cluster, it doesn’t go via an empty vacuum. Hot gas fills the space between galaxies in a cluster, and it pushes again in opposition to the galaxy very like the wind you are feeling if you go for a motorcycle experience. And similar to that wind might push again your hair, the new intracluster medium rams into the galaxy’s cooler, denser gas. Astronomers have seen gas streaming behind galaxies on this style earlier than, nicknaming them “jellyfish” for his or her look.

Even bereft of its star-forming reservoir, the galaxy will go on crusing via the cluster, its stars and darkish matter sticking with it. The stars will get older (and redder), and no new stars will substitute them.

In this case, although, Ge and colleagues couldn’t discover an apparent dad or mum galaxy, simply the cloud that it had left behind. Its orphaned gas ought to combine into the warmer and sparser surrounding medium over time, evaporating utterly inside 30 million years. (The intracluster gas is so unfold out, it will probably’t cool effectively, so it simply stays sizzling. In mixing with it, the orphan cloud must warmth up, too.)

Yet, it hasn’t — the cloud appears to have warmed up some, but it surely’s nonetheless intact. Based on spectroscopic observations of the motions throughout the cloud, and the dearth of a dad or mum galaxy, the researchers estimate that it’s half a billion years outdated. To assist its survival, the crew suggests a magnetic area may thread the gas. A area of 6 microgauss can be sufficient to carry the cloud collectively — that’s about 100,000 occasions weaker than Earth’s magnetic area however about the identical power as the sphere within the interstellar gas that surrounds the Sun.

An Unusual Orphan

While astronomers have seen different lonely clouds hanging round within the nearer Virgo Cluster, none of them emit X-rays the way this one does. The X-rays point out that there’s actually sizzling gas too, along with the merely heat ionized 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 concerned within the examine.

“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 extra clouds like this one? Or rephrasing the query, why did this cloud survive when others prefer it presumably didn’t? “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 (additionally at University of Alabama in Huntsville) says acquiring extra details about the cooler gas within the cloud will likely be key to unraveling its mysteries, observations the crew is now engaged on buying.

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