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Clusters of galaxies are the largest structures in the Universe linked together by gravity. They can contain thousands of galaxies, huge oceans of hot gas, invisible islands of dark matter, and – sometimes – the glowing ghost of a jellyfish or two.
In the Abell 2877 galaxy cluster, located in the southern sky about 300 million light years from Earth, astronomers have discovered one of these jellyfish.
Visible only in a narrow band of radio light, the Cosmic Jelly is over a million light-years wide and features a large lobe of supercharged plasma, dripping with tentacles of hot gas.
The gelatinous appearance of the structure is both “ghostly” and “strange,” according to the authors of a new article published on March 17 in the Astrophysics Journal.
However, how quickly the structure disappears from view is even more astonishing than the shape of the space jelly, the authors said.
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“This radio jellyfish is sort of a world record,” study lead author Torrance Hodgson, of the International Radio Astronomical Research Center (ICRAR) in Perth, Australia, said in a statement.
“While it is brilliant at regular FM radio frequencies, at 200 megahertz the emission almost disappears. No other extragalactic emission like this has been observed to disappear so quickly.”
The ghost of the jellyfish past
The Universe swims with energetic structures that are only visible in radio wavelengths, like the mysterious X-shaped galaxies rolling in space, or the twin drops at the center of the Milky Way.
However, no structure of this size has ever been observed in such a narrow band of the radio spectrum.
According to the researchers, this probably means that this cosmic jellyfish is in fact a strange bird known as the “radio phoenix”.
Like the mythical bird that died in the flame and rose from its ashes, a radio phoenix is a cosmic structure born from a high-energy explosion (like a black hole explosion), fades for millions of years. years as the structure expands and its electrons lose energy, then is eventually reenergized by another cosmic cataclysm (such as the collision of two galaxies).
To create a radio phoenix, this last cosmic event must be powerful enough to send shock waves surging through the dormant electron cloud, causing the cloud to compress and electrons again with energy.
According to the study’s authors, this could cause a structure like the jellyfish cluster to glow in some radio wavelengths, but darken quickly in others.
“Our working theory is that about 2 billion years ago, a handful of supermassive black holes from multiple galaxies spewed out powerful jets of plasma,” Hodgson said.
The energy of this plasma waned for millions of years, until “just recently two things happened – the plasma started to mix together as very soft shock waves were passing through. the system, ”Hodgson said.
“This briefly rekindled the plasma, lighting up the jellyfish and its tentacles so that we could see them.”
The researchers used a computer simulation to show that this explanation is a plausible origin story for this large jellyfish in the sky, although several big questions – such as the origin of the “soft shock waves” – remain unanswered.
The team hopes to take a closer look at jellyfish in the future, following the completion of the Square Kilometer Array – an array of hundreds of radio telescope antennas slated for construction in the Australian outback.
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