This is an ORC - an "odd radio cluster". It's a faint circle of radio emissions surrounding a distant galaxy. 5 ORCs have been found. This one is about a million light years in diameter, roughly 10 times the size of a galaxy like ours. So it was probably formed by some sort of explosion, and took a long time to get this big. But the details remain a mystery!
Astronomers have just discovered that one ORC is emitting X-rays. So at least in that one ORC, the diffuse gas must be hot: about 8 million Celsius.
Here are some theories of what an ORC might be:
• it's a spherical shock wave from a cataclysmic event in the host galaxy, such as a merger of two supermassive black holes
• it's the shock wave formed by a 'starburst wind' created by a burst of star formation in the host galaxy
• it's the jet produced by a supermassive black hole, seen head on.
• it's a wormhole
As often the case, the most exciting theory is the least likely unless the others get ruled out.
But we can still have some fun. So watch an animated gif of what the birth of an ORC might look like!
Here's a possible scenario for the formation of an ORC: some sort of explosion in the host galaxy, producing a shock wave that expanded outwards for a billion years.
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