The discovery of a gigantic black hole in a small galaxy has challenged theories about how black holes grow.
The monster black hole, 220 million light years away in galaxy NGC 1277, is the second most massive ever located and as large as our solar system.
It is 4000 times larger than the black hole at the centre of the Milky Way, and has a mass 17 billion times that of our sun, a report in Nature said.
Astronomers using the Hobby-Eberly telescope in Texas found it during a search for the largest black holes in the Universe. Their survey turned up nearly 900 host galaxies, with some of the largest black holes in small galaxies.
That is a surprise because existing models of black hole growth consider black holes evolve in tandem with host galaxies.
Dr Remco van den Bosch, from the Max Planck Institute for Astronomy in Germany, said galaxy NGC 1277 seemed to be very old.
"So somehow this black hole grew very quickly a long time ago, but since then that galaxy has been sitting there not forming any new stars or anything else," he told the BBC.
"We're trying to figure out how this happens, and we don't have an answer for that yet. But that's why it's cool."
"This is a really oddball galaxy," study team member Karl Gebhardt of the University of Texas at Austin said in a statement. "It's almost all black hole. This could be the first object in a new class of galaxy-black hole systems."
The black hole's event horizon - the point at which nothing can escape - has a diameter 11 times greater than Neptune's orbit around the sun.
Gebhardt said the ratio of the black hole's mass to that of its galaxy was unprecedented.
"It is definitely the black hole with the largest ratio of black hole to galaxy mass," Gebhardt told The Huffington Post. "The ratio is about 100 times larger than typical galaxies."
The team of astronomers has five other small-galaxy candidates that, with the help of more data, could disprove the rule that big black holes only happen in big galaxies.
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