Put it away: Dwarf's 'supermassive' marvel is actually smallest thing boffins have ever seen

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At the heart of every large galaxy resides a supermassive black hole, and astroboffins have found the smallest one yet – about 340 million light years away.

"In a sense, it's a teeny supermassive black hole," said Elena Gallo, assistant professor of astronomy at the University of Michigan College of Literature, Science, and the Arts.

The mini-hole sits at the center of galaxy RGG 118, and is about 50,000 times the mass of our Sun, making it half the size of any similar body observed in space. By contrast, the black hole at the center of our galaxy is nearly 100 times as large, and the biggest black hole yet discovered is 200,000 times more massive.

(El Reg calculates the Schwarzschild radius of the tiny black hole to be just 147km, which means the black hole must be less than 294km in diameter.)

That mass estimate comes from watching the visible motion of gas within RGG 118 and correlating it against the X-ray brightness of hot gas swirling toward the black hole.

In doing so they found the outward push of radiation pressure of this hot gas is about 1 per cent of the black hole's inward pull of gravity, which ties in with what we know about other supermassive black holes.

"It might sound contradictory, but finding such a small, large black hole is very important," said Vivienne Baldassare of the University of Michigan, who authored the paper in The Astrophysical Journal Letters. "We can use observations of the lightest supermassive black holes to better understand how black holes of different sizes grow."

As far as we know, black holes come in two categories – supermassives and stellar mass varieties. The latter type probably forms when large stars fizzle out and collapse in on themselves. But supermassives are more of a mystery.

Some posit that these universal giants are formed when super-massive stars collapse, while others suspect they are formed when stellar counterparts merge and form into a larger rip in the fabric of space.

"We have two main ideas for how these supermassive black holes are born," Gallo said. "This black hole in RGG 118 is serving as a proxy for those in the very early universe, and ultimately may help us decide which of the two is right." ®

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