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Middleweight black hole shows its jets

Black holes Evidence of hot jets of plasma have confirmed the presence of a middleweight black hole, boosting one theory about how much larger black holes at the centre of galaxies form.

Astrophysicist Dr Sean Farrell, from the University of Sydney, and colleagues, report their findings today in the journal Science.

"The really far-reaching consequences of finding and confirming this intermediate-black hole is that it gives us a possibility for how we can form the supermassive black holes in the middle of galaxies," says Farrell.

Black holes are regions of space that are so dense that nothing, not even light, can escape.

Scientists believe black holes that are about 3 to 20 times the mass of our Sun form when a massive star collapses at the end of its life. These stellar-mass black holes generally occur in binary star systems in our own galaxy.

But scientists are less certain about how black holes that are millions to billions of times the mass of our Sun form. Such black holes are believed to be at the centre of most galaxies.

Merger theory

One theory is that these supermassive black holes are created by the merging of smaller black holes.

If this process occurs, then one would expect building blocks in the form of black holes whose mass is between that of the stellar-sized and the supermassive black holes (about 100 to a 100,000 times the mass of our Sun).

But to date, says Farrell, this theory has been hotly debated, largely because the evidence for such intermediate-mass black holes has been weak.

Previous research by Farrell and colleagues discovered x-ray evidence of a possible middleweight black hole positioned above the disc of a galaxy called ESO 243-49, nearly 300 million light years away.

Now, they have confirmed this is indeed an intermediate-mass black hole using radio astronomy.

They have detected outflows of hot plasma coming from both above and below the black hole.

Farrell says when gas falls into a black hole it swirls around at first to form a disc, much like water going down a plug hole and magnetic fields in the disc get all twisted up.

When these fields break and recombine with other magnetic fields, this creates an enormous pressure away from the black hole and results in the thin compact streams of really hot gas.

"We think they're launched relatively close to the black hole but not from beyond the boundary where light can't escape," says Farrell.

"We don't really know how they form but we know they are a common property of black holes."

Black hole continuum

The first ever discovery of jets of plasma from an intermediate-mass black hole suggest there is a continuum in black holes from stellar-sized to supermasisve and they all basically behave the same way, lending weight to the 'merger' theory.

"It shows that the theory of supermassive blackholes forming through the merger of lower-mass black holes is actually viable," says Farrell.

Just as there is a relationship between the brightness of x-rays and the mass of a black hole, there is also a relationship between the brightness of radio waves from the jets and the mass of the black hole.

"This gives us a ruler for measuring the mass of the black hole," says Farrell.

"And the mass that we get from this is consistent with it being intermediate mass so that's a really nice confirmation. We think this one weighs 10,000 to 20,000 times the mass of the Sun."

The researchers used the Australia Telescope Compact Array, which is the only radio astronomy facility capable of viewing the black hole, which is only visible from the southern hemisphere.

"It's the only instrument on Earth that we could have used to do what we have done," says Farrell.