Finally, a black hole Goldilocks would appreciate. Most
black holes are either incredibly massive or amazingly compact and lightweight,
but now scientists have discovered a medium-sized black hole that's just
right.
The finding is the most solid evidence yet for a long-sought-after
class of intermediate-sized black holes, researchers said. These middleweights,
at about 500 times the mass of the sun, could represent a missing link between
common stellar black holes, created by the death of a single star, and the
supermassive variety that can pack the mass of millions or even billions of
suns.
The discovery is an object on the outskirts of the ESO
243-49 galaxy, about 290 million light-years away. Astronomers detected a
strong source of X-ray light, without a counterpart in optical light. These
characteristics make the object much more likely to be a black hole than a
foreground star or a background galaxy, researchers said.
"It's very difficult for us to say definitively 100
percent this is what it is," said study leader Sean Farrell, an astronomer
at the University of Leicester in England. "But certainly it's the
strongest candidate that we've seen so far."
Black holes are objects so dense that once captured, even
light cannot escape their gravitational draw. Since they reflect no light, they
are impossible to see directly. Astronomers can detect them through the strong
radiation released by the whirlpooling disks of matter that fall into them,
though.
In this particular case, the researchers think the X-rays
they observed are being emitted as friction heats up gas and dust in the disk.
The discovery, made with the European Space Agency¹s XMM-Newton X-ray space
telescope, is detailed in the July 1 issue of the journal Nature.
Black holes in the smaller class form when a single massive
star dies and gravity forces some of its matter to collapse into itself.
Scientists aren't sure about the origins of supermassive
black holes, which populate the centers of many large galaxies. But one
hypothesis suggests these behemoths are created when smaller black holes in
dense environments, such as globular
clusters, collide and merge.
"This is where our result comes into play in
significance," Farrell told SPACE.com. "If we can show that
black holes between the two mass limits do exist then that gives us a natural
progression of how something can go from stellar to supermassive in size."
These intermediate black holes are relatively rare because
in most environments, such as a normal galaxy, stars aren't packed in tightly
enough to cause frequent collisions, Farrell said.
The newly-discovered intermediate black hole candidate was
seen on the edge of a galaxy, where a globular cluster could be expected to
reside. Because the galaxy is so far away, the cluster itself wouldn't be
visible, but a middle-sized black hole's X-ray radiation could reach us.
Previous studies have found other medium-sized black
hole candidates, but these are more ambiguous because they are fainter in
X-ray light, Farrell said.
"There have been a number of theories to explain high
[X-ray] luminosities without needing intermediate mass black holes," he
said. "Where our paper stands out is that we've got a detection of an
ultra-luminous source that is 10 times brighter than the previous record
holder. It gets increasingly difficult to explain this higher luminosity."
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