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Do intermediate-mass black holes exist?

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Searching for the seeds of supermassive black holes

The imbh project scoured the night sky for evidence of white dwarfs being nibbled away by diminutive black holes.

Space

The universe is filled with black holes millions of times the mass of our sun, yet nobody has yet spotted a young black hole in the process of growing to this enormous size. Finding these intermediate-mass black holes (IMBHs) was the objective of the EU-funded imbh project. “We can see supermassive black holes from very far out, and we know they appear very early in the universe,” says project coordinator Peter Jonker. “But it’s difficult to understand how those formed so rapidly, when the universe started out as a homogenous soup. You can start with a small black hole, 10 times the mass of the sun, and it then has to grow to a billion times the mass of the sun in only a few hundred million years.”

Missing pieces

There is a lot of circumstantial evidence that an intermediate step must exist, but no hard proof, says Jonker. To find it, he and his colleagues at the Netherlands Foundation of Scientific Research Institutes searched for dynamical evidence of IMBHs in data gathered by the European Space Agency’s Gaia satellite. They were assisted by a curious property related to how white dwarf stars are predicted to interact with IMBHs. White dwarfs are the remnants of stars similar in size to our own sun, which have blown away their outer layers, leaving a glowing helium core the size of the Earth. When they approach a supermassive black hole, these stars typically vanish behind the event horizon before they are torn apart by gravitational forces. That’s not the case for IMBHs. “It’s slightly counterintuitive, that the lighter the black hole is, the more easily it can tear stars apart,” adds Jonker. A white dwarf is a compact star, with strong gravity. Only IMBHs can rip apart a white dwarf before it disappears behind the event horizon. Astronomers often see stars of all varieties in the process of being consumed by black holes. The light emitted during this process can give clues as to the constituent elements of the star: if it is made of helium, astronomers can be confident they are looking at the death of a white dwarf.

Fast food

The process is incredibly quick, unfolding over a matter of days, yet the team were able to spot several possible examples. “So far we are still not certain we saw such events; there are good candidates, but these have good alternative explanations we can’t rule out yet,” he explains. The work was supported by the European Research Council. “This helped tremendously,” says Jonker. “Without that funding it would not have been possible.” Jonker and his team continue to search for evidence of IMBHs, using both satellite imagery and recordings of gravitational waves. This, he says, is a necessary step to learning how supermassive black holes like the one at the centre of the Milky Way form – and how they grow so fast.

Keywords

imbh, intermediate, mass, black, hole, white, dwarf, supermassive, soup, helium

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