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The Milky Way has thousands of black holes

New Scientist

UP TO 25 000 black holes are hiding in the heart of our Galaxy, claim two astronomers in the US. "The black holes are buzzing like flies around the centre," says Jordi Miralda Escud¿ of Ohio State University.

Although only half a dozen black hole candidates are known to exist in the Milky Way, some astronomers think the Galaxy is swarming with black holes created by exploding stars. Miralda and his Ohio State colleague Andrew Gould say that these holes will cluster in the centre of the Galaxy because they tend to transfer some of their orbital energy to smaller objects every time they have a stellar encounter. "The black holes therefore lose energy and fall to the centre," says Miralda.

The migration is very slow, however. Miralda and Gould calculate that in the 10-billion-year lifetime of our Galaxy, only those holes born within 15 light years of the centre would have had enough time to make it to the middle. To gauge how many have made it, the astronomers estimated how many black hole-spawning supernovae have gone off within this 15-light-year radius over the past 10 billion years. "We assumed about a fifth of the stars bigger than eight times the Sun's mass left black holes at the end of their lives," says Miralda. "That gave us the figure of 25 000."

According to the astronomers, the black hole cluster will be depleted as holes are occasionally gobbled up by the giant black hole thought to dominate the middle of the Milky Way. But it will be replenished as others slowly migrate in. Other galaxies should also have similar black hole clusters in their hearts, they believe.

Experts in the field agree that such gatherings are plausible. "I certainly think the black hole cluster story is interesting," says Martin Rees of Cambridge University.

Detecting the black holes may be difficult, however. One possibility is picking up a burst of gravitational waves-the death cry of a black hole as it swoops close to the central giant hole before being swallowed. However, this will require a space-based gravitational wave detector such as the European Space Agency's LISA, which isn't due to be launched for another 10 years.

But there could be other ways to observe the cluster. The stellar encounters that sap the holes' energy can also boost the orbital energy of stars, throwing them out into the farther reaches of the Galaxy. Therefore Miralda and Gould predict that astronomers should find very few old, low-mass stars in the central region, as most would have been ejected over time. What's more, they predict that any remaining stars should have been kicked into highly elliptical orbits by black hole encounters.

"There is a good chance of seeing both these effects," says Miralda. He and Gould have submitted a paper to The Astrophysical Journal.

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Author: Marcus Chown

New Scientist issue: 8th April 2000

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