UP TO 25 000 black holes are hiding in the heart of our Galaxy, claim two
astronomers in the US. 鈥淭he black holes are buzzing like flies around the
centre,鈥� says Jordi Miralda Escud茅 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. 鈥淭he black holes therefore lose energy and fall to the
centre,鈥� 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. 鈥淲e assumed about a fifth of the stars bigger than eight
times the Sun鈥檚 mass left black holes at the end of their lives,鈥� says Miralda.
鈥淭hat gave us the figure of 25 000.鈥�
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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. 鈥淚 certainly
think the black hole cluster story is interesting,鈥� says Martin Rees of
Cambridge University.
Detecting the black holes may be difficult, however. One possibility is
picking up a burst of gravitational waves鈥� 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鈥檚 LISA, which isn鈥檛 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鈥� 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鈥檚 more, they predict
that any remaining stars should have been kicked into highly elliptical orbits
by black hole encounters.
鈥淭here is a good chance of seeing both these effects,鈥� says Miralda. He and
Gould have submitted a paper to The Astrophysical Journal.