Astronomers have spied a small black hole nestled in the middle of a packed star cluster, a region not typically thought to be very black hole-friendly.
About 10 times more massive than our Sun, the black hole was found inside a globular cluster in the elliptical galaxy NGC 4472, located some 50 million light-years away from Earth in the Virgo Cluster. A black hole is an object whose mass is so concentrated that nothing, not even light, can escape the strong pull of its gravity from within a certain distance.
The discovery, detailed in the Jan. 4 issue of the journal Nature, is surprising because some theories predict that gravitational interactions among black holes inside a cluster would simply kick most or all of the black holes out of the cluster.
Scientists think the ejection process works like this: because black holes are usually among the most massive objects in a globular cluster, they sink to the cluster's center. There, they typically pair up with either a star or with another black hole that has also happened to meander into the cluster's middle. A binary system made up of a black hole and a star can be stable, but when two black holes pair up, strong gravity interactions between them are thought to give one or both the boot.
The fact that NGC 4472's black hole has resisted ejection suggests it is paired up with a star, and not another black hole, the researchers say.
"If you look at the numerical simulations that say it's hard to keep black holes in clusters, what they generally say is that the ejection happens because of binary black holes," said study leader Thomas Maccarone, an astronomer at the University of Southampton in the UK.
There might be another way that black holes can stay put within a cluster. "Some people have predicted that in some cases you might start off with one extremely massive black hole, like 50 times the mass of the Sun. If that happens, the ejection scenarios don't work," Maccarone told SPACE.com. "That black hole may end up merging with all of the smaller black holes in there, until it builds up to something that's 1,000 times the mass of the Sun or more."
At such significant masses, black holes, even solitary ones, would be difficult to eject, Maccarone said.
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