By Govert Schilling
ScienceNOW Daily News
9 January 2008
Black holes--regions of space where gravity is so strong that even light can't escape--come in various sizes. The smaller ones are a few times more massive than the sun and form when massive stars burn out and implode. In contrast, supermassive black holes reside in the cores of galaxies and can be up to billions of solar masses. And astronomers have circumstantial evidence for the existence of a third class: intermediate-mass black holes that weigh in at a few hundred to a few thousand suns. They might form within dense, spherical swarms of hundreds of thousands of stars known as globular clusters, although no one is sure how (ScienceNOW, 17 September 2002). If such intermediate-mass black holes do exist, then a new analysis suggests that many of them are spit out by the clusters and end up roaming through space alone, Kelly Holley-Bockelmann of Vanderbilt University in Nashville, Tennessee, reported at the meeting.
Why would a globular cluster eject its intermediate-mass black hole? The answer, according to Holley-Bockelmann and her colleagues, lies in mergers with smaller, stellar-mass black holes that would also form in the cluster and slowly sink to its core. According to general relativity, the merging of the two black holes produces a burst of gravitational waves, which are literally expanding ripples in spacetime itself. If the merger is off-center, or if the two black holes spin at different speeds or in different directions, the gravitational waves emerge asymmetrically. The asymmetric waves would act like a rocket on the combined black hole, flinging it out of the cluster with a velocity of thousands of kilometers per second.
As the Milky Way contains some 200 globular clusters, hundreds of rogue black holes may be crisscrossing our galaxy, says Holley-Bockelmann. Spotting them may be difficult, however. They would be visible only if they encounter interstellar gas clouds to feed on, in which case they would heat some of the gas and cause it to emit x-rays. Alternatively, says Holley-Bockelmann, a roaming hole might reveal itself if it passes in front of a more distant star. The black hole's gravity would then distort the image of the background star in telltale ways in a process called gravitational lensing (ScienceNOW, 6 October 2006).
"Learning more about intermediate-mass black holes and their possible fates is very interesting" because so little is known about them, says theoretical astrophysicist J. Craig Wheeler of the University of Texas, Austin. He notes that many other processes in the Milky Way may also send smaller black holes drifting through space: "There might be 100 million stellar-mass black holes out there."
So should we worry about a hefty intermediate-mass black hole coming our way? No, says Holley-Bockelmann, there are too few of them. "The most dangerous effect I can think of is a rogue black hole stirring up our solar system's Oort cloud of comets, which would send huge numbers of comets on a potential collision course with Earth. But the chance of that happening is only one in a quadrillion or so. There's no need to build black-hole shelters."
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