Two merging black holes can generate gravitational waves so powerful that the merged hole shoots out of its host galaxy at a speed of up to 2,500 miles per second, according to a new simulation.
This research, led by Manuela Campanelli at the Rochester Institute of Technology, demonstrates for the first time that the violent recoil that follows a merger is capable of ejecting the supermassive black holes known to lie at the heart of most light-emitting galaxies. These black holes may be cruising through the universe, virtually undetectable unless they should crash into something and gain matter.
The study found the optimal conditions for producing recoil speeds high enough to free a supermassive black hole from its host galaxy. In this scenario, the two black holes orbit around one another. They have equal masses and spin at the highest possible rate. They must be tilted onto their sides, with their axes of rotation lying in the plane of their orbit, and they must spin in opposite directions. They spiral toward one another, and when they merge, they are kicked in a direction perpendicular to the orbital plane.
Some astrophysicists have argued that such conditions are rather unlikely. The probability that black hole ejection will occur remains an open question for future research. Even if supermassive black holes have been removed from galactic cores, the odds that one of them will streak through our solar system are small enough that we need not fear a sudden obliteration.
A second study, conducted by Abraham Loeb of Harvard University, examines the possibility of detecting a black hole that has been kicked by gravitational recoil. If the black hole is surrounded by a ring of gas, it will emit light and resemble a star-like object known as a quasar.
A quasar exists when the supermassive black hole at the center of a galaxy rapidly acquires gas. As a result, the gas near the black hole heats up and radiates several times as much energy as the Milky Way. A quasar that is displaced from galactic core may well be a kicked black hole. Unfortunately, it would require a real stroke of luck to catch one in action - the gas fueling the light would only last about ten million years, so an ejected black hole would be dark by the time it left its galaxy.
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