Astronomers Steve Allen of Stanford University and Christopher Reynolds of the University of Maryland along with an international team studied nine older black holes ensconced within relatively nearby galaxies, between 50 million and 400 million light-years away. They picked these particular specimens because they seemed to emit little radiation despite having plenty of fuel--gravitationally captured gas--surrounding them. "They attracted our attention because they were too boring," Reynolds says.
But what these black holes lack in emitted light, they make up for in explosive jets of high-energy particles--traveling at up to 90 percent of the speed of light--that carve cavities in the surrounding disk of gas. By directly observing these bubbles and the total amount of gas available, the scientists were able to determine just how well these galactic engines turn matter into energy. "In technical language, they convert 2.5 percent of the rest-mass energy of the captured gas to jet power," Allen says, or about a trillion-trillion-trillion watts of generation.
This means that all black holes, young or old, are tremendously efficient at churning out energy. "They never seem to lose their efficiency, they just put out the energy in different ways in different phases," Reynolds notes. The finding overturns theoretical models of how such older black holes work and suggests that the high-energy jets may be responsible for continued heating of galactic gas, delaying its cooling and collapse into new stars and limiting the growth of the largest galaxies.
"Though we don't definitely know the mechanism by which these jets are produced, our findings are supportive of the idea that magnetic field lines interact in a way that causes them to work like the elastic bands of a giant sling shot throwing incoming material back out from the black hole," Reynolds adds. The study will be published in Monthly Notices of the Royal Astronomical Society.