Universe's 'greenest' systems are black holes

By Aditi Risbud

It might not help at the gas pump, but a new finding suggests the most efficient "engine" in our universe resides in the distorted depths of space. Using data from NASA's X-ray observatory Chandra, a team of researchers has made the first direct estimate of the "greenness," or efficiency, of black holes. This work helps illuminate how black holes generate enormous energy flows and influence their environment.

"It's critical to know the fuel efficiency of black holes," says Assistant Professor Steve Allen of the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University and the Stanford Linear Accelerator Center. "Without this information, we cannot figure out what is going on under the hood, so to speak."

Allen announced the results at a NASA press conference on Monday with team member Christopher Reynolds of the University of Maryland-College Park. Allen says the efficiency of a black hole is 25 times greater than that of a nuclear power plant.

"This is an amazing process of efficiency," he says. Allen will publish the findings in an upcoming issue of the Monthly Notices of the Royal Astronomical Society, with co-authors Reynolds, Gregory Taylor of the University of New Mexico, and Robert Dunn and Andrew Fabian of the University of Cambridge, U.K.

"If a car was as fuel efficient as these black holes, it could theoretically travel over a billion miles on a gallon of gas," Reynolds says.

Supermassive black holes sit at the centers of galaxies and are a billion times the mass of our sun. Studying these behemoths, Allen and his colleagues found that matterin the form of hot gases at a temperature of tens of millions of degreesgets sucked toward the black hole. The gases swirl to form a disk that spins faster and gets hotter as the matter approaches the "point of no return," or event horizon. But black holes are "messy eaters," and some of what's sucked in gets spewed out. Most of the energy released from matter sucked in toward the black hole shoots out in the opposite direction, forming jets of high-energy particles traveling at nearly the speed of light.

The Chandra data also implies a significant fraction of the gas first attracted to the black holes is then used with great efficiency to power the particle-spewing jets. The jets then blow cavities into the surrounding environment, not only transferring energy to the gases in the galaxy but keeping these gases from cooling and forming stars. This process prevents the formation of billions of stars, says Allen, thereby regulating the growth of galaxies.

"There's been a big puzzle in cosmology to understand why the biggest galaxies in the universe are only as big as they are and not bigger," says Allen, whose study was funded by the Department of Energy's Office of Science. "This is an important stepa piece of the puzzle."

Aditi Risbud is a science-writing intern at Stanford News Service.