Monday, September 15, 2008

When black holes go on diets

Perhaps black holes can only get so big, and no bigger.

But so big, is really, really big.

(Image: Ultra-massive black holes like the one shown seem to have a limit in the mass they can attain—but an impressive limit. (Credit: NASA, ESA, and The Hubble Heritage Team.)

The maximum black hole size seems to be around ten billion times the mass of our sun. In another way of looking at it, perhaps black holes can get no bigger than one percent of the mass of the galaxies they inhabit.

Astronomer physicists from the University of Hawai'i and Yale suggest that while they may not technically go on diets, black holes at least stop increasing their size once they become so grotesquely large.

Black holes were once in the realm of science fiction, but they're now accepted in physics. They are intensely powerful features whose gravity is so great that it sucks in everything nearby. Black holes grow in size like people do—by eating.

But there seems to be a point at which they simply can't do it any more. It seems that when black holes approach a mass of 10 billion times that of our sun, they stop, according to Ezequiel Treister, a Chandra postdoctoral fellow at the University of Hawai'i's Institute for Astronomy and Priyamvada Natarajan, an associate professor of astronomy and physics at Yale University.

Their study was published in Monthly Notices of the Royal Astronomical Society (MNRAS).

The scientists used both optical and X-ray studies of black holes to show that, at some point, black holes continue to exist, but stop growing.

It's not clear why. Natarajan suggests that the radiation from the supermassive black hole is so intense that it destroys its own food sources.

No matter where you look in the universe, their appetite seems to abate.

"They shut off at every epoch in the universe," Natarajan said.

When you look out into the universe, you also look back in time. The farther away things are, the longer it's been since what you're looking at actually happened. The researchers found that there seemed to be limits in black hole size, no matter how long ago they existed—no matter how far back in time you look.

“The only way to match the observations at early times with late times is by having black holes stunt their own growth,” Treister said in a prepared statement.

© 2008 Jan W. TenBruggencate

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