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How did supermassive black holes get so big? New data give a clue.

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"That's a huge amount of rotational energy," she says.

Indeed, it represents 84 percent of the maximum spin rate predicted by Einstein's theory of general relativity, adds Guido Risaliti, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who was the lead author of the paper in Nature.

The supermassive black hole's high spin rate provides a direct clue as to how it grew, researchers say.

"We believe that these black holes were born when the universe was only about 10 percent of its current age," said Arvind Parmar, mission manager for the European Space Agency's XMM-Newton orbiting X-ray telescope, during a press briefing Thursday afternoon. Back then, Dr. Parmar says, these objects would have tipped the cosmic scales at 20 or 30 times the sun's mass.

They can grow as galaxies collide and their central black holes merge. If both black holes are spinning in the same direction, the merger would result in a black hole with amped-up spin. Likewise, if the black hole continuously feeds on material in its host galaxy in what's called ordered accretion, the spin would accelerate as well. If feeding is random, however, spin rates would be relatively slow.

Thus, for this black hole, the results imply either constant feeding, a merger, or both, Parmar suggests.

Now that researchers have demonstrated that a supermassive black hole's spin can be measured, the next step is to observe these objects in ever more-distant galaxies that span a large stretch of cosmic time.

"This will allow us to probe the importance of accretion and the importance of mergers in creating the universe we see today," he says.

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