A monster black hole rotates at roughly half the speed of light, astronomers reported on Wednesday, twisting space as it turns. (Read "Star Eater" in National Geographic magazine.)
Most galaxies, including our own spiral Milky Way, possess such monster black holes. Located at the center of a quasar galaxy some 6.1 billion light-years away, the jumbo black hole in the study, which is about 200 million times as massive as our sun, is the most distant one with its spin revealed to astronomers.
And the study quasar is at least four times farther away than any other galaxy whose central black hole spin has been determined, notes astrophysicist Emanuele Berti of the University of Mississippi at Oxford. That puts it closer to the era of the earliest galaxies more than 13 billion years ago. (See: "Hubble Reveals Universe's Earliest Galaxies.")
For that reason, the fast-spinning observation points to how galaxies, such as ours, grew larger billions of years ago.
"Information about the formation of a galaxy is contained in the spin of its supermassive black hole," says Berti, who was not on the study team. "That is why this observation is interesting."
In the study, the team measured the distortion of spectral lines from x-rays emitted by the black hole. That distortion reveals the spin rate of the black hole.
The x-rays themselves are emitted from a belt of star stuff circling the black hole at nearly the speed of light, Reynolds says. That material is heated to tremendous temperatures and giving off powerful bursts of energy.
Black Hole Gastronomy
The supermassive black holes at the center of galaxies are thought to grow through mergers of smaller ones, as well as the gravitational accretion of gas, dust, and stars. The black hole at the center of the Milky Way, Sagittarius A*, weighs about four million times as much as the sun. Some large galaxies have central black holes billions of times heavier than the sun.
But they all started out smaller. Whether the growth of the black hole was smooth or bumpy tells us something about how galaxies grew larger around the supermassive black holes, say Berti and other astrophysicists.
"If the growth is chaotic, then the spin of the black hole heads toward zero," Berti says. In that case, the black hole acts like a top given random kicks from side to side, which rob it of energy to spin.
If instead, the structure of a growing galaxy is orderly, with vast clouds of hydrogen gas (and the occasional smaller black hole) flowing into its center to feed the central black hole steadily, its spin will increase.
"We're seeing a very rapid spin, which points to a more coherent structure to early galaxies during their formation," Reynolds says. How our own galaxy and stars like our sun formed might be better explained by understanding this formation.
Quasar galaxies such as the one in the study (dubbed RXJ113212 1231) are some of the most powerful sources of energy in the universe, their central black holes emitting powerful jets that shine like beacons in astronomical surveys.
The astronomers caught a lucky break to view the x-ray emissions from the quasar with NASA's Chandra x-ray telescope, Reynolds says, because it required a gravitational "lens" to view.
"The alignment of the quasar with the gravitational lens had to be just right to see it," Berti says. The gravity of closer galaxies actually bends the light from the quasar, focusing it to make it viewable from Earth—the so-called gravitational lens effect.
The team hopes to hunt out more such quasars, including ones even farther away, and divine the spin of their central black holes. In that way, an understanding of the formation conditions of early galaxies might come into focus.
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