Black Holes in Quasars Spin Fast

By Ken Croswell

April 9, 2006

3C 273, the brightest quasar in Earth's sky, has a redshift of 0.158 and a distance of 2 billion light-years. Image by the Hubble Space Telescope. NASA and John Bahcall.

The black holes that power quasars rotate rapidly, say astronomers in China, California, and Scotland. In fact, the fast spins make quasars emit much more light than they would if their black holes weren't spinning at all.

Quasars are the most luminous objects in the universe, pouring out hundreds of times more radiation than the Milky Way--but from a region that's smaller than our solar system. Ironically, a quasar owes its brilliance to what may seem its opposite: a black hole millions or billions of times more massive than the Sun. The black hole sits at the quasar's center and swallows gas, stars, and planets.

Before taking the final plunge, this material orbits the black hole in an accretion disk, like water spiraling around a drain. As this material falls, its gravitational potential energy gets converted into orbital energy and extreme heat. The high heat causes the material to shine so brightly that astronomers can see it across billions of light-years of space. Then the glowing material plummets into the black hole and out of sight.

Now Jian-Min Wang and Yan-Mei Chen of the Key Laboratory for Particle Astrophysics in Beijing, China; Luis Ho of the Carnegie Observatories in Pasadena, California; and Ross McLure of the University of Edinburgh in Scotland have analyzed thousands of quasars from the Sloan Digital Sky Survey. With redshifts between 0.4 and 2.1, the quasars are located 4 billion to 10 billion light-years from Earth.

Wang's team compared the quasars' luminosities with the masses of the black holes powering them. Other astronomers had derived the black hole masses by examining the quasars' spectra. In particular, the broader the spectral lines due to hydrogen and magnesium, the more mass the quasar's central black hole has.

To determine how fast the black holes spin, Wang and colleagues exploited a basic black hole property: the faster an accreting black hole spins, the brighter its accretion disk. This result follows from the general theory of relativity, which says that objects can orbit up to six times closer to a spinning black hole than they can to a nonspinning black hole of the same mass. Thus, the faster a black hole spins, the closer the accretion disk can get to the black hole. The innermost part of the accretion disk is the hottest, so it accounts for most of the quasar's luminosity--and shines brightest when the black hole spins fastest.

To anyone who knows Einstein's famous equation E = mc2, the formula for the luminosity of an accreting black hole will look familiar: L = ηMc2, where M is the amount of mass the black hole is swallowing and c is the speed of light. The Greek letter out front, η (eta), indicates how efficiently the black hole converts the infalling matter into energy. If the black hole does not spin, then η is only 5.7 percent. However, the faster the black hole spins, the greater η is. For the fastest possible spin, η is 42 percent, which means that nearly half the infalling mass gets transformed into energy. Thus, a black hole can change matter into far more energy than a normal star can. For example, the Sun's nuclear reactions will convert less than 1 percent of its mass into energy during its life.

By analyzing the large sample of quasars, Wang and colleagues conclude that quasars are quite efficient at converting mass into energy. In fact, the astronomers put the quasars' efficiency between 30 and 35 percent, close to the maximum of 42 percent, thus "strongly suggesting that their black holes are rotating very fast." The astronomers will publish their study in a future issue of Astrophysical Journal Letters.

"This is very interesting work," says Ramesh Narayan of the Harvard-Smithsonian Center for Astrophysics, who is an expert on black hole accretion. "It's quite an interesting result, especially when the efficiency is this large--30 or 35 percent. That's uncomfortably large." Such a high efficiency implies that the black holes rotate at more than 99 percent of the maximum allowed rate.

Wang and colleagues concede that they may have overestimated the black holes' efficiencies, because the black holes may be twice as massive as astronomers had previously calculated. If so, the efficiency would be around 20 percent.

"Frankly," comments Narayan, "20 percent is still extremely interesting--in fact, it's probably more interesting than 35 percent, in the sense I would probably believe it more. You have to be spinning rather close to the maximum speed allowed for a black hole to get an efficiency even of 20 percent." This efficiency means the black hole rotates at 96 percent of the limit.

In recent years, astronomers have found other signs that quasar black holes spin fast. In 2002, Martin Elvis of the Harvard-Smithsonian Center for Astrophysics and his colleagues published work that claimed quasars make a substantial contribution to the universe's x-ray background. The quasars' large contribution implied an efficiency of at least 15 percent, which meant their black holes must spin rapidly.

The rapid spins help reveal how quasar black holes grew so massive. In particular, says Wang's team, the black holes probably acquired most of their mass by accreting gas as opposed to swallowing other black holes. Theoretical studies show that accretion of gas makes black holes spin faster. In contrast, when two black holes merge into one, the final black hole may end up spinning more slowly than either of the original black holes.

Closer to home, the massive black hole at the center of the Milky Way also seems to rotate fast. Although the Milky Way is not a quasar, its heart harbors a black hole with several million solar masses. By detecting infrared emission near this black hole's event horizon--the point of no return for infalling matter--Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and his colleagues concluded that the Milky Way's central black hole spins at about half the maximum possible rate.

Ken Croswell earned his Ph.D. in astronomy from Harvard University for his study of the Milky Way Galaxy. He is the author of books about the Milky Way, The Alchemy of the Heavens, and cosmology, The Universe at Midnight, as well as Magnificent Universe.

"Ken Croswell's The Alchemy of the Heavens is one of the very best popular astronomy books in decades."--Keay Davidson, San Francisco Examiner. See all reviews of The Alchemy of the Heavens here.

"The Universe at Midnight is vastly entertaining and enjoyable, as well as informative."--Sir Patrick Moore, New Scientist. See all reviews of The Universe at Midnight here.

"Magnificent Universe by Ken Croswell is elegant and eloquent."--Kathy Sawyer, Washington Post. See all reviews of Magnificent Universe here.