SCIENCE -- December 5, 2011 at 2:50 PM ET
Researchers Discover Monstrous Supermassive Black Holes
Researchers have discovered a monster black hole that appears to be the most massive found to date, as massive as 21 billion suns. This is one of two black holes found in elliptical galaxies some 300 million light years away -- both believed to be the biggest yet.
A black hole occurs when gravity causes an object to collapse under its own weight to a point, creating an object that is fantastically small, yet tremendously dense. They are voracious eaters, sucking in everything they can absorb, and once formed, the gravitational pull is so strong that nothing -- not even light -- can escape. This region where nothing can escape - the point of no return - is known as the "event horizon."
These black holes have event horizons as large as five times the size of our solar system.
"That tells you how scary it is - how big that region is in space from which nothing can get out," said Chung-Pei Ma, a professor of astronomy at University of California, Berkeley, and one of the authors of the study, which was published online Monday in the journal Nature.
The biggest of the two black holes weighs as much as 21 billion suns, though its data contains more uncertainties than the littler one, which weighed in at about 10 billion suns, said Nicholas McConnell, a U.C. Berkeley graduate student and the study's first author. To put the sheer size of these in perspective, the previous record holder, found 33 years ago in the galaxy Messier 87, has a mass of about 6 billion suns.
The mass of a black hole tends to correspond to the mass of its galaxy. Generally, the bigger the galaxy, the bigger the black hole at its center, a sign that the formation and evolution of a black hole and parent galaxy are intimately related, Ma said.
But these black holes are bigger than they should be given the size of their host galaxies. Ma said that black holes from neighboring elliptical galaxies most likely fused together when their galaxies merged. And they may have gained additional mass by sucking in surrounding gas.
That something different is occurring with these giants warrants further study, said Kevin Schawinski, a Yale university astrophysicist. "We actually think that this link between galaxies and the black holes that live at their center is a really fundamental relationship. And the fact that we now see in the most massive galaxies hints of a departure - that's interesting."
Scientists calculate the mass of black holes by studying the matter swirling around them. In this case, researchers used state-of-the-art spectrographs on Hawaii's Gemini North and Keck 2 telescopes to determine the speed and motion of stars orbiting around the black holes. The more massive the black hole, the faster objects will orbit around it.
McConnell compared the technique to a police officer using a radar gun to determine how fast a car is speeding down the highway. While the radar reflects off the car back to the police officer, the starlight intrinsically moves toward us. But it's light from a source moving toward a detector. "Within that light, the wavelengths give you information about the speed at which the object is moving," he said.
A next step, Ma said, will be to simulate how these galaxies and their supermassive black holes grow with time by modeling on supercomputers how these holes merge and feed.
Ma's team had been analyzing data from the past four years when McConnell alerted her to numbers indicating a potential monster black hole, she recalled.
"When you see a number like that, your first reaction is, 'Ooh, could that number be wrong?'" she said. "But the number didn't want to go away. It was pretty consistent; the statistics really preferred a big black hole. That was the information the stars were telling us: We're orbiting a big, black hole that you can't see."
The stars measured were a few hundred light years away from the black hole -- not close enough to make the job easy, McConnell said.
"But it is close enough that we're getting a signature that can't be explained only from dark matter or only from other stars. We'd love to get closer, to make the uncertainties of our measurements go down. And we've measured starlight close enough to definitely say that we've discovered something very massive."
This post has been updated since its original version.
Photo credit: An artist's rendition of stars moving in the central regions of a giant elliptical galaxy that harbors a supermassive black hole. Image by Gemini Observatory/AURA artwork by Lynette Cook