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Many black holes University of Hawaii astronomers and colleagues have discovered that supermassive black holes produce as much energy as all stars combined -- and they're still growing.
formed later
Space 'magnets' still growing,
research showsBy Helen Altonn
Star-BulletinReporting the findings today at a Texas symposium, UH Institute for Astronomy researcher Amy Barger said many supermassive black holes formed recently, rather than during the explosive early stages of galaxy formation as believed earlier.
"It's kind of curious. It wasn't what I expected," said Institute for Astronomy astronomer Lennox Cowie, who participated in the research.
Barger, with the institute and the University of Wisconsin, led the study. Other team members were Richard Mushotzky of Goddard Space Flight Center and Eric Richards of Marshall Space Flight Center."At least 15 percent of supermassive black holes have formed since the universe was half its present age," Barger said in presenting the results at the 20th Texas Symposium on Relativistic Astrophysics in Austin.
"This challenges the widely held view, based on the relationship between the sizes of black holes and their host galaxies, that the black holes formed when the galaxies formed," she said.
"Instead, it seems that the black holes are still growing at the present time."
Cowie and Barger have been working with data from the orbiting Chandra X-ray Observatory, which provided the first technology to detect the origin of the high-energy X-ray background.
The astronomers used Chandra to find distant objects, then obtained the first multi-wavelength observations to tell what is going on in the galaxies in the X-ray background.
In January, they found "hidden galaxies" that have cores that shine brightly in X-rays but not in visible light.
Supermassive black holes are believed created from collapsed gas clouds. They contain millions to billions of stars confined to a region no larger than the Earth's solar system.
Astronomers now believe there's a supermassive black hole at the core of most galaxies, including the Earth's.
Black holes are considered "active" when they are swallowing great quantities of matter. Heated to millions of degrees under the force of gravity, the matter shines brightly in X-ray light, as well as in other wave bands.
After finding distant galaxies with active black holes, Barger has been measuring all their properties to determine how much energy is being emitted, Cowie said.
The astronomers used the Keck 10-meter telescope for optical observations and the James Clerk Maxwell telescope for submillimeter observations. Both are on Mauna Kea.
They also obtained radio observations with the Very Large Array of the National Radio Observatories.
The radio observations, taken in New Mexico, provided the deepest images of the sky ever taken and allowed the scientists to look at long wavelengths, Cowie said.
The team was able to map the energy produced by supermassive black holes throughout the history of the universe and determine when they formed.
"We looked at all the wavelengths we could find," Cowie said. "It's a remarkable set of data.
"It's like archaeology," he said, explaining as much information as possible was needed to unravel the history of the massive black holes.
"We need to understand the whole spectrum of energy actually coming out from an object to decide how much energy is released."
The astronomers found that 10 percent of the black holes are active at any one time, indicating their growth is a slow, ongoing process with the galaxies spending more than a billion years in creating black holes at their centers.
The time intervals for black hole growth are much longer than would be expected if the black holes were formed in violent mergers of galaxies as has been speculated, the researchers said.
Cowie said a tight correlation found between the mass of a black hole and a galaxy by former UH astronomer John Kormendy, now at the University of Texas, "kind of leads you to think intuitively that they must have both formed at the same time and the two properties are a function of local condition."
But the new observations show many supermassive black holes are formed later and they're fed by properties that aren't controlled by the galaxies, he said.
A massive black hole accretes material in its vicinity that produces the power for the radiation seen from quasars or other very energetic events in the center of the galaxy, he said.
"It's just gravitational energy going into the black hole that's being radiated. That feeding mechanism, if it continues, can continue to grow the object."
Barger has received two elite fellowships to study the distant universe, in the field of extragalactic astronomy.
She is one of 10 people in the world to receive a Hubble Space Telescope fellowship and one of five chosen for a Chandra X-ray Observatory fellowship.
Ka Leo O Hawaii
University of Hawaii
