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Tuesday March 13 04:30 PM EST
Black Holes Abundant, Varied in Early Universe, Detailed Study Shows (the universe of the very large)
By Robert Roy Britt
Senior Science Writer, SPACE.com
The deepest and most detailed look ever made of the distant cosmos has revealed that black holes were thick as thieves in the early universe, and that they behaved in more varied ways than researchers expected.
At a press conference Tuesday at NASA Headquarters, researchers said they had spotted the most distant quasar ever seen -- a discovery that may precede a slew of such findings in coming months and is expected to help illuminate the chemical makeup of the early universe.
Combining the power of the orbiting Hubble and Chandra observatories and other ground-based telescopes, two groups of researchers surveyed two patches of sky and combined visible, infrared and X-ray images to provide what they say is proof that black holes of all sizes ruled the early universe.
The research found evidence that as many as 200 million supermassive black holes and even more small, star-sized black holes existed in the early universe, which is thought to be between 12 billion and 15 billion years old.
Scientists have grown to suspect that black holes exist at the centers of most or all galaxies, but it is not known when they developed or whether they have increased or decreased in number or intensity since then. The new study "is the deepest and most comprehensive look we've had at the distribution of black holes and the variety of behavior in them," said Harvey Tananbaum, director of the Chandra X-ray Center.
The two telescopes surveyed a pair of relatively small patches of sky, one in the Northern Hemisphere and one to the south. Chandra's southern view covered an area about the size of the full Moon as it appears from Earth. But each set of observations "goes very deep in that patch," Tananbaum said in a telephone interview. "Presumably what we're seeing in these two patches is typical of the rest of the sky."
The studies found black holes that are up to 12 billion light-years away, which means the light and X-ray energy researchers studied was emitted 12 billion years ago, when the universe was very young.
"The Chandra data show us that giant black holes were much more active in the past than at present," said Riccardo Giacconi, a Johns Hopkins University researcher who led the one of the study.
Giacconi said the patch of sky his group studied found some 350 supermassive black holes -- at a rate of about one per day. Extrapolated to the whole sky, he said this would amount to 200 million of the objects throughout the early universe.
Black holes everywhere
Black holes are massive objects that cannot be seen. Scientists theorize their existence based on the effects they have on their surroundings. The most notable effect is gravity. A black hole's tremendous mass is concentrated in an infinitely small space, and they exert incredible gravity on surrounding matter and energy. Once in the grip of a black hole, nothing can escape, not even light.
Supermassive black holes at the centers of galaxies are often actively gobbling stars and gas in mass quantities. Tremendous energy is created as this matter swirls inward and approaches the speed of light. The colossal friction generates the X-ray emissions that researchers use to identify black holes.
Other black holes, like the one at the center of our Milky Way Galaxy, are relatively quiescent and in some cases do not emit significant amounts of X-rays.
By combining the images, researchers figured out what sorts of galaxies emitted X-rays most intensely, indicating that their black holes were active. "It's often not the biggest and the brightest, the nearest, or the ones with the most stars" that emit the most X-rays, Tananbaum said. "It's sometimes a rather faint, nondescript [galaxy] that may be sitting not too far from much brighter, otherwise more spectacular looking galaxy."
Next page: Revealing more than just black holes
Capping four decades of research
For Riccardo Giacconi, who led the Chandra study, the new findings cap a four-decade quest for answers.
In 1962, Giacconi and others realized that no matter where one points a telescope, X-rays filled the skies above Earth's atmosphere. Back then, instruments could not resolve whether these X-rays permeated the universe evenly or if they came from individual sources. As data rolled in, the case grew for the latter in the 1980s and 90s.
The new findings are the strongest confirmation yet that, in fact, X-rays from distant sources are generated by individual galaxies presumed to have supermassive black holes at their centers.
"For me these observations were the fulfillment of a dream," said Giacconi, who researchers called "the Granddaddy" of the effort.
The new studies accounted for the black holes responsible for between 70 and 90 percent of the X-rays coming from the two patches of sky. But these are the brighter sources, and there must be far more sources as yet unaccounted for, "galaxies just like our own that are relatively feeble X-ray sources," Tananbaum said. "It may be that 99 percent of the galaxies make up the last 2 percent" of X-ray emissions.
More than just black holes: A quasar
Another discovery in the Chandra data was an ancient "type II quasar," by far the most distant ever found, that was shrouded in gas and dust. Tananbaum said the researchers found high concentrations of iron in the quasar, which provides a glimpse of the chemical makeup of the early universe.
"The discovery of this object, some 12 billion light-years away, is a key to understanding how dense clouds of gas form galaxies, with massive black holes at their centers," said Colin Norman of Johns Hopkins, who worked with Giacconi and others on the study.
Quasars are little-understood objects that "shine" bright in X-rays, sometimes with more power than 1,000 galaxies. When researchers first detected their radio emissions, they thought they were stars. Their name is short for quasi-stellar radio sources. They are now known to be old, distant, and powerful.
Astronomers think quasars are powered by massive black holes. A type II quasar is surrounded by, or buried in, an occluding cloud of gas and dust that reduces its brightness in visible light, so they can only be studied well in X-rays.
Norman said the gas and dust is likely the material that feeds the voracious black hole.
"The thinking is that these may be quasars in the early stages of their evolution," Norman said. "As the quasar matures, the extremely powerful radiation it emits disperses the shroud of gas and dust. We're very eager to get a sense of how prevalent they are, and to compare that to type I quasars."
An answer may come fast. The findings reported Tuesday represent a first run at just a portion of the more than more than 500 hours of observations made over the course of more than a year, and Tananbaum and others said other astronomers were already clamoring to get their hands on the data. More detailed findings are pending throughout the year, including the possibility of many more quasars in the far corners of the cosmos.
Tananbaum said the observations will explain more than just black holes.
Researchers now have a wealth of data about galaxies and galaxy clusters, and what they looked like when the universe was less than half its current age. Such information will improve understanding of how and when the universe began and evolved.
"For the first time, we are able to use X-rays to look back to a time when normal galaxies were several billion years younger," said Ann Hornshemeier, a Penn State graduate student who led the other study. "In essence, it is like seeing galaxies similar to our own Milky Way at much earlier times in their lives."
Small black holes, too
Hornshemeier said the study also revealed a handful of galaxies that contain smaller X-ray emitting objects called stellar black holes. If the studied patch of sky is representative of the entire universe, Hornshemeier said there could be as many as 300 billion stellar black holes lurking out there.
These black holes typically have the mass of a few Sun-sized stars and are thought to be spread throughout our own galaxy. Finding them in distant galaxies provides a glimpse of what our own Milky Way might have looked like in its early years, she said.
One particular galaxy that looks like a younger version of our own, but is seen as it was 5 billion years ago, is two or three times more luminous in X-rays than the Milky Way, Hornshemeier said.
Data for the studies were also provided by the European Southern Observatory, the Hobby-Eberly telescope in Texas and the Keck Observatory atop Mauna Kea in Hawaii.
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