ST. LOUIS, MISSOURI--The deepest and most sensitive sky surveys ever attempted have revealed that the distant cosmos is burning with ultrabright X-ray sources, coming from more super-massive black holes than anyone had predicted. And that's not even the biggest surprise. It turns out almost all quasars, those blinding beacons of the universe, emerged only in the most ancient galaxies.
This "antihierarchy," as it is being called, suggests nearly all of the biggest, most active, and most violent black holes--the ones that produced the quasars--appeared not too long after the first stars and galaxies. The universe has been calming down, so to speak, ever since.
To conduct the new surveys, astronomer Neil Brandt of Pennsylvania State University in University Park trained the Chandra X-ray Observatory on two patches of sky for 23 days. When he examined the resulting images--one in the Northern hemisphere called the Chandra Deep Field-North, and another in the Southern hemisphere called the Extended Chandra Deep Field-South--and compared the data with corresponding surveys by optical telescopes, he and Günther Hasinger from the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, found about 600 black holes of the super-massive variety. Extrapolated to the whole sky, he says, there could be as many as 300 million of the cosmic monsters lurking in the hearts of galaxies.
The data from Chandra, and from other x-ray sky surveys of lower resolution, also show a striking pattern: The brightest galactic nuclei, which indicate the presence of quasars, are present in the most distant parts of the universe and decrease in intensity as the x-ray sources grow nearer. "This is remarkable," Brandt said at a presentation here at an annual meeting of the American Association for the Advancement of Science (which publishes ScienceNOW). "You're seeing an antihierarchical evolution, with the bigger black holes first, and then getting smaller and smaller." That means, he says, that the early universe was full of black holes and thus a much more violent place than astronomers had imagined.
"It's fabulous--really fantastic stuff," says Jeffrey McClintock of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, who has been studying stellar-mass black holes within the Milky Way galaxy. "I didn't know before this presentation that the most luminous quasars, and hence the most massive black holes, were there at the beginning, while the little puny ones were created more recently. That's very exciting."