The x-ray background is just what it sounds like: an even veil of x-rays spread across the universe. And last January, scientists using the Chandra X-ray Observatory honed in on the individual objects--distant galaxies with active black holes--that produce it. Now in a follow up study, astronomers have reexamined these point sources in optical, submillimeter and radio wavelengths--the first such multi-spectrum observation of the x-ray background. The results, some surprising, map out the energy production from supermassive black holes throughout history.

Amy Barger of the University of Hawaii's Institute for Astronomy and her colleagues, who announced the findings on Tuesday, used the Keck 10-meter telescope to obtain optical measurements showing when the supermassive black holes have been active. As it turned out, only 10 percent of them are active at any given time, suggesting that their growth must be a slow, ongoing process, and many have formed more recently than expected. Submillimeter recordings from the James Clerk Maxwell telescope and radio data from the Very Large Array of the National Radio Observatories further indicated that supermassive black holes contribute about as much energy to the universe as do all the stars combined.

"At least 15 percent of supermassive black holes have formed since the universe was half its present age," Barger says. "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. Instead, it seems that the black holes are still growing at the present time."