Black Holes More Powerful Than Expected, And Still Growing

Photo:Near-infrared and optical image of a high-energy X-ray source first identified by Chandra

Date:15 December 2000

Black holes may generate as much energy as all the stars in the universe combined, according to a new study that also indicates the objects may be younger than previously thought. And they are getting more massive all the time.

The study used ground-based telescopes to look at a handful of supermassive black holes -- objects that have never actually been seen but are suspected of being as massive as billions of stars, all fitting into an area no larger than our solar system. Astronomers think most galaxies harbor a black hole at their centers.

The objects in the new study were selected from 20 strong X-ray sources previously identified by the Chandra X-ray Observatory in a January report, which proposed an explanation for a ubiquitous "background" of X-rays in the universe. The source of this background radiation has been puzzling researchers for more than three decades, and it is now thought to emanate from individual galaxies with active black holes.

Amy Barger of the University of Hawaii's Institute for Astronomy and the University of Wisconsin, along with her colleagues, looked at these same sources with ground-based telescopes. Focusing on a phenomenon called redshift, which measures the shift in the spectrum of light emitted by a receding object to determine its distance and movement, the scientists looked at the far-off objects, and therefore back in time. They figured out when the black holes had released the energy being measured today, then created a historical timeline of black hole activity.

Slow growth

Black holes can be relatively quiet, like the one at the center of our galaxy, or they can be "active," meaning that as they suck matter into themselves and spit out tremendous streams of X-ray energy that travel for billions and billions of miles (kilometers).

Barger said 10 percent of black holes are active at any time, implying that black hole growth is a slow, ongoing process that can take billions of years. This is much longer than would be expected if the black holes were formed in violent galaxy mergers, as some researchers have speculated.

"At least 15 percent of supermassive black holes have formed since the universe was half its present age," said Barger. "This challenges the widely held view...that the black holes formed when the galaxies formed. Instead, it seems that the black holes are still growing."



photo:The X-ray sources used in the new study were found in this previous Chandra X-ray telescope image.

In other words, these objects are younger than scientists had previously thought.

Barger's team has drawn reasonable conclusions with the data available, said Niel Brandt, a Penn State researcher who was not involved in the study but has seen the data and is working on similar research. But much more data from many more X-rays sources are needed before the conclusions are confirmed, Brandt said.

"Barger and collaborators have found several important and enticing clues suggesting that the growth of massive black holes in the universe may well be a more gradual process than often proposed," Brandt told SPACE.com. "It will be exciting to extend these results to higher redshift and more X-ray sources with the substantially deeper Chandra surveys currently in progress."

If the results are proved out, then scientists will have new puzzles to solve.