A team of astronomers has found that supermassive black holes contribute about as much energy to the Universe as all the stars combined. Many have formed recently rather than in the early, violent stages of galaxy birth. And, at any give time in the history of the Universe, about 10 percent of all supermassive black holes are actively pulling in huge quantities of gas and whole stars.

Chandra's view of hard X-ray sources. Photo: NASA/Institute for Astronomy

These are but a few of the revelations brought forth by the first multi-wavelength observations of the distant and exotic objects that constitute what astronomers call the X-ray background, which produces an evenly spread veil of X-rays across the entire Universe.

Dr. Amy Barger of the University of Hawaii's Institute for Astronomy and the University of Wisconsin, who led the observations, presented these new results yesterday at a press conference at the 20th Texas Symposium on Relativistic Astrophysics, in Austin. Her colleagues are Drs. Lennox Cowie, also at the UH Institute for Astronomy, 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," said Barger. "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."

Supermassive black holes, likely created from collapsed gas clouds, contain the mass of millions to billions of stars confined to a region no larger than our Solar System. Astronomers now believe that most galaxies, including our own, harbor a supermassive black hole at their core.

Black holes are deemed "active" when they are actively accreting great quantities of matter. This matter, heated to millions of degrees under the crushing force of gravity, shines particularly bright in X-ray light as well as in other wavebands.

Barger was part of the team that, in January 2000, announced that the Chandra X-ray Observatory resolved the X-ray Background into individual point sources, distant galaxies with active black holes. Barger looked at these same sources with ground-based optical, submillimeter and radio telescopes. Optical measurements reveal the times at which the black hole activity occurred. Submillimeter and radio measurements provide crucial information on the amount of energy emitted in the formation of the supermassive black holes.

With such follow-up, the team was able to map the energy production by supermassive black holes throughout the history of the Universe and determine the time history of the supermassive black hole formation process. The discovery that 10 percent of the black holes are active at any time implies that black hole growth is a slow, ongoing process with the galaxies spending more than a billion years in creating the black holes at their centers.

Moreover, the time intervals found for the black hole growth are much longer than would be expected if the black holes were formed in violent galaxy mergers, as has often been speculated.

The Keck 10-meter telescope and the James Clerk Maxwell telescope, both located on Mauna Kea in Hawaii, were used to obtain the optical and submillimeter observations, respectively. The radio observations were obtained with the Very Large Array of the National Radio Observatories.

The 20th Texas Symposium is on Relativistic Astrophysics runs from December 10-15 at the Hyatt Regency Hotel and Austin Convention Center. The Astronomy and Physics departments of the University of Texas at Austin are hosting the event.

The Institute for Astronomy at the University of Hawaii conducts research into galaxies, cosmology, stars, planets, and the Sun. Its faculty and staff are also involved in astronomy education, and in the development and management of the observatories on Haleakala and Mauna Kea.