Astronomical hunt ends in success

The Chandra Deep Field South: Objects at the edge of the Universe

By BBC News Online science editor Dr David Whitehouse

Astronomers have spotted the first example of the type 2 quasar, an object on the fringes of the known Universe whose existence they had suspected for two decades.

The scientists say the discovery is an important step in their efforts to understand how black holes and galaxies formed when the cosmos was young.

The new quasar was found nestling among the many objects seen in the Chandra Deep Field South Exposure, taken by Nasa's orbiting Chandra X-ray Observatory.

The exposure covers a patch of sky about the apparent size of the Moon and reveals sources of X-rays in the distant Universe in unprecedented detail. Like a conventional quasar, a type 2 quasar is a strong source of X-ray and other radiation.

Powerful radiation

It is believed that both types of quasars are powered by massive black holes that reside at their centres. Unlike a type 1 quasar, however, a type 2 is surrounded by a cloud of gas and dust that absorbs visible light. The cloud has little effect on X-ray radiation from the quasar, hence its detection.

Astronomers first predicted the existence of type 2 quasars in the early 1980s, when they were beginning to devise an explanation for quasars and other distant, bright objects thought to be powered by massive black holes.

"The thinking is that these may be quasars in the early stages of their evolution," said Colin Norman, professor of physics and astronomy at Johns Hopkins University.

"As the quasar matures, the extremely powerful radiation it emits disperses its 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."

More discoveries

To detect the type 2 quasar, astronomers compared data from the Chandra Deep-Field South Exposure with optical observations made by the Very Large Telescope (VLT) in Chile, one of the most powerful ground-based telescopes.

The new type 2 quasar is located in the Southern Hemisphere constellation of Fornax. To reach us, its light has travelled for approximately nine billion years. This means that the activity researchers detected took place when the Universe was approximately one-tenth of its current age.

Veteran astronomer Ricardo Giacconi, of Johns Hopkins University, said: "The identification of a type 2 quasar is just the first of many exciting discoveries that I am sure will come from this data.

"The gas surrounding these objects is thought to contain a large fraction - perhaps most of the normal matter in the universe."