The Chandra telescope scrutinised X-ray signals from an exceptionally clear patch of the southern sky known as the Chandra Deep Field South. The high-resolution telescope identified over 300 separate X-ray sources. Astronomers at the Very Large Telescope then obtained the infrared and visible spectra of over 100 of the sources.

The X-ray sources are 'active galactic nuclei' (AGNs) and astronomers believe that they are the main contributors to the X-ray background. At about 8000 million light years away, we see the AGNs as they were when the universe was about half its present age. "In essence, it is like seeing galaxies similar to our own Milky Way at much earlier times in their lives", says Ann Horschemeier of Pennsylvania State University. The AGNs are enveloped in clouds of gas and dust, and are probably powered by enormous black holes. "The Chandra data show us that giant black holes were much more active in the past than at present", says Riccardo Giaconni of Johns Hopkins University.

The deep field study also identified for the first time a so-called type-II quasar, which has a black hole at its core. The quasar is extremely remote and shrouded in gas and dust. "The discovery of this object is key to understanding how dense clouds of gas form galaxies with massive black holes at their centres", says Colin Norman of Johns Hopkins University.