Quasars are star-like objects that are thought to be fuelled by supermassive black holes. Padovani and colleagues searched for a rare type quasar known as an obscured quasar. However, these objects - as their name suggests - are partially hidden by clouds of dust and gas, which makes them too faint to be observed by classical methods.

To overcome this problem, the ESO team used the Astrophysical Virtual Observatory (AVO) - a database that combines visible data from the Hubble Space Telescope, near-infrared data from the Very Large Telescope (VLT), and X-ray data from the Chandra observatory. Padovani and co-workers found 68 candidate obscured quasars, and 31 of these had characteristics that suggested they were associated with supermassive black holes.

“This discovery means that so far we might have been underestimating the number of powerful supermassive black holes by at least a factor of two, possibly up to a factor of five,” Padovani told PhysicsWeb. “This has important implications for our understanding of active galactic nuclei and quasars and therefore, given the likely connection between quasar and galaxy formation, for galaxy formation as well.”

The results also demonstrate the potential of the virtual observatory methods. “One major obstacle in taking spectra of these sources is their faintness, which puts them beyond the reach of even very large (8 to 10 metre) telescopes,” said Padovani.

The team now plans to use data from the VLT and the Spitzer Space Telescope, which operates in the far-infrared, to place tighter constraints on the redshift of the sources.