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Galactic spring-clean unveils missing quasars

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A team of astronomers might have solved one of the mysteries of astrophysics with the discovery of a clutch of quasars, hiding behind clouds of dust. The discovery was made using data from NASA's Spitzer Space Telescope, and could explain previously mysterious levels of cosmic background radiation.

Quasars are very distant, but very strong sources of radio signals. The working theory is that they are located in the central cores of very distant galaxies, where matter falling into a supermassive black hole is turned into a blinding torrent of radiation.

Astronomers suspect that all quasars are surrounded by a dusty ring, and that this hides about half of them from our Earth-based line of sight. However, the cosmic X-ray background, primarily made up of emissions from quasars, suggests that there must be many more of the voracious objects than could be accounted for by those currently known about.

Examining data from Spitzer, the team found 21 examples of these lost quasars, hidden behind both a dust ring, and the dust of the galaxy itself. The objects were confirmed as quasars by the National Radio Astronomy Observatory's Very Large Array radio telescope, New Mexico, and the William Hershel Telescope on La Palma.

Alejo Martinez-Sansigre from the University of Oxford noted: "We were missing a large population of obscured quasars, which had been inferred from studies at X-ray frequencies. This newly discovered population is large enough to account for the X-ray background."

The trick now, he says, will be to work out why there are more hidden quasars than unobscured ones. ®

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