Spectra of quasar accretion discs separated from surrounding nebulosity

July 24, 2008
July 24, 2008--Astronomers used polarizing filters to discern the characteristic blue emission spectra of quasar accretion discs from the surrounding clouds. The observations at the Very Large Telescope reconciles quasar observations with models.

July 24, 2008--Quasars are the brilliant cores of remote galaxies, at the hearts of which lie supermassive black holes that can generate a trillion times more energy than the Sun. These mighty power sources are fueled by interstellar gas, thought to be sucked into the hole from a surrounding "accretion disc." A paper in this week's issue of the journal Nature, partly based on observations collected with the European Southern Observatory's (ESO's) Very Large Telescope in Paranal, Chile, verifies a long-standing prediction about the intensely luminous radiation emitted by these accretion discs.1

"Astronomers were puzzled by the fact that the best models of these discs couldn't quite be reconciled with some of the observations, in particular, with the fact that these discs did not appear as blue as they should be," explains lead-author Makoto Kishimoto at the Max Planck Institute for Radio Astronomy (Bonn, Germany).

Such a discrepancy could be the signal something was very wrong with the models. With his colleagues, Kishimoto investigated this discrepancy by studying the polarized light from six quasars. This enabled them to demonstrate that the disc spectrum is as blue as predicted.

"The crucial observational difficulty here has been that the disc is surrounded by a much larger torus containing hot dust, whose light partly outshines that of the disc," says Kishimoto. "Because the light coming from the disc is scattered in the disc vicinity and thus polarized, by observing only polarized light from the quasars, one can uncover the buried light from the disc."

In a similar way that a fisherman would wear polarized sunglasses to help get rid of the glare from the water surface and allow him to see more clearly under the water, the filter on the telescope allowed the astronomers to see beyond surrounding clouds of dust and gas to the blue color of the disc in infrared light.

The observations were done with the FORS and ISAAC instruments on one of the 8.2-m Unit Telescopes of ESO's Very Large Telescope, located in the Atacama Desert in Chile, as well as several other telescopes, including the Science and Technology Facilities Council (STFC) United Kingdom Infra-Red Telescope (UKIRT).

The standard picture of the accretion disc is therefore vindicated. The authors believe that further measurements could eventually provide valuable insight into how and where the disc ends, and how material is being supplied to the disc.

"The characteristic blue spectra of accretion disks in quasars as uncovered in the infrared," by Makoto Kishimoto et al., appears in the 24 July 2008 issue of the journal Nature. In addition to Kishimoto, the team includes Robert Antonucci, Omer Blaes, and Christian Leipski (University of California, Santa Barbara), Andy Lawrence (SUPA, University of Edinburgh, Scotland), Catherine Boisson (LUTH, Observatoire de Paris, France), and Marcus Albrecht (Universidad Catolica del Norte, Chile).

Images of the quasar make it appear as a reddish star-like object (see figure). To uncover the true color of the quasars' disc, the astronomers used a polarizing filter to suppress the emission generated from the surrounding messy dust clouds. When the radiation from these clouds are filtered from view, the true color of the accretion disc appears bluish.

For more information, see the ESO news release.

1. M. Kishimoto et al., Nature 454, 452 (July 24, 2008).


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