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The characteristic blue spectra of accretion disks in quasars as uncovered in the infrared


Quasars are thought to be powered by supermassive black holes accreting surrounding gas1,2,3. Central to this picture is a putative accretion disk which is believed to be the source of the majority of the radiative output2,3,4. It is well known, however, that the most extensively studied disk model5—an optically thick disk which is heated locally by the dissipation of gravitational binding energy—is apparently contradicted by observations in a few major respects6,7. In particular, the model predicts a specific blue spectral shape asymptotically from the visible to the near-infrared5,8, but this is not generally seen in the visible wavelength region where the disk spectrum is observable9,10,11,12,13. A crucial difficulty has been that, towards the infrared, the disk spectrum starts to be hidden under strong, hot dust emission from much larger but hitherto unresolved scales, and thus has essentially been impossible to observe. Here we report observations of polarized light interior to the dust-emitting region that enable us to uncover this near-infrared disk spectrum in several quasars. The revealed spectra show that the near-infrared disk spectrum is indeed as blue as predicted. This indicates that, at least for the outer near-infrared-emitting radii, the standard picture of the locally heated disk is approximately correct.

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Figure 1: Overlay of the polarized- and total-light spectra observed in six different quasars.
Figure 2: Spectral index of polarized light spectra.

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The UK Infrared Telescope (UKIRT) is operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the UK. We thank the Department of Physical Sciences, University of Hertfordshire, for providing the IRPOL2 polarimetry facility for the UKIRT. This research is partially based on observations collected at the European Southern Observatory, Chile.

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Correspondence to Makoto Kishimoto.

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The file contains Supplementary Notes, Supplementary Figures 1-8 and Supplementary Tables 1-8. This Supplementary Information contains details on observations and data reductions, and also on the removal of instrumental polarization. (PDF 1683 kb)

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Kishimoto, M., Antonucci, R., Blaes, O. et al. The characteristic blue spectra of accretion disks in quasars as uncovered in the infrared. Nature 454, 492–494 (2008).

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