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| PRI (MPIfR) 07/2008 (1) | Press Release | July 23, 2008 |
The central regions of active galaxies, are thought to be powered by supermassive black holes accreting gas from their environment. An important ingredient of the so-called "standard model" of Active Galactic Nuclei or AGN is a massive accretion disk which is believed to be the source of most of the radiation from the AGN.
Until recently, the presence of such accretion disks was only assumed
theoretically.
An international team of astronomers, led by Makoto Kishimoto from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, found a clever way to get around observational problems caused by the dust
environment of the nucleus. They could eliminate the influence of dust
contamination by observing polarised emission directly from the central
region of the AGN.
Thus they could show
that the spectrum of the central source is as blue as expected from theory,
verifying a long-standing prediction about the intensely luminous radiation emitted
by these accretion disks.
The results are published in this week's issue of the journal "Nature".
Figure 1:
Schematic display of the polarisation observation. The red star-like object in the upper left is one of the quasars observed. The light is thought to originate from an accretion disk around a black hole with a strong contamination from messy dust clouds, as shown by the drawing on the upper-right panel. When we put in a polarisation filter, these clouds are suppressed from view, giving us the true color of the accretion disk, as shown in the two lower panels.
Image:
M. Kishimoto with cloud image by M. Schartmann
(Click image for higher resolution).
Quasars are the brilliant cores of remote galaxies, at the hearts of which lie supermassive black holes that can generate enough power to outshine the Sun a trillion times. These mighty power sources are fuelled by interstellar gas, thought to be sucked into the hole from a surrounding "accretion disk".
Such black holes and their accretion disks are thought to be in a messy environment - surrounded by many clouds of dust. This has confused astronomers who tried to study the spectrum of the black hole vicinity - the strong emission from these clouds badly contaminates their precious
spectrum.
"Astronomers were puzzled by the fact that the most extensively studied models of these disks couldn't quite be reconciled with some of the observations,
in particular, with the fact that these disks did not appear as blue as they
should be", explains Makoto Kishimoto from MPIfR.
However, an international team of astronomers, led by Kishimoto, found a clever way to get around this.
Since the disk light is scattered in the vicinity of the disk and thus
appears polarised,
they could use the polarised light to separate the disk from the surrounding
dust clouds.
Figure 2:
Artists's impression of an Active Galactic Nucleus (AGN).
A supermassive black hole in the very center is surrounded by an accretion
disk and messy dust clouds.
Strong jets radiate perpendicular to the accretion disk.
Image: NASA E/PO - Sonoma State University, Aurore Simonnet
(Click image for higher resolution).
For their observations in the infrared, they used polarising filters at some of the largest telescopes on Earth - one of the 8.2m VLT telescopes at the Paranal observatory of ESO in Chile as well as the United Kingdom Infrared Telescope (UKIRT) on Mauna Kea in Hawaii. This enabled them to get rid of emission from hot dust outside the accretion disk, and they could demonstrate that the disk spectrum is as blue as predicted.
Dr. Robert Antonucci of the University of California at Santa Barbara, a fellow investigator, says: "Our understanding of the physical processes in the disk is still rather poor, but now at least we are confident of the overall picture." The disk behaviour found in the paper is expected to originate in the outermost region of the disk, where important questions are yet to be answered: how and where the disk ends and how material is being supplied to the disk. "In the near future, our new method may pioneer the way to address these questions", says Makoto Kishimoto.
The characteristic blue spectra of accretion disks in quasars as uncovered in the infrared,
Makoto Kishimoto, Robert Antonucci, Omer Blaes, Andy Lawrence, Catherine Boisson, Markus Albrecht & Christian Leipski, Nature, paper 2007-12-12964B, 24 July 2008.
Infrared Sunglasses See Black Hole Disks, Press Release Joint Astronomy Centre (JAC), Hawaii.
Infrared Sunglasses See Black Hole Disks, Press Release Royal Observatory Edinburgh (ROE).
Accretion Discs Show Their True Colours, ESO Press Release 21/08, July 23, 2008.
Polarizing Filter Allows Astronomers to See Disks Surrounding Black
Holes,
UCSB Press Release, July 15, 2008.
Max Planck Institute for Radio Astronomy (MPIfR).
Infrared Interferometry Group at MPIfR.
European Southern Observatory (ESO).
ESO Very Large Telescope (VLT) at Cerro Paranal, Chile.
Astrophysics Group University of Santa Barbara (UCSB), California.
Joint Astronomy Centre (JAC), Hawaii.
United Kingdom Infra-Red Telescope (UKIRT), Hawaii.
Royal Observatory Edinburgh (ROE).
Dr. Makoto Kishimoto,
Max-Planck-Institut für Radioastronomie, Bonn.
Fon: +49-228-525-186
Fax: +49-228-525-229
E-mail: mk (at)
mpifr-bonn.mpg.de
Dr. Norbert Junkes,
Public Outreach,
Max-Planck-Institut für Radioastronomie, Bonn.
Fon: +49-228-525-399
Fax: +49-228-525-438
E-mail: njunkes (at)
mpifr-bonn.mpg.de