The True Colour of Black Hole Disks (Forwarded)

Press and Public Relations Department
Max Planck Society for the Advancement of Science
Munich, Germany

Contact:

Dr. Makoto Kishimoto
Max Planck Institute for Radio Astronomy, Bonn
Tel.: +49-228-525-186 Fax: +49-228-525-229

Dr. Norbert Junkes, Public relations
Max Planck Institute for Radio Astronomy, Bonn
Tel.: +49-228-525-399 Fax: +49-228-525-438

July 23rd, 2008

News SP / 2008 (163)

The True Colour of Black Hole Disks

A new observing technique reveals the spectral behaviour of accretion disks
around black holes in quasars

The central regions of active galaxies are thought to be powered by
supermassive black holes accreting gas from their surrounds. 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 theoretically assumed. 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".

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.

For their observations in the infrared the researchers 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.

Related links:

[1] Infrared Sunglasses See Black Hole Disks, Press Release Joint Astronomy
Centre (JAC), Hawaii
http://outreach.jach.hawaii.edu/pressroom/2008_irpol/

[2] Accretion Discs Show Their True Colours, ESO Press Release XX/08, July
23, 2008
http://www.eso.org/public/outreach/press-rel/pr-2008/

[3] Polarizing Filter Allows Astronomers to See Disks Surrounding Black
Holes, UCSB Press Release, July 23, 2008
http://www.ia.ucsb.edu/pa/display.aspx?pkey=1815

[4] Max Planck Institute for Radio Astronomy (MPIfR)
http://www.mpifr.de/english/

[5] Infrared Interferometry Group at MPIfR
http://www.mpifr.de/div/ir-interferometry/

[6] European Southern Observatory (ESO)
http://www.eso.org/public/

[7] ESO Very Large Telescope (VLT) at Cerro Paranal, Chile
http://www.eso.org/sci/facilities/paranal/

[8] Astrophysics Group University of Santa Barbara (UCSB), California
http://www.physics.ucsb.edu/~astrogroup/

[9] Joint Astronomy Centre (JAC), Hawaii
http://www.jach.hawaii.edu/

Original work:

Makoto Kishimoto, Robert Antonucci, Omer Blaes, Andy Lawrence, Catherine
Boisson, Markus Albrecht & Christian Leipski
The characteristic blue spectra of accretion disks in quasars as uncovered
in the infrared
Nature, paper 2007-12-12964B, 24 July 2008

IMAGE CAPTIONS:

[Fig. 1:
http://www.mpg.de/bilderBerichteDoku.../Web_Zoom.jpeg
(157KB)]
Artist's impression of an Active Galactic Nucleus (AGN). A supermassive
black hole in the very centre 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

[Fig. 2:
http://www.mpg.de/bilderBerichteDoku.../Web_Zoom.jpeg
(60KB)]
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 the accretion disk 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 colour of the accretion disk, as shown in the two lower panels.

Image: M. Kishimoto with cloud image by M. Schartmann