The true colour of black hole disks

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

July 23, 2008

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".

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.

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.

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.

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.

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