Posted: July 24, 2008
For the first time astronomers have found a way to obtain a clean view of the discs that theorists say should surround black holes, by using a polarising filter that only shows up the discs’ infrared properties.
Artist impression of an Active Galactic Nucleus, which harbours a black hole at its centre surrounded by an accretion disc and 'messy' dust clouds. Image: NASA E/PO - Sonoma State University, Aurore Simonnet.
The central regions of active galaxies are thought to be powered by supermassive black holes accreting gas from their local environment, and are the subject of intense research in astronomy, but the surrounding gas and dust clouds constitute a messy environment and contaminates spectral observations of the black hole vicinity.
“Theory says that matter getting sucked into a black hole should swirl inwards in a disc, which heats up as it goes in,” Professor Andrew Lawrence of the University of Edinburgh tells Astronomy Now. “The disc should be much hotter in the middle, which overall should lead to the radiation from the disc having a very blue colour in infrared (IR) light. Unfortunately, in the IR you can't really see the disc around the black hole because further out there is dust and ‘muck’ which shines very brightly in the IR. The glare from this means you can't see the fainter stuff coming from the disc.“
Lawrence, along with an international team of astronomers lead by Dr Makoto Kishimoto of the Max Planck Institute for Radio Astronomy in Germany, found a way around the problem by focusing on the small amounts of scattered light emanating from the local region around the black holes. Scattered light is polarised, which is how polaroid sunglasses help get rid of glare on car windshields, for example. By applying this concept to black holes, essentially by donning a telescope with a pair of giant polaroid sunglasses, the team could eliminate the dust contamination and reveal the much sought after blue spectral features.
Schematic display of the polarisation observation. The red star-like object in the upper left is one of the quasars that the researchers observed. The light is thought to originate from an accretion disc around a black hole with a strong contamination from messy dust clouds, as shown by the drawing on the upper-right panel. When a polarisation filter was added, these clouds were suppressed from view, revealing the true colour of the accretion disc, as shown in the two lower panels. Image: M. Kishimoto with cloud image by M. Schartmann.
The observations were conducted using the IRPOL (InfraRed POLarimeter) polarising filter on the United Kingdom Infrared Telescope (UKIRT) located on Mauna Kea in Hawaii and one of the European Southern Observatory’s 8.2-metre VLT telescopes at the Paranal Observatory in Chile.
“In the specific quasars we picked the dust is not polarised but the disc radiation is, so when you pick out only the polarised light, the glare from the dust goes away and you see only the light from the swirling disc, and finally we can test the theory,” says Lawrence. “Nobody has tried this before in the IR, to remove the dust-glare, but the amount of polarisation is very small, so it’s very hard work - these are difficult measurements.”
The astronomers have finally demonstrated that the disc spectrum is blue, which supports the theory that has so often been contradicted by previous observational attempts. But important questions remain: how and where does the disc end and how is material being supplied to the disc? The team's new method may provide answers to these questions in the near future.
"After many years of controversy, we finally have very convincing evidence that the expected disc is truly there,” says Kishimoto. “However, this doesn't answer all of our questions. While the theory has now been successfully tested in the outer region of the disc, we have to proceed to develop a better understanding of the regions of the disc closer to the black hole. But the outer disc region is important in itself - our method may provide answers to important questions for the outer boundary of the disc."
A paper describing these results is published in today’s issue
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