Infrared Sunglasses See Black Hole Disks

For the first time, astronomers have found a way to get a clean view of the disks which surround supermassive black holes in the centre of galaxies. By using a polarizing filter on the UK Infrared Telescope (UKIRT) on Mauna Kea, Hawaii, they have been able to see through the clouds of dust which surround these black holes.

It is believed that most, if not all, galaxies have a supermassive black hole in their centre, and this is an area of intense research within astronomy. Studying these black holes and discovering more about their structure can be difficult due to the fact that they are so far away from us. Also, the clouds of gas and dust which surround the black holes make it difficult to achieve a clean, uncontaminated spectrum of the black hole vicinity.

Andy Lawrence, of Edinburgh's Institute for Astronomy, and co-investigator on the project, says "For decades there has been a theory that this type of black hole should have a particular disk...but until now the theory and observations have been contradictory due to the contamination by the dust clouds."

The team, led by Makoto Kishimoto, of the Max Planck Institute for Radio Astronomy, have found a way around this problem. Some of the black holes have a very small amount of scattered light, which turns out to be coming from the vicinity of the black hole itself, rather than the clouds of gas and dust. This scattered light is polarized, so by using a polarizing filter on UKIRT, the team have been able to achieve an uncontaminated spectrum of the black hole vicinity, which proves that the long standing predictions are correct.

In a similar way that a fisherman would wear polarized sunglasses to help get rid of the glare from the water surface and allow him to see more clearly under the water, the filter on the telescope allows the astronomers to see beyond the surrounding clouds of dust and gas.

Until now, the astronomers have been hindered by the dust and gas as the clouds have prevented them from seeing the expected blue colour of the disk in infrared light. The team have finally uncovered this blue colour by eliminating the dust contamination using the polarizing filter.

This disk property is thought to originate at the outermost region of the disk, where important questions have yet to be answered: how and where the disk ends, and how material is being supplied to the disk. The team's new method could provide answers to these questions in the near future.

Dr Makoto Kishimoto, lead investigator of this project, says: "After many years of controversy, we finally have very convincing evidence that the expected disk is truly there. However, this doesn't answer all of our questions. While the theory has now been successfully tested on the outer region of the disk, we have to proceed to develop a better understanding of the regions of the disk closer to the black hole. But the outer disk region is important in itself - our method may provide answers to important questions for the outer boundary of the disk."



Removing dust from Black Hole images

This figure schematically shows how the team's polarization observation works. The red star-like object in the upper left panel 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 a polarizing filter in, these clouds are suppressed from view, giving us the true color of the accretion disk, as shown in the two lower panels. (Figure by M. Kishimoto, with cloud image by Schartmann).


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This press release refers to a paper published in Nature: "The characteristic blue spectra of accretion disks in quasars as uncovered in the infrared" (paper 2007-12-12964B, 24 July 2008).