Polarized sunglasses see black hole disks

For the first time astronomers have found a way to get a clean view of the elusive disks of matter surrounding supermassive black holes. By using a polarising filter on the Science and Technology Facility Council's UK Infrared Telescope (UKIRT) in Hawaii, they have been able to see through the clouds of dust which surround these black holes. This work is published on 24th July 2008 in Nature. In a similar way that a fisherman would wear polarised 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 allowed the astronomers to see beyond surrounding clouds of dust and gas to the blue colour of the disk in infrared light.

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 as 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 the University of Edinburgh's Institute for Astronomy, and co-investigator on the project, says "For decades there has been a theory that supermassive black holes should be accumulating materials in the form of a disk …but until now this has been impossible to test due to the contamination by the dust clouds."

The team, led by Makoto Kishimoto of the Max Planck Institut fuer Radioastronomie, have found a way around this problem. Some of the black holes have a very small amount of scattered light coming from the vicinity of the black hole itself, rather than the clouds of gas and dust around it. This light has become polarised after hitting matter within the disk. By using a filter that only allows this polarised light to come through and blocks out the unpolarised light from the gas clouds, they were able to visually eliminate them and reveal the disk.

This new method could help astronomers in their understanding of the outermost region of the disks where important questions are still to be answered: how and where the disk ends, and how material is being supplied to the disk.

Dr. Chris Davis of the Joint Astronomy Centre, the facility operating UKIRT, says: "UKIRT has long been at the forefront of infrared astronomy, and has been a leader in the niche area of infrared polarimetry for almost two decades. Without facilities like the infrared polarimeter (IRPOL), even with the very largest telescopes in the world, exciting discoveries like those of Kishimoto and his colleagues could not be made."

Source: Science and Technology Facilities Council