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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."

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Black Hole

A black hole is a body of zero dimension but large mass and therefore large gravitational force. Black holes in our own Galaxy typically have ten to several hundred solar masses. Supermassive black holes in the centres of galaxies are known to have several million solar masses. Their gravity is so large, that not even light can escape from them, so they cannot be seen using conventional methods, hence the name. They have to be detected by observing the effects their gravity has on their surrounding environment. Black holes are also defined by their event horizon, an imaginary sphere surrounding the black hole which marks the point-of-no-return for light. If light gets closer to the black hole than this, it can not escape. The radius of the event horizon is also determined by the mass of the black hole. A black hole of a hundred solar masses would have an event horizon of radius 270 kilometres (about 170 miles.) Black holes can be formed from collapsing massive stellar, cluster or galactic cores. They are surrounded by an accretion disk of material spiralling into the black hole.

Quasar

A quasar (quasi-stellar object) is a very bright point-like source of light emanating from the centres of massive galaxies. The quasar's power is provided by the black hole at the galactic core. The light received from the quasar has contributions from both the black hole jets and the accretion disk. If the jet should be pointed at us, the quasar will appear even brighter. Every quasar has a black hole at its core. A galaxy and its black hole have to have sufficient mass, and sufficient "food matter" for the black hole, in order to generate enough power to host a quasar. Quasars were more common in the early universe, as this energy production ends when the supermassive black hole has consumed all of the matter near it.

Infrared Light

Infrared wavelengths are longer wavelengths than light waves. They are typically measured in microns, also called micrometres. One micron is one millionth of a metre, one 10000th of a centimetre, or one 25000th of an inch.

UKIRT

The world's largest telescope dedicated solely to infrared astronomy, the 3.8-metre (12.5-foot) UK Infrared Telescope (UKIRT) is sited near the summit of Mauna Kea, Hawaii, at an altitude of 4194 metres (13760 feet) above sea level. It is operated by the Joint Astronomy Centre in Hilo, Hawaii, on behalf of the UK Science and Technology Facilities Council. More about the UK Infrared Telescope.

Reference

This press release refers to a paper published in Nature: "The characteristic blue spectra of accretion disks in quasars as uncovered in the infrared"