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Issued by: Inge Heyer, Science Outreach Specialist Images, notes, and contact details appear below. 24 July 2008 Infrared Sunglasses See Black Hole DisksFor the first time astronomers have found a way to get a clean view of the disks surrounding black holes. Infrared light from clouds of dust usually contaminate their view heavily, but by using a polarizing filter in the infrared, they have finally been made clearly visible to us. Supermassive black holes in the nuclei of galaxies are the subject of intense research in astronomy, but they are too far away from us to see any details even with the biggest telescopes on Earth. Furthermore, such a black hole and its surrounding gas and dust clouds constitute a messy environment. This has confused astronomers who tried to observe the spectrum of the black hole vicinity, as the strong emission from these clouds badly contaminates the spectrum. An international team of astronomers has found a way around this problem. Some of the black holes feature a very small amount of scattered light, which turns out to be coming from the near vicinity of the black holes and not from the surrounding dust clouds. Scattered light is polarized, which is how polaroid sunglasses help get rid of glare on car windshields, for example. By using a polarizing filter with large telescopes to detect this small amount of scattered light and measure it with unprecedented accuracy, one could use the natural scattering mirror as a periscope and see the light from the immediate vicinity of the black hole. To do this one needs a polarimeter. The United Kingdom Infrared Telescope (UKIRT) on Mauna Kea in Hawaii offers just such an instrument to its users. Built at the University of Hertfordshire (UK) and affectionately known as IRPOL (infrared polarimeter), this instrument offered the needed capability which is not often seen on large telescopes. Dr. Kishimoto and co-workers used UKIRT and IRPOL and other telescopes to search for proof for many years: proof that such a luminous supermassive black hole is accreting materials in a particular form of disk, where the disk shines directly using the gravitational binding energy of the black hole. Theorists have long thought that such disks should exist, and while there is a well-developed theory for it, until now theory and observations have been contradictory. The dust cloud contamination has previously presented difficulties in identifying an expected disk prediction, a particular blue color of the disk in infrared light. The team has finally uncovered this blue color by eliminating the dust contamination using the polarizing filter. Using similar techniques the team has already reported the discovery of an important predicted spectral feature in the blue part of the spectrum, which could not be discovered in any other way. Furthermore, this disk property 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. The team's new method may provide answers to these questions in the near future. Dr. Makoto Kishimoto of the Max Planck Institut fuer Radioastronomie, principal 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 in 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." 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." 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 IRPOL, even with the very largest telescopes in the world, exciting discoveries like those of Kishimoto and his colleagues could not be made." 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).
The United Kingdom Infrared Telescope on Mauna Kea, Hawaii.
IRPOL on the United Kingdom Infrared Telescope on Mauna Kea, Hawaii.
IRPOL in its travelling case.
Looking at UKIRT on Mauna Kea through IRPOL.
Looking at the sunset on Mauna Kea through IRPOL.
Notes for EditorsBlack HoleA black hole is a body of zero dimension but large mass and therefore large gravitational force. Black holes typically have ten to several hundred solar masses. Supermassive black holes 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 kilometers (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. QuasarA quasar (quasi-stellar object) is a very bright point-like source of light emanating from the centres of massive elliptical 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. Light YearOne light year is about 10 million million kilometres or 6 million million miles. Infrared LightInfrared 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. UKIRTThe 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: http://outreach.jach.hawaii.edu/articles/aboutukirt/ Science and Technology Facilities CouncilThe Science and Technology Facilities Council is an independent, non-departmental public body of the Office of Science and Innovation which itself is part of the Department of Innovation, Universities and Skills. It was formed as a new Research Council on 1 April 2007 through a merger of the Council for the Central Laboratory of the Research Councils (CCLRC) and the Particle Physics and Astronomy Research Council (PPARC) and the transfer of responsibility for nuclear physics from the Engineering and Physical Sciences Research Council (EPSRC). We are one of seven national research councils in the UK. The Science and Technology Facilities Council is government funded and provides research grants and studentships to scientists in British universities, gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Organisation for Nuclear Research, CERN, the European Space Agency and the European Southern Observatory. It also contributes money for the UK telescopes overseas on La Palma, Hawaii, Australia and in Chile, the UK Astronomy Technology Centre at the Royal Observatory, Edinburgh and the MERLIN/VLBI National Facility. Media ContactsPlease note that it is best to contact these individuals by email.
Science ContactsPlease note that it is best to contact these individuals by email.
ReferenceThis press release refers to a paper published in Nature: Web links
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