Brussels, 10 Aug 2005
Most of the largest black holes in the Universe have been devouring stars without being noticed - until now.
A UK-led international team of astronomers has reported in Nature that it has tracked down an elusive population of black holes that are growing rapidly, hidden behind clouds of dust. The results suggest that most black hole growth takes place this way, solving a mystery that has intrigued astronomers for years.
Super massive (made up from a mass of between 1 million and 1 billion times larger than usual) black holes undergo periods of exponential growth, during which we see them as quasars in the distant Universe. Quasars are circled by a giant ring of gas and dust. They exist primarily at the heart of distant galaxies, and can annually consume up to the equivalent mass of one thousand stars. As their black holes suck in material from their dusty rings, the material lights up brilliantly, making quasars the brightest objects in the Universe. This bright light comes in many forms, including x-rays, visible and infrared light. Quasars are thought to be precursors to large, normal galaxies.
Astronomers have puzzled for years over the question of how many of these cosmic giants are out there. Black holes cannot be seen directly, because they trap light and anything else that gets too close. Astronomers can infer their presence by noting the behaviour of material nearby: gas is superheated and accelerated to a significant fraction of light-speed just before it is consumed. This activity releases x-rays that escape the black hole and reveal its presence. Quasars outshine everything else in the universe in x-rays. By counting the background buzz of x-rays, it is possible to predict the approximate total number of quasars.
But this estimate never matched previous x-ray and visible-light observations of actual quasars, which number far fewer than expected. As quasars can consume the equivalent mass of between ten and a thousand stars in one year, they create immense clouds of gas and dust around them, through which astronomers cannot peer. Astronomers believe that quasars are surrounded by a dusty ring which hides them from sight from Earth in roughly half of cases, while others are buried in dust-drenched galaxies.
'From past studies using x-rays, we expected there were a lot of hidden quasars, but we couldn't find them,' said study leader Alejo Martínez-Sansigre of the University of Oxford Astrophysics group, UK.
The Spitzer telescope appears to have found both types of missing quasars by looking in infrared light. Unlike x-rays and visible light, infrared light can travel through gas and dust. Researchers found 21 examples of these quasars in a small patch of sky. All the objects were confirmed as quasars by the National Radio Astronomy Observatory's Very Large Array radio telescope in New Mexico, the US, and by the Particle Physics and Astronomy Research Council's William Herschel Telescope in Spain. Of the 21 quasars uncovered by Spitzer, ten are believed to be inside fairly mature, giant elliptical galaxies. The rest are thought to be encased in thick, dusty galaxies that are still forming stars.
Extrapolating from the 21 quasars of the study to the rest of the sky, the team believes that there are a lot more of quasars hidden by dust than are not hidden, and that most super-massive black holes grow in short, efficient bursts at the heart of growing galaxies.
Professor Martinez-Sansigre explains 'This newly discovered population is large enough to account for the x-ray background, and now we wish to find out why there are more obscured quasars than unobscured ones.'
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Remarks: Reference publication: The obscuration by dust of most of the growth of supermassive black holes. Martínez-Sansigre A, Rawlings S, Lacy M, Fadda D, Marleau FR, Simpson C, Willott CJ, Jarvis MJ. Nature. 2005 Aug 4;436(7051):666-9