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Swirling Dust Near Black Hole Too Thick for Theory
By Robert Roy Britt
Senior Science Writer
posted: 07:00 am ET
07 May 2004
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Images: A close-up view of a donut-shaped disk of dust around a black hole confirms several expectations about the massive structure but has astronomers wondering how the disk could be so thick. The black hole packs a mass equal to about 10 million Suns. That's more than twice the mass of the black hole in our Milky Way but many times less than others in the more distant universe. It anchors a nearby galaxy called NGC 1068. Unlike some supermassive black holes, this one is not surrounded by a tremendously bright region. Astronomers long ago speculated that this and similar black holes might be shrouded in a disk of dust that partially hides the tremendous luminosity that a black hole's sloppy eating habits generate. The new observations, released this week, are the first to accurately measure this disk of dust, called a torus. It is about 11 light-years across and a surprising 7 light-years thick. "The torus probably represents accumulated debris, gas and dust, thrown off from stars farther away from the center of the galaxy that has worked its way inward," explained study leader Walter Jaffe of Leiden University in The Netherlands. "When this material is thrown off by stars it spins around the galaxy just like the original stars did." Jaffe said centrifugal force should cause the disk to flatten out. Similar thin disks develop around stars, and the rings of Saturn are another example of this flattening process at work. "One of the difficult-to-explain aspects of our measurement is that the disk is in fact thick, with height not much less than its radius," Jaffe said in an e-mail interview. "We do not know why it hasn't settled down completely into a very thin disk." The observations were made using an interferometer at the European Southern Observatory's Very Large Telescope. Other findings were not surprising. The torus of dust is about room temperature at the outer edge, and about 1,832 degrees Fahrenheit (1,000 Celsius) on the inner edge. Inside about 1 or 2 light-years, the disk has been evaporated by hotter temperatures created by frenetic activity around the black hole. "This is why the dust structure is a torus, with a hole in the middle, rather than a complete disk," Jaffe said. The black hole, whose width is not known, is buried well deep inside the torus. The torus is composed of tiny dust particles similar to rock minerals on the Earth, including silicon, oxygen, and probably aluminum and magnesium, Jaffe said.
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