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Black holes go middle class.

Astronomers generally accept the existence of two kinds of objects that wield enormous gravitational pull: baby black holes just a few times more massive than the sun and giant black holes weighing as much as a billion suns.

Two groups of researchers now claim to have found a new, intermediate class of these bizarre beasts--black holes 100 to 10,000 times as massive as the sun.

"This is a new mass range that doesn't have a clear explanation," says study collaborator Edward J.M. Colbert of NASA's Goddard Space Flight Center in Greenbelt, Md.

One theory suggests that the midsize black holes arise from the merger of many smaller black holes. In another proposal, middleweights formed outright early in the universe and will ultimately pack on enough mass to become supermassive black holes, like the one believed to lurk at the core of our own galaxy.

The two teams studied X-ray emissions from galaxies suspected of harboring dense concentrations of matter near their cores. Both groups reported their results in mid-April at a meeting of the American Astronomical Society in Charleston, S.C.

As dust and gas disappear into a black hole, they emit a swan song of X rays. Studying the X-ray spectra with the Japanese satellite ASCA, Colbert and Richard Mushotzky of Goddard found that 10 of 17 nearby spiral galaxies emit a pattern of radiation expected from middleweight black holes. Colbert says that half of all spirals may contain midsize black holes. The team's survey of elliptical galaxies proved inconclusive.

Andrew Ptak and Richard Griffiths of Carnegie Mellon University in Pittsburgh found the same X-ray signature in the starburst galaxy M82. Starburst galaxies contain many massive stars, which provide the raw material for black holes.

Ramesh Narayan of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., says that the findings are intriguing. However, he cautions that determining the mass of a black hole by studying its X rays is not nearly as accurate as inferring its weight from its tug on neighboring objects.
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Publication:Science News
Article Type:Brief Article
Date:May 1, 1999
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