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Apr. 13, 1999:
The field of black holes, formerly dominated by
heavyweights packing the gravitational punch of a billion Suns and
lightweights just a few times heavier than our Sun, now has a new
contender -- a just-discovered mysterious class of "middleweight"
black holes, weighing in at 100 to 10,000 Suns.
Astronomers at NASA and Carnegie Mellon University have independently found evidence for the new type of black holes in spiral-shaped galaxies throughout the Universe. The newfound black holes, formed by an unknown process, are 100 to 10,000 times as massive as the Sun, yet each occupies less space than the Moon. Right: M82 is a nearby galaxy now thought to harbor a middling-weight black hole in its nucleus. It is a member of a group of galaxies dominated by itself, M81, and NGC 3077. M82 is thought by some to be limping away from a close encounter with M81. This galactic collision might have stirred up the inner stars and gas in M82, causing the unusual dark lanes of dust visible in the above photograph. More information
The astronomers identified the new class of black holes through X-ray light, the final cries of energy emitted from gas and particles spiraling into a black hole. The discovery will be announced today at the meeting of the High Energy Astrophysics Division of the American Astronomical Society in Charleston, SC.
"Our intent was to understand what was producing an unusual class of X-ray luminosities near the centers of many galaxies," said Colbert. "With data from the Einstein satellite from the 1970s, we couldn't determine whether they had features associated with supermassive black holes or stellar black holes. So we took a fresh look with newer data." Colbert and Mushotzky found telltale clues for a new type of black hole in the spectrum, or colors, of the invisible X-ray light. Such colors are judged by comparing the intensity of X- rays with shorter wavelengths to those with longer wavelengths, just as blue skylight is mostly composed of shorter wavelengths than the light from a red sunset. 
Supermassive black holes are thought to power a phenomenon
called Active Galactic Nuclei, which are extremely compact and
energetic objects seen in the core of one percent of all galaxies
and are typically very bright X-ray sources. The luminosities
that Colbert and Mushotzky analyzed have colors different from
those found in Active Galactic Nuclei, suggesting the source is
something other than a typical supermassive black hole.
Right: NGC 4945 is a spiral galaxy in the Centaurus Group of galaxies, located only six times further away than the prominent Andromeda Galaxy. NGC 4945 is thought to be quite similar to our own Milky Way Galaxy. X-ray observations reveal, however, that NGC 4945 has an unusual, energetic, Seyfert 2 nucleus that might house a large black hole. More information Ptak and Griffiths acted on the belief among astronomers that black holes of various sizes must exist and likely reside in "irregular" galaxies (galaxies not spiral or elliptical in shape). M82 is one such galaxy, called a starburst galaxy because of the high rate of star formation found inside. Such a scenario leads to a higher rate of supernovae, or star explosions, the precursor of stellar black holes. "Millions of black holes and neutron stars have formed in M82 over the last 10 million years," Ptak said. "Now, we are noticing that some of these may be coalescing into a larger-mass black hole." Ptak said this is the most viable current theory for intermediate black hole formation. Colbert also said the intermediate class suggested by his and Mushotzky's observations might be formed by "the continual merging of stellar black holes." In other words, stellar black holes that approach each other too closely under certain circumstances can merge to form a more massive single black hole. This process might build objects that produce the peculiar colors of these X-ray glows. Ptak and Griffiths used data from the Japan-U.S. Advanced Satellite for Cosmology and Astrophysics (ASCA). Colbert and Mushotzky used data from the German/US/UK ROSAT satellite and ASCA. Japanese researchers led by Dr. Tsunefumi Mizuno at the University of Tokyo have reported results similar to Colbert and Mushotzky's. Dr. Takehishi Go Tsuru at Kyoto University and colleagues have found data supporting Ptak and Griffiths' work. |
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NASA Press release - Apr. 13, 1999 Unusual M82: The Cigar Galaxy - Astronomy Picture of the Day, Mar. 15, 1998 Nearby Spiral Galaxy NGC 4945 - Astronomy Picture of the Day, Apr 12, 1999 To learn more about black holes, browse Yahoo!'s listing of black hole links. |
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