New Class of "Mid-Size" Black Holes Discovered
Contact:
Christopher Wanjek
wanjek@gsfc.nasa.gov
301-286-4453April 13, 1999
Charleston, S.C. -- Astronomers at NASA and Carnegie Mellon University have independently found evidence for the new type of "mid-size" black holes in spiral-shaped galaxies throughout the Universe. The newfound black holes are 100 to 10,000 times as massive as the Sun yet occupy a region smaller than the Moon.
The astronomers identified the 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 the subject of a press conference at the meeting of the High Energy Astrophysics Division of the American Astronomical Society in Charleston, S.C., April 13, 1999 at 10:20AM EST.
A black hole is a region of space where the force of gravity is so powerful that nothing, not even light, can escape its pull. Until now, scientists knew about two types of black holes: stellar and supermassive. Stellar black holes are the remains of dead stars several times heavier than the Sun, compressed to a diameter of a few miles or less. Supermassive black holes have mind-boggling masses of one million to one billion Suns and may have formed in the early universe from giant gas clouds or from the collapse of clusters of immense numbers of stars.
Drs. Edward Colbert and Richard Mushotzky, astronomers at NASA's Goddard Space Flight Center, Greenbelt, Md., first saw hints of the new class of black holes while studying X-rays from 39 relatively nearby galaxies. Drs. Andrew Ptak and Richard Griffiths at Carnegie Mellon University studied X-ray light from a galaxy not included in Colbert and Mushortzky's set, galaxy M82. Both teams found unique X-ray light indicative of a new black hole class.
"Our intent was to understand what was producing an unusual class of X-ray luminosities near the centers of many galaxies," said Colbert. "With older 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 "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 (AGN), which are extremely bright X-ray sources seen in the core of one percent of all galaxies. The luminosities that Colbert and Mushotzky analyzed have colors different from those found in AGN. This suggested that something other than a typical supermassive black hole might be responsible.
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 supernova, 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." This, Ptak said, is the most viable theory at the moment for intermediate black hole formation. Colbert also suggested that the intermediate class 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 eventually build objects that produce the intermediate-strength 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 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.
Colbert is a research associate with the National Academy of Sciences/ National Research Council. Mushotzky is an astrophysicist in the Laboratory for High Energy Astrophysics at NASA/Goddard. Ptak is a research scientist and Griffiths is an astrophysics professor at Carnegie Mellon University. The results from both teams will be published in the Astrophysical Journal and the Astrophysical Journal Letters, respectively.
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