Discovering Middleweight 
Black Holes

• Middleweight Black Holes
• NASA Press Release

    In 1999, evidence for a class of black holes with mass between 100 and 10,000 solar masses was independently discovered by Edward Colbert and Richard Mushotzky of NASA Goddard Space Flight Center, and Andrew Ptak and Richard Griffiths of Carnegie Mellon University.  The  black holes were identified by X-ray emission wavelengths different from those of stellar black holes or supermassive black holes. 

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. 

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. 

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. 

Dr. Edward Colbert and Dr. 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. Dr. Andrew Ptak and Dr. Richard Griffiths at Carnegie Mellon University studied X-ray light from a galaxy not included in Colbert and Mushotzky's set, galaxy M82. Both teams found unique X-ray light indicative of a new black hole class. The results from both teams will be published in the Astrophysical Journal and the Astrophysical Journal Letters, respectively. 

"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.

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.