Middleweight Black Holes
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.
Read the NASA press release.
Continue to Discovery of Black Holes Review.
FROM A NASA HQ PRESS RELEASE
A New Class of Black Holes?
Astronomers may have discovered a new type of middle-weight black
hole in the centers of some galaxies.
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
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
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.
Continue to Discovery of Black Holes Review.
NASA Science News - Press Release