Donald Savage
Headquarters, Washington, DC                     April 13, 1999
(Phone:  202/358-1547) 

Bill Steigerwald
Goddard Space Flight Center, Greenbelt, MD
(Phone:  301/286-5017)

RELEASE:  99-51

ASTRONOMERS DISCOVER 'MIDDLEWEIGHT' BLACK HOLES

       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.

                             -end-

EDITOR'S NOTE:  Images to support this story are available on the 
web at:  FTP://PAO.GSFC.NASA.GOV/newsmedia/HEAD/NBH