Donald Savage
Headquarters, Washington      September 17, 2002
(Phone: 202/358-1547)

Nancy Neal
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-0039)

Ray Villard
Space Telescope Science Institute, Baltimore
(Phone: 410/338-4514)

RELEASE: 02-174

HUBBLE DISCOVERS BLACK HOLES IN UNEXPECTED PLACES

     Medium-size black holes actually do exist, according to 
the latest findings from NASA's Hubble Space Telescope, but 
scientists had to look in some unexpected places to find 
them.

The previously undiscovered black holes provide an important 
link that sheds light on the way black holes grow. Even more 
odd, these new black holes were found in the cores of 
glittering, "beehive" swarms of stars -- called globular star 
clusters -- that orbit our Milky Way and other galaxies. 

The new findings promise a better understanding of how 
galaxies and globular clusters first formed billions of years 
ago. Globular star clusters contain the oldest stars in the 
universe. If globulars have black holes now, then globulars 
most likely had black holes when they originally formed. The 
new results indicate that the very sedate, elderly 
environments of globular clusters house these exotic objects, 
quite unlike the violent cores of some galaxies.

"These findings may be telling us something very deep about 
the formation of star clusters and black holes in the early 
universe," said Roeland Van Der Marel of the Space Telescope 
Science Institute in Baltimore. "Black holes are even more 
common in the universe than previously thought."

"Not only will we learn about the formation of the black 
holes, but these new data from Hubble help us connect 
globular clusters to galaxies, providing information on one 
of the most important unsolved problems in astronomy today: 
how galaxy structure forms in the universe," added Michael 
Rich of the University of California, Los Angeles.

This is reinforced by the uncanny fact found by these 
investigations that a black hole's mass is proportional to 
the mass of the stellar environment it inhabits. Supermassive 
black holes found by Hubble in the centers of galaxies 
represent about 0.5 percent of the galaxies' mass.  
Amazingly, the black holes now found in star clusters, which 
are 10,000 times less massive than a galaxy, also obey this 
trend. It appears that there is some yet-to-be-discovered 
underlying process that ties a black hole to its host in a 
fundamental way. Nature is providing a big clue as to how 
these systems and their black holes form.

"The intermediate-mass black holes that have now been found 
with Hubble may be the building blocks of the supermassive 
black holes that dwell in the centers of most galaxies," said 
Karl Gebhardt of the University of Texas at Austin.

Van Der Marel led a team that uncovered a black hole in the 
center of the globular star cluster M15, 32,000 light-years 
away in the constellation Pegasus. His collaborator Joris 
Gerssen, also of the Space Telescope Science Institute, 
pinned down the black hole's mass at 4,000 times that of our 
Sun. 

In a separate observing program, a team led by Rich, and 
including Gebhardt and Luis Ho of the Carnegie Institution of 
Washington, found a 20,000-solar-mass black hole in the giant 
globular cluster G1, located 70 times farther -- 2.2 million 
light-years away -- in the neighboring Andromeda galaxy. By 
contrast, stellar-mass black holes are only a few times the 
mass of our Sun, and galactic-center black holes can be 
millions or billions of times more massive than our Sun.

Previously, X-ray observations from the ROSAT Observatory and 
NASA's Chandra Observatory have identified ultra-bright X-ray 
sources that could also be interpreted as intermediate-mass 
black holes in star-forming galaxies. However, alternative 
interpretations for these X-ray sources continue to exist. By 
contrast, Hubble's measurements are based on the velocities 
of stars whirling around in the dense cores of globular 
clusters, which yield a direct measurement of the black hole 
masses.

The M15 globular star cluster is close enough that individual 
star speeds can be measured. By contrast, the G1 observations 
rely on measurements of the collective properties of many 
stars. In either case, a black hole can be identified by 
using a common Hubble black-hole-hunting technique. Stars 
close to the black-hole "whirlpool" orbit at a faster rate. 

Astronomers have searched for black holes in globular 
clusters for nearly 30 years. The roadblock has been the fact 
that ground-based telescopes cannot easily resolve the stars 
closest to the suspected black hole. As far back as the 
1970s, hunting for globular-cluster black holes was 
recognized as a task suited for Hubble Space Telescope's 
exquisite resolution, which is needed for looking close to a 
black hole. 

Images and additional information are available at: 
http://oposite.stsci.edu/pubinfo/pr/2002/18
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