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Media Relations
University of California-Berkeley

Media Contacts:

Robert Sanders
(510) 643-6998, (510) 642-3734
rls{at}pa.urel.berkeley.edu

FOR IMMEDIATE RELEASE: Thursday, April 10, 2003

"Skinny" galaxy harbors massive black hole at core
By Robert Sanders, Media Relations

BERKELEY -- Scientists have uncovered a supermassive black
hole at the core of a svelte, spiral galaxy, a finding
that questions a recently devised rule of thumb in which
only galaxies with bulging cores have such black holes.

Dr. Alex Filippenko, professor of astronomy at the
University of California, Berkeley, and Dr. Luis Ho,
an astronomer at the Observatories of the Carnegie
Institution of Washington, in Pasadena, discuss these
results in the May 1 issue of The Astrophysical Journal
Letters. 

The scientists determined that galaxy NGC 4395, a flat
"pure-disk" galaxy with no central bulge, has a central
black hole approximately 10,000 to 100,000 times the
mass of our sun. This suggests that other pure-disk
galaxies, thought to be devoid of supermassive black
holes, may indeed have one lurking within -- quite
possibly the featherweights of the supermassive black
hole club. 

"The supermassive black hole in NGC 4395 is the
smallest one yet found in the center of a galaxy,"
said Filippenko. "This would be consistent with the
galaxy having a small bulge. However, the bulge is
not just small, it seems to be nonexistent."

Supermassive black holes typically range from millions
to billions of times the mass of the sun, dwarfing the
more common stellar black holes that are created by
the runaway gravitational collapse of the cores of
massive stars.

NGC 4395 is 11 million light years away in the Northern
Hemisphere constellation of Canes Venatici ("the
Hunting Dogs"). The galaxy has long been known to emit
significant amounts of light, including visible light
and X rays, from its central core region. This is a
telltale sign of the presence of a central, giant
black hole actively sucking in enough matter to create
a swirling, superheated accretion disk. Yet, no
supermassive black hole had been detected.

Filippenko and Ho used the Keck I 10-meter telescope
on Mauna Kea in Hawaii and the Japanese-U.S. ASCA
X-ray telescope to determine that NGC 4395 indeed
has a supermassive black hole, albeit an unusually
light one.

"Here is an example of a massive black hole that is low
in comparison to all previously reported supermassive
ones, but it is definitely much more massive than
stellar-class black holes, and is located in a galaxy
that has no bulge," Ho said. "Thus, having a well-
developed bulge is evidently not a necessary condition
for the formation of massive, central black holes."

Filippenko and Ho confirmed the mass range using the
velocity dispersion technique for estimating the mass
of black holes, and in the process found that this
powerful tool for inferring black hole mass works on
scales far lower than previously thought.

Velocity dispersion refers to how fast, on average,
the stars in a galactic core region are orbiting
about the central black hole. It is akin to taking
an average speed of a swarm of bees circling about
their hive on paths (orbits) of differing size and
orientation. Although predicted theoretically back
in the 1990s, confirmation of a very tight
correlation between velocity dispersion and
supermassive black hole mass came in 2000. But until
now, that correlation was always assumed to apply to
black holes weighing in at millions or billions of
solar masses. Using this technique on the NGC 4395,
Filippenko and Ho found its mass to be 66,000 solar
masses -- nearly in the middle of the range found
using X-ray luminosity.

This means that the black hole mass/velocity
dispersion relation may apply even on scales of
relatively small star clusters that contain a central,
relatively light, supermassive black hole, Ho said.

The scientists also suggest that NGC 4395 may represent
a unique step in the evolution of supermassive black
holes, in which a bulge will develop as the black hole
grows. The velocity dispersion measurements were based
on a star cluster near the central black hole, a
possible indication that a bulge may someday form.

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