A Report from The Galactic Center Workshop “The Central 300 Parsecs”

The center of our Galaxy is a dynamic
interplay of interdependent activity where
stellar processes play a far more active role
than scientists originally believed. The big
surprise is that these processes leave our
Galaxy’s central black hole starving for
“food,” a new study suggests.

“It’s almost as if the Galactic Center were a
living organism,” Dr. Heino Falcke told a
group of more than 100 astronomers and
astrophysicists at the Galactic Center
Workshop being held this week (Nov. 4-8) in
Kona, Hawaii.

Dr. Falcke made his introductory remarks on
what astronomers are learning about the
center of the Milky Way on the opening day of
the workshop, entitled “The Central 300
Parsecs of the Galaxy” being held at the
Keauhou Beach Resort in Kailua-Kona, Hawaii.
(Three hundred parsecs is equal to
approximately 1,000 light years across.)

The international conference is sponsored by
the Gemini Observatory, the National Science
Foundation and several other observatories on
Mauna Kea.

More than 50 papers will be presented
concerning research and findings of
scientists from all over the world who have
made our Galactic Center one of the primary
areas of astrophysical research.

“The Galactic Core is like a boiling cauldron
of raw creation, where all components such as
stars, cold and hot gas, magnetic fields and
the central black hole show an intimate
interplay,” said Dr. Falcke.

A scientist with the Max Planck Institute for
Radio Astronomy in Bonn, Germany, and a
professor at the University of Nijmegen,
Falcke is a co-editor of the “Galactic Center
Newsletter” and has spent the last ten years
studying the center of our Galaxy, developing
one of the leading models for the central
black hole.

One of the most significant aspects of
discussions that is emerging from the
conference is the role that an enormous black
hole at the center of our Galaxy plays in the
present-day life of our Galaxy. The one-
million-mile-wide black hole is embedded in a
“boiling cauldron” of hot gas heated by stars
created in the center of the Galaxy. “It now
appears that the black hole seems to be
rather isolated and starved,” Dr. Falcke
said.

New results on the supermassive black hole at
the Galactic Center were presented at the
conference on Wednesday by Dr. Geoffrey Bower
and his colleagues, show that the infall rate
of matter onto the black hole is much smaller
than previously expected.

“Rather than being a powerful monster in the
Galactic Center, this black hole is more like
the Cowardly Lion,” said Dr. Bower. Dr. Bower
is a researcher at University of California,
Berkeley who has studied the radio emissions
of the central black hole for several years.

“It has been a mystery for a long time that
we have seen so little light from the
Galactic Center black hole – our observations
now show that the black hole is on a
starvation diet,” he said.

Dr. Bower used the Berkeley-Illinois-Maryland
Association array (BIMA) to measure the
effect of infalling gas on the polarized
radiation that is emitted very close to the
surface of the black hole. The estimated
infall rate derived from these observations
is 1,000 times less than previous studies had
suggested. These new findings effectively
explain the extremely low luminosity of the
central black hole which had previously
puzzled scientists.

For a long time it was believed that the
accretion (inflow) rate onto the black hole
should be as high as ten Earth masses per
year. The new studies being reported at the
Galactic Center Workshop indicate that the
rate is much less than 0.1 percent of the
mass of the Earth which is swallowed per year
(or one-ten-millionth of a Solar mass per
year). The gravitational energy of the
infalling matter is converted into radiation
as it approaches the black hole.

“This is a very significant result which has
resolved a long standing question in galactic
center and black hole research,” adds Dr.
Falcke, who is a co-author with Dr. Bower of
the paper submitted to the Astrophysical
Journal.

In their observations, the scientists
detected surprisingly strong polarized
emission at high radio frequencies that was
emitted from the immediate vicinity of the
black hole. Using a well-known effect called
“Faraday rotation” which changes the
polarization of radio emission as it travels
through intervening material, they were able
to estimate the amount of material in the so-
called accretion flow around the Galactic
Center black hole.

The BIMA array is located in Hat Creek,
Calif., and operated by UC Berkeley, the
University of Illinois and the University of
Maryland.

Co-authors of Bower=92s paper are Falcke, and
Don Backer and Mel Wright of UC Berkeley.

Related Image and Movie:

Gemini Adaptive Optics image of Galactic
Center region:
http://www.gemini.edu/project/announcements/press/geminigc.html

Movie of dive into the black hole:
http://www.mpifr-bonn.mpg.de/staff/hfalcke/bhflight.html