Black Hole at Milky Way's Center is on Starvation Diet

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


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