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A Pancake, Not A Doughnut, Shapes Distant Galactic Center (Forwarded)



 
 
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Old July 23rd 03, 05:21 PM
Andrew Yee
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Default A Pancake, Not A Doughnut, Shapes Distant Galactic Center (Forwarded)

Public Affairs Department
Harvard-Smithsonian Center for Astrophysics
Cambridge, Massachusetts

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Harvard-Smithsonian Center for Astrophysics
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Harvard-Smithsonian Center for Astrophysics
Phone: 617-495-7463, Fax: 617-495-7016


For Release: July 22, 2003

Release No.: 03-17

A Pancake, Not A Doughnut, Shapes Distant Galactic Center

Cambridge, MA -- While a person's shape can be affected by pancakes, especially
if you eat too many, you may not expect the same to be true on a cosmic scale.
As it turns out, at least for the Circinus spiral galaxy, a pancake can shape an
entire galactic nucleus. Astronomer Lincoln Greenhill (Harvard-Smithsonian
Center for Astrophysics) and colleagues have found direct evidence for a
"pancake" of gas and dust at the center of Circinus -- a thin, warped disk
surrounding the galaxy's central, supermassive black hole.

That disk shapes the galaxy's nucleus. It shadows different regions from the
"glare" of the black hole, a glare created by the glow of accreting gas. And
when some of this material is blown away from the black hole, as by radiation,
the disk channels it, leaving shadowed regions in relative peace. This idea
stands in contrast to the prevailing wisdom that shadows and outflows are caused
by vast, thick "doughnuts" of dust and gas.

"We caught the Circinus galaxy and its black hole red-handed," said Greenhill.
"Most astronomers think that the center of an active galaxy has an outflow
directed and channeled by a doughnut-shaped torus of dust and gas. Our detailed
radio images show that the culprit is a warped disk. And if that's true for the
Circinus galaxy, then the same may be true for other active galaxies."

Greenhill and his fellow astronomers identified the disk using the Australia
Telescope Long Baseline Array, which is a network of radio telescopes 600 miles
across. Only radio imaging can reveal directly such tiny structures inside
galactic nuclei. The Circinus disk in particular is so deeply buried in a jumble
of stars, gas, and dust that no optical telescope can detect it. They estimate
the disk contains enough mass to form perhaps as many as 400,000 stars like our
Sun, were it given a chance.

The Australian array picked up microwave signals from clouds rich in water vapor
within both the warped edge-on disk and the outflow. The locations and
velocities of the clouds provide strong evidence that the disk is channeling
ejected material into two broad cones extending above and below the galactic plane.

"Water masers have been observed in broad, wide-angle outflows in star formation
regions within our Galaxy, but this is the first time they have been observed
associated with the nuclear region of an active galaxy," said Simon Ellingsen
(University of Tasmania), a co-author of the study. "These observations also are
the first to show that this wide-angle outflow originates within about a third
of a light-year from the galactic nucleus."

A black hole is a massive object so compact and with such a powerful
gravitational field that nothing can escape its pull once past the black hole's
event horizon. However, material can and does escape from regions near the black
hole due to radiation pressure and inefficiencies of the accretion flow, among
other things. The escaping material carries away angular momentum, allowing the
remaining matter to fall into the black hole. The black hole in Circinus
presents a stark contrast to other supermassive black holes whose outflows are
channeled into long, narrow jets of material that blast out from the galactic
nucleus.

"In the center of the Circinus galaxy, we see a black hole that spews out gas
and dust in a broad spray like clouds of vapor from a steam locomotive. This
presents us with a paradox. X-ray radiation from the nucleus of Circinus --
radiation driven by the black hole -- is as intense as for black holes in other
active galaxies. In that way, the Circinus black hole appears to be typical.
However, while other black holes drive narrow relativistic jets of plasma, the
Circinus black hole drives a comparatively meek wind -- one that can support the
formation of delicate molecules and dust," said Greenhill.

Greenhill and his colleagues plan to continue studying the nucleus of the
Circinus galaxy to investigate the mechanism responsible for generating the outflow.

NOTE TO EDITORS: A high-resolution artwork image is available at
http://cfa-www.harvard.edu/press/pr0317image.html

This research was published in the June 10, 2003 issue of The Astrophysical Journal.

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA scientists
organized into six research divisions study the origin, evolution, and ultimate
fate of the universe.


 




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