Astronomers Get Closest Look Yet At Milky Way's Mysterious Core(Forwarded)

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EMBARGOED For Release: 1:00 p.m., EST, Wednesday, 2 November 2005

Astronomers Get Closest Look Yet At Milky Way's Mysterious Core

Astronomers have gotten their deepest glimpse into the heart of our Milky
Way Galaxy, peering closer to the supermassive black hole at the Galaxy's
core then ever before. Using the National Science Foundation's
continent-wide Very Long Baseline Array (VLBA), they found that a
radio-wave-emitting object at the Galaxy's center would nearly fit between
the Earth and the Sun. This is half the size measured in any previous
observation.

"We're getting tantalizingly close to being able to see an unmistakable
signature that would provide the first concrete proof of a supermassive
black hole at a galaxy's center," said Zhi-Qiang Shen, of the Shanghai
Astronomical Observatory and the Chinese Academy of Sciences. A black hole
is a concentration of mass so dense that not even light can escape its
powerful gravitational pull.

The astronomers used the VLBA to measure the size of an object called
Sagittarius A* (pronounced "A-star") that marks the exact center of our
Galaxy. Last year, a different team announced that their measurements
showed the object would fit inside the complete circle of Earth's orbit
around the Sun. Shen and his team, by observing at a higher radio
frequency, measured Sagittarius A* as half that size.

A mass equal to four million Suns is known to lie within Sagittarius A*,
and the new measurement makes the case for a black hole even more
compelling than it was previously. Scientists simply don't know of any
long-lasting object other than a black hole that could contain this much
mass in such a small area. However, they would like to see even stronger
proof of a black hole.

"The extremely strong gravitational pull of a black hole has several
effects that would produce a distinctive 'shadow' that we think we could
see if we can image details about half as small as those in our latest
images," said Fred K.Y. Lo, Director of the National Radio Astronomy
Observatory and another member of the research team. "Seeing that shadow
would be the final proof that a supermassive black hole is at the center
of our Galaxy," Lo added.

Many galaxies are believed to have supermassive black holes at their
centers, and many of these are much more massive than the Milky Way's
black hole. The Milky Way's central black hole is much less active than
that of many other galaxies, presumably because it has less nearby
material to "eat." Astronomers believe that the radio waves they see
coming from Sagittarius A* are either generated by particle jets that have
been detected in many more-active galaxies or from accretion flows that
are spiraling into the central black hole. By observing the object at
higher radio frequencies, scientists have detected a region of radiation
ever closer to the black hole. The results announced last year were based
on observations at 43 GigaHertz (GHz), and the latest observations were
made at 86 GHz.

"We believe that if we can double the frequency again, we will see the
black-hole shadow produced by effects of Einstein's General Relativity
theory," Lo said.

In a few years, when the Atacama Large Millimeter Array (ALMA) comes on
line, it may be used in conjunction with other millimeter-wave telescopes
to make the higher-frequency observations that will reveal the telltale
black-hole shadow.

At a distance of 26,000 light-years, the Milky Way's central black hole is
the closest such supermassive object. That makes it the most likely one to
finally reveal the concrete evidence for a black hole that astronomers
have sought for years.

Shen and Lo worked with Mao-Chang Liang of Caltech, Paul Ho of the
Harvard-Smithsonian Center for Astrophysics (CfA) and the Institute of
Astronomy & Astrophysics of the Academia Sinica in Taiwan, and Jun-Hui
Zhao of CfA. The astronomers published their findings in the November 3
issue of the scientific journal Nature.

The National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc.

IMAGE CAPTION:
[http://www.nrao.edu/pr/2005/sagastar/sgraW1ly.jpg (38KB)]
The Milky Way's nucleus, as seen with the VLA. Sagittarius A* is the
bright white dot at center. CREDIT: NRAO/AUI/NSF, Jun-Hui Zhao, W.M. Goss