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EMBARGOED FOR RELEASE ON
DECEMBER 15, 1999
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First Image of a Black Hole's "Shadow" May Be
Possible Soon
A "picture" of the massive black hole thought to be lurking at
the heart of our home galaxy may be within astronomers' reach in
the next few years, according to a report in the Jan. 1, 2000,
edition of "Astrophysical Journal Letters."
The paper predicts that upcoming improvements in scientific
techniques
could permit astronomers to see how a narrow escape from the
black hole's
clutches twists, dims, and amplifies radio waves.
Such observations should reveal a circular shadow at the heart of
the
galaxy -- the first image of a black hole's event horizon --
according to a
computer model created by theorists at The Johns Hopkins
University, the
Max-Planck-Institut fuer Radioastronomie in Germany, and the
University of
Arizona.
The event horizon is thought to be the defining feature of a
black hole, a
point-of-no-return surrounding the hole inside which even light
cannot
escape the black hole's gravity. Imaging this would be a final
step in the
black hole's journey from curious theoretical oddity to cosmic
reality.
"Regardless of the structure of the region around the black hole
that we
tried in our computer models, we saw a shadow in the simulated
images,"
says Eric Agol, a postdoctoral researcher at Hopkins and an
author of the
paper. "This paper is our way of trying to interest astronomers
in working
together to perform the actual observations, which could produce
very
exciting results."
Agol cautions that the same plasma that emits radio waves near
the black
hole might also block the radio waves needed to "see" the hole –
an effect
not included in the models. This could be circumvented by
observing at
even shorter wavelengths, where the plasma becomes transparent
and the
black hole shadow will appear. "This would make it harder to see
it from
the ground, but it should always be possible to see it from
space," Agol
says, noting that some shorter wavelengths are blocked by Earth's
atmosphere.
So far, scientists have only been able to indirectly detect black
holes by
observing their effects on the orbits of nearby stars or by
detecting the
powerful radiation given off by gas and other material being
pulled into
the black hole.
Astronomers have seen these effects in the centers of other
galaxies. The
Milky Way's center can't be seen in visible light because there's
too much
interstellar gunk in the 25,000 light years between Earth and the
galactic
center. But longer-wavelength radiation like infrared radiation
and radio
waves can make it through relatively unscathed.
"At Sagittarius A star [Sagittarius A*], a point at or near our
galaxy's
center, astronomers have found a compact source of very strong
radio
emission, perhaps created by highly ionized gas surrounding a
black hole,"
says Heino Falcke, research scientist at Max-Planck-Institut and
lead
author on the paper. "Infrared observations of the same region
show rapidly
moving stars pulled around by a very concentrated mass at the
same position
as the radio source Sagittarius A*. This is probably the best
evidence
that we have for a black hole so far, but not decisive proof."
To zoom in further on the radio wave emission in this area,
scientists have
used a technique known as Very Long Baseline Interferometry
(VLBI). By
coordinating and comparing the results they receive from
different radio
telescopes, they can produce an image with greater detail and
resolution
than the individual radio telescopes could on their own.
"The resolving power is equivalent to what you'd get if you had a
radio
telescope as large as the telescopes you're combining and the
area between
them," says Falcke. "This can be as large as the size of the
Earth."
Astronomers at the Max-Planck-Institut and elsewhere have been
working to
use VLBI to observe shorter wavelengths of radio emission, a
technique
known as millimeter-VLBI. By pushing VLBI to the shortest
wavelengths and
highest spatial resolutions available in astronomy, they have
already come
very close to the resolution that should be needed to see the
shadow.
"I think we didn't realize before how close the technique is to
detecting
this shadow," Falcke says. "With the currently available
resolution, we
could ‘see' from Berlin, Germany, a radio source in Los Angeles
the size of
a mustard seed. Now we have to improve things just to the point
where we
can image a dent on the seed."
"The improvements necessary to test this prediction are within
reach and
should become feasible over the next few years," says Anton
Zensus,
director at the Max-Planck-Institut and leader of the VLBI
group.
For the paper, the authors took what astronomers currently know
about the
mass of Sagittarius A* and plugged it and other potential
features of the
black hole, such as its rotation, into a "relativistic
ray-tracing" program
Agol had developed. The program traces the path of
electromagnetic
radiation through space warped by the tremendous gravity of a
black hole.
"You can think of it as taking each photon of radiation emitted
somewhere
near the black hole and following its path to the observer,"
explains
Fulvio Melia, astrophysicist from the University of Arizona and
co-author
on the paper. "The program calculates the effects of the black
hole on the
radiation's path and wavelength, effects that are very precisely
predicted
by Einstein's Theory of General Relativity."
"A similar, simplified calculation was made by physicist James
Bardeen in
the 1970s," says Agol. "At that time, we didn't have as much
information
on the galactic center, so his work was considered by many to be
a purely
theoretical exercise."
Given the resolution achievable at short radio wavelengths, the
new
calculations showed a distinctive pattern in radiation from
Sagittarius A*:
a circular shadow.
"With the major observatories working together, and a further
improvement
of millimeter-VLBI, we should soon be able to actually image the
shadow of
a black hole. This would be the final test of whether black holes
and event
horizons exist," says Falcke.
Since demand is high for time at radioastronomy observatories, he
acknowledges, that would take no small amount of money, effort
and
sacrifice. But because of the potentially tremendous step forward
this
effort might produce, he and the other authors strongly feel the
challenge
is worthwhile.
This research was supported by Melia's Sir Thomas Lyle Fellowship
and
grants from NASA, DFG (Deutsche Forschungsgemeinschaft), and the
National
Science Foundation.
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