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. 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. Also, in many other galaxies,
the gravitational energy of the black hole is powering superfast
“jets” of subatomic particles at nearly the speed of light. Such
jets in other galaxies extend outward for thousands of light-years.
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* probably
are generated by particle jets much shorter than those of
more-active galaxies. By observing the object at higher
radio frequencies, scientists have detected parts of the
jets ever closer to the black hole. The results announced
last year were based on observations at 43 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.
