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Optical
image of the central part of the Centaurus A Galaxy where a
supermassive black hole hides.
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During
the past three decades, astronomers have found that the centers
of giant galaxies are often locales of immense violence, from which
enormous bursts of energy flood outward. In many cases, this energy
is in the form of synchrotron radiation, produced by a process first
observed in particle accelerators called synchrotrons. This process
occurs when charged particles move at nearly the speed of light
through strong magnetic fields. Therefore, wherever radiation is
produced by the synchrotron process, we can infer that violent events
have recently accelerated particles there to velocities very close
to the speed of light (300,000 kilometers per second). The synchrotron
process can produce all types of radiation, from high-energy gamma
and x rays down to low-energy radio waves. Consequently, it can
be studied with a host of different instruments, each capable of
gaining a different perspective on the objects that produce this
radiation.
The
most intense outflows of radiation at the centers of some galaxies
apparently arise from the effects produced by supermassive black
holes. Such objects,no larger than our solar system, contain a mass
hundreds of millions of times greater than the Sun's. A supermassive
black hole bends the space nearby, attracting matter that spirals
at ever-increasing speed before disappearing within the event horizon,
at a distance from the black hole’s center that marks a point of
no return. Surrounding its event horizon, a supermassive black hole
develops an accretion disk of orbiting matter, within which particle
collisions occur at enormous velocities. From such an accretion
disk, the central region of a large galaxy can produce more energy
per second than the entire output of an ordinary large galaxy!
In
most cases, the accretion disk diverts some of the matter spiraling
inward, producing twin jets that eject matter at enormous velocity
in opposite directions above and below the disk.When we have a side
view of these jets, we see large amounts of radiation. But when
we happen to look directly down one of them, along the fire hose
of emission from the vicinity of the black hole, we receive a truly
enormous blast of radiation, fortunately dimmed because the jets
are so far away from us.
During
the coming decade, with advanced instruments, we will be able to
make better observations of the accretion disk that surrounds the
supermassive black hole at a giant galaxy’s center. These observations
will allow us to improve our currently modest understanding of how
the disk diverts the material to produce jets of outflowing matter
in opposite directions. Detailed studies of how particles move close
to these black holes will allow us to test the predictions of Einstein’s
general theory of relativity. This test will go beyond the “weak
limit ” that describes most objects in the cosmos to probe phenomena
in gravitational fields sufficiently strong to create huge warps
in space.
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