What Happens At The Heart Of A Galaxy
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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