Black Holes

So where does the flow of matter in the jets come from? What can eject two jets of material traveling nearly at the speed of light, and why would it continue to do this for millions of years ? The answer comes from recognizing that the observed properties of the sources define the underlying nature of the hidden "engine" driving the radio source. Whatever it is, the engine shows two preferred directions, oppositely oriented in the sky. It is also very steady. Such a thing is a spinning ob ject, acting like a gyroscope, which can keep spinning very steadily for very long periods of time unless acted on by some external force tending to pull it out of alignment.

The invisible object is actually very small, but enormously massive. The only type of astronomical object that can satisfy the demands of the observations is a gigantic black hole. A black hole containing 10 million solar masses would be 3 light-min across, or approximately the size of Venus' orbit about the sun. A black hole containing 5 billion solar masses, such as those believed to exist at the centers of some radio galaxies, may be 28 light-h across, or more than twice the size of the solar system.

A spinning black hole literally distorts the space around it, and any matter that comes relatively close will feel its tug, just as any particle feels the tug of gravitating objects in its neighborhood. For example, interstellar gas near the object will move inward and will first settle into an accretion disk, which spins around the black hole. Interactions between particles of gas will force them to settle into this disk and the same forces will cause the rapidly orbiting material to move gradually closer to the central hole. The gas will grow hotter as more and more energy is created due to collisions and interactions between particles in the swirling disk around the black hole. This gas will grow so hot, as much as a billion degree K, that it will actually expand and form a fat torus—a doughnut-shaped region—rapidly spinning around the black hole. This is illustrated in Figure 12.3. Inside this torus

Figure 12.3. A schematic model for radio galaxies and quasars, both of which contain radio loud active galactic nuclei (AGNs), which explains most of the properties of these objects. A black hole at the core is surrounded by luminous material in an accretion disk shaped like a thick torus or doughnut. Narrow radio emitting jets of matter traveling close to the speed of light streak outward while small clouds of gas orbit the central region to produce optically observable broad emission lines, farther out and indicated by the light colored objects, other orbiting clouds produce narrow emission lines. As they fall in they will be swept into the accretion disk. (Image courtesy of C. Megan Urry.) This image originally appeared in the Publications of the Astronomical Society of the Pacific (Urry, C. M. and Padovani, P., 1995, PASP, 107, 803). Copyright 1995, Astronomical Society of the Pacific; reproduced with permission of the Editors.

Figure 12.3. A schematic model for radio galaxies and quasars, both of which contain radio loud active galactic nuclei (AGNs), which explains most of the properties of these objects. A black hole at the core is surrounded by luminous material in an accretion disk shaped like a thick torus or doughnut. Narrow radio emitting jets of matter traveling close to the speed of light streak outward while small clouds of gas orbit the central region to produce optically observable broad emission lines, farther out and indicated by the light colored objects, other orbiting clouds produce narrow emission lines. As they fall in they will be swept into the accretion disk. (Image courtesy of C. Megan Urry.) This image originally appeared in the Publications of the Astronomical Society of the Pacific (Urry, C. M. and Padovani, P., 1995, PASP, 107, 803). Copyright 1995, Astronomical Society of the Pacific; reproduced with permission of the Editors.

will be magnetic fields that are literally tied to the black hole because some of the gas will have plunged into the hole and dragged the magnetic fields with it. Those magnetic fields at first remain connected to the gas outside and will rapidly wind up. Then, when they have become over-wound, they snap. As a result the fields will realign themselves, but since they are constantly being wound up they will snap again, and in this way energy from the rotating black hole is converted, through the magnetic field reconnection process, into the energy of particles where the field is so badly twisted and distorted. This process continues as long as the appetite of the black hole and the availability of gas allow it.

Around the poles of the black hole there is a critical funnel-shaped region of space in which matter finds it has two options. If it has insufficient energy it will plunge into the black hole and wave the universe goodbye. If, however, it has enough energy the particle may suddenly be free to escape from the funnel and blast out into space! This tends to happen in a series of outbursts in which blobs of matter are driven outward at near the speed of light

If the shape of this funnel is narrow enough, provided the torus of gas around the black hole is thick enough, this matter will escape as if ejected from a nozzle (Figure 12.3). A similar beam of high-energy particles leaving the nucleus of a spiral galaxy would tend to collide with the surrounding interstellar gas, so abundant in spiral galaxies, and this gas would obstruct the flow. Hence black holes at the centers of spiral galaxies don't usually create jet-like radio sources. They are more likely to be observed as Seyfert galaxies. Only in relatively gas-and dust-free elliptical galaxies will the gas stream outward and be likely to escape unimpeded, as observed in classic radio galaxies and quasars.

The energy of this ejected material comes from the black hole itself. Based on an efficiency of 10% for the energy generation process, the most active galactic nuclei in distant radio galaxies must have processed the equivalent mass of 100 million suns through a region not much larger than the solar system. Since the sources are believed to have lifetimes of about 100 million years, it requires only one solar mass to be processed per year and only a fraction of that escapes up the funnel. That is enough to cause the AGN to glow. The black hole has to be about 100 million solar masses to do the job of creating a double radio source.

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