Generating Energy

We saw in the last chapter that the rotation curves of many galaxies show evidence for large mass accumulations in the central regions. In fact, the masses contained are so large and in such a small volume that astronomers have in many cases concluded that a black hole must be present at the galaxy's center.

If a black hole were present in the center of active galaxies (and the quasars that they might have evolved from), many of the observed properties of these strange galaxy types might be explained. For one, the tremendous nonstellar energy output originating in a compact area points to the gravitational field and accretion disk of a black hole.

But not just any black hole.

These black holes dwarf the stellar remnant black holes that we discussed in Chapter 19, "Black Holes: One-Way Tickets to Eternity," and are called supermassive black holes.

The center of our own Galaxy, the Milky Way, may contain a black hole with a million (or more) solar masses. To account for the much greater luminosity of an active galaxy, the mass must be higher—perhaps a billion solar masses.

If black holes are the engines, we can explain the luminosity of active galaxies and quasars as the result of gas that spirals toward the black hole at great velocity, becoming heated in the process and producing energy in the form of electromagnetic radiation, x-ray photons are often produced and these are visible with the new Chandra X-ray Observatory.

In the black hole model, the radio jet arises when the hot gas streams away from the accretion disk in the direction with the least resistance—perpendicular to the accretion disk. These jets, then, can stream away from the disk in two directions, giving rise to the oppositely directed jets.

Remember the fluctuations in brightness seen in Seyfert galaxies and quasars? These fluctuations may be explained by brightness fluctuations in the accretion disk—the swirling disk of gas spi-raling toward the black hole.

Astronomer's Notebook

Remember the fluctuations in brightness seen in Seyfert galaxies and quasars? These fluctuations may be explained by brightness fluctuations in the accretion disk—the swirling disk of gas spi-raling toward the black hole.

This image shows the jet associated with the quasar 3C273. On the left is the optical image (from the Hubble Space Telescope). In the middle is the Chandra X-ray Observatory image, and on the right is the radio image of the jet as seen by the Multi-Element Radio Linked Interferometer Network (MERLIN). Notice that the different wavelengths emphasize different parts of the jet.

(Image from NASA, Chandra and Merlin)

If black holes exist at the centers of all or most galaxies, then the differences between quasars, radio galaxies, Seyfert galaxies, and normal galaxies is not the engine, but the fuel. When fuel is plentiful (as it is early in the universe), the cores of all galaxies might burn bright as a quasar. Then they would move through a quieter, yet still active phase as Seyfert or radio galaxies. Finally, at our time in the universe, with fuel less plentiful, most black holes lie dormant. Models of galaxy evolution are still hotly debated (at least by astronomers), but the existence of central black holes may answer many questions.

A closing thought, then. Let's say you take the trouble to find the quasar 3C 273 for yourself. You see a quasar—but the light that you see left the galaxy some 2 billion years ago. The quasar you see may now (now as it is experienced at the distant location of the 3C 273) be a more mature galaxy, not unlike the Milky Way. Perhaps on some planet orbiting an average star somewhere in the distant universe, an amateur astronomer is pointing a telescope at our Milky Way, which appears to him or her as a faint, bluish blob of light: a quasar.

In the following chapters, we ask some of the big questions, the first of which is "Are We Alone?" And we will see just how likely it is that someone else might be watching us.

The Least You Need to Know

V Quasars were first discovered at radio wavelengths and are some of the most distant astronomical objects visible.

V Quasars may be the ancestors of all galaxies, the violent beginnings of us all.

V Active galaxies are any galaxies that more luminous than what we call normal galaxies, with bright star-like cores and broad, strong emission lines.

V Seyfert galaxies are the active subset of spiral galaxies, and radio galaxies are the active subset of elliptical galaxies.

V Radio jets originate at the cores of active elliptical galaxies and terminate in wispy patches of emission called lobes.

V Supermassive black holes are the most likely source of energy for quasars and active galaxies.

Telescopes Mastery

Telescopes Mastery

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