(Background) This artist's conception depicts one possible scenario for the "central engine" of an active galaxy. Two jets of matter are shown moving outward perpendicular to a flattened accretion disk surrounding a supermassive black hole.
(Inset A) Numerical simulations performed on supercomputers can help us visualize key events in the universe. Here a uniform distribution of some thousand stars is spread over a dimension of several hundred parsecs.
(Inset B) As the simulation proceeds, the hypothetical stars congregate preferentially toward the core of the star cluster, forming a "luminosity cusp" of light.
(Inset C) If we could see inside the cusp, an accretion disk would probably be evident, swirling around a giant black hole.
(Inset D) As the accretion disk rotates, tilts, and acquires more matter, jets of matter emerge to cool the environment surrounding the hole.
Studying this chapter will enable you to:
Specify the basic differences between active and normal galaxies.
Describe the important features of Seyfert and radio galaxies.
Explain what drives the central engine thought to power all active galaxies.
Describe the observed properties of quasars and discuss the special properties of the radiation they emit.
Discuss the place of active galaxies in current theories of galactic evolution.
Our journey from the Milky Way to the Great Wall in the past two chapters has widened our cosmic field of view by a factor of 10,000, yet the galaxies that make up the structures we see show remarkable consistency in their properties. The overwhelming majority of galaxies fit neatly into the Hubble Classification Scheme, showing few, if any, unusual characteristics. However, sprinkled through the mix of normal galaxies, even relatively close to the Milky Way Galaxy, are some that are decidedly abnormal in their properties. Although their optical appearance is often quite ordinary, these abnormal galaxies emit huge amounts of energy—far more than a normal galaxy—mostly in the invisible part of the electromagnetic spectrum. Observing such objects at great distances, we may be seeing some of the formative stages of our own galactic home.
