Introductory Astronomy: Active Galaxies

Radio astronomers find faint radio sources scattered all over the sky. These sources do not register on optical plates, leading us to believe that they are emitting enormous amount of radio energy even if they are too far away to be seen visually. Astronomers now believe that these distant radio sources are actually individual galaxies whose centers are emitting huge amounts of energy. These galaxies are called Active Galaxies, or AGN (Active Galactic Nuclei), because their nuclei are active.

There are several different kinds of active galaxies:

Each of these galaxies can be understood in terms of a unified model of Active Galactic Nuclei. In this model, active galaxies contain supermassive black holes at their centers. As surrounding galactic material moved toward the center, it would form an accretion disk around the black hole. Friction in the accretion disk could slow the material enough to cause the material to fall into the black hole. This would cause the inner part of the accretion disk to become very hot. The hot, inner part of the accretion disk would then be capable of producing jets of hot gas and radiation streaming out of the nucleus along the axis of rotation of the disk. These jets would be observable at radio wavelengths. If the disk were thick, the jets would be focused into narrow beams. Such beams have been observed in active galaxies, for example, in the center of the giant elliptical galaxy M87 and NGC4261.

Observations of M87 suggest that the cores of these galaxies are extremely crowded and that the stars near the core are moving very rapidly. In order for gravity to bind such high velocity stars, the core must be very massive. Astronomers estimate that a black hole at the center of M87 would have to be about 5 x 10^9 solar masses! Matter flowing into such a massive black hole would certainly be capable of producing the intense energy and the relativistic jets observed.

An observation of a suspected black hole at the core of a NGC4261.

According to the unified model, we observe "different" kinds of active galaxies only because our angle of view changes. For example, we see two kinds of Seyfert galaxies because we are looking either edge on through the disk (Type II Seyferts) or at an inclined angle so the jet is nearly pointed towards us (Type I Seyferts). Likewise, BL Lac objects are simply supermassive black holes at the centers of elliptical galaxies in which we look directly into the jet. The lobes of radio galaxies are simply the jets produced by the heated accretion disk, and quasars are extremely bright because they are the cores of galaxies which have extremely massive black holes at their centers.

The unified model suggests that normal galaxies can become active when something triggers the flow of matter toward the black hole, for example, a collision with another galaxy.

The unified model may not be the answer, however. Some astronomers believe that active galaxies are bright because they have extensive star formation occuring at their centers. Lots of star formation would produce lots of light, particularly if many high mass stars were formed.

Whatever they may be, active galaxies are fascinating and powerful!

After you review these sections, try a few sample questions to test your understanding. These questions are typical of questions given in introductory astronomy course exams. They are meant only to give you an idea of what kinds of questions MIGHT be on your exam. Just because these questions are here does NOT mean that you will have questions like them on your exam, NOR does it even mean that you will have questions on these topics on your exam. They are just PRACTICE questions!