Introductory Astronomy: Seyfert Galaxies

Seyfert Galaxies are spiral galaxies with unusually bright, tiny cores that fluctuate in brightness. They do not have radio lobes. Most are powerful sources of infrared radiation. In addition, some emit intensely in the radio, X ray, and gamma ray regimes. Approximately 2% of all spiral galaxies are Seyfert galaxies. This means either that about 2% of all spiral galaxies have active core or that most spiral galaxies have potentially active cores which erupt 2% of the time. The heart of our own galaxy is believed by some astronomers to contain a supermassive (although quiet) black hole and suggests that all spirals erupt now and then. We may be living in what was once (or what will one day be) a Seyfert galaxy!

There are two kinds of Seyfert galaxies called Type I and Type II. Both types have emission spectral lines in their nuclei, which is evidence of highly excited gas. The difference arises in the shape of the emission lines. The emission lines of Type I Seyferts are very broad, suggesting gas velocities of over 1000 km/sec. The emission lines of Type II Seyferts are much narrower, which suggests that the gas in these galaxies is moving much more slowly.

As discussed on the front page, these differences can be explained in terms of our viewing angle. Astronomers believe that we are viewing Type I Seyferts nearly along the jets. Here, we would expect to see broad lines because the gas is very hot and moving rapidly. Type II Seyferts are viewed almost directly through the accretion disk. The disk blocks the light form the central black hole and the jet. The light we see comes from slower-moving gas farther from the black hole and, thus, produces a narrower line.