Seyfert Galaxies (5) ...

If the preceding picture is correct, it provides a unified model of Seyfert galaxies. Seyfert 1s and Seyfert 2s are really the same thing, but just seen from different perspectives, as illustrated schematically in the adjacent right diagram.

Although this is a highly oversimplified model, most astronomers now believe that it is an essentially correct description of Seyfert galaxies. They are powered by a monstrous central black hole surrounded by a hot accretion disk and a dusty torus, and whether we see a Seyfert 1 or Seyfert 2 galaxy depends on whether the central monster is hidden from our view by the dusty torus.

Seyfert Galaxies, Quasars, and Normal Spirals
Seyfert galaxies and quasars have many features in common (for example, emission lines and strong nonthermal continuum radiation across a broad range of wavelengths). In fact, if the most luminous of nearby Seyfert galaxies were placed at the much larger distance of the quasars, we would see primarily the central source and a bright Seyfert would be virtually indistinguishable from a low-luminosity quasar. This suggests that perhaps quasars and AGNs are the same phenomena: active galactic nuclei powered by spinning supermassive black holes. On the other hand, Seyfert galaxies bear some relation to the center of our own galaxy, where we also see evidence for non-thermal emission, broad lines, and a large mass concentration. As we have already noted, there is some evidence that a black hole of perhaps 10 million solar masses may reside at the center of the Milky Way. The difference is primarily one of scale. A Seyfert galactic nucleus is 1000 times brighter than the nucleus of the Milky Way.

Feeding the Monster
We are thus led to an hypothesis. Perhaps all, or at least most, large galaxies have supermassive black holes at their centers. The question of whether the galaxy exhibits an active nucleus is then primarily one of "feeding the monster" at the center. In a quasar, the black hole is accreting matter at a higher rate, leading to very high luminosity (typical estimates for quasars indicate that the black hole must accrete a few solar masses of matter per year to account for their luminosity). In a normal galaxy like the Milky Way, the black hole is rather quiet because it is presently accreting little matter. Seyfert and other active galaxies are somewhere in between: the black hole is active because it is accreting matter, but at a rate lower than that for a quasar.

This picture is supported by the finding that Seyfert galaxies are more abundant than average in interacting pairs of galaxies, and that approximately a quarter of observed Seyferts exhibit evidence for tidal distortion. Both of these observations suggest that interactions with other galaxies may "turn on" AGNs by increasing the fuel flow to the central black hole. In the next module we shall pursue a unified AGN model where all active galaxies and quasars are powered by central supermassive black holes. In this unified model, the differences among AGNs arise primarily from differences in the orientation angle and local environment for the central engine. The first determines whether the view of the central engine is blocked by dust; the second determines the rate at which fuel flows to the central engine.