Seyfert Galaxy

From Slackerpedia Galactica

Seyfert Galaxies are spiral galaxies with bright, point-like nuclei. They are a kind of Active Galactic Nuclei, and are named after the astronomer Carl Seyfert, who studied them in the mid-20th Century.

Seyferts are classified as "Type I" or "Type II" except when they're not. The binary differentiator between the two classes is whether or not the emission lines observed are strongly or weakly Doppler broadened. The energy source in all AGNs is a central, supermassive black hole being fed by an accretion disk. These disks get hot, and so emit lots of emission line radiation (of Hydrogen, for example). The material in this disk follows Keplerian orbits about the black hole, so the stuff in close moves very fast and gets strongly red and blue shifted (Doppler-broadened) along our line of sight; thus the hydrogen emission lines are very broad for the stuff close in. The stuff farther out is moving slower, so it is only weakly red and blue shifted along our line of sight, and the emission lines are narrow. The physical picture then is that for the systems with broad lines, we can see the inner disk, but for the ones with only narrow lines, the inner disk is obscured by dust.

Of course, after figuring all that out, they promptly found stuff that didn't match either of these, and so they introduced waffle types, like Type 1.5 or Type 1.9. Then, they found that when you look in polarized light, you can see the broad line component sometimes, even in the Type II Seyferts. This happens when light from the inner accretion disk, which might be visible along a different line of sight than ours, gets scattered into our line of sight by something like a galactic Corona (which is how we see our Sun's corona in white light).