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Feb - Mar 99
Astronomical
Feature of the Month
SEYFERT GALAXIES
They are the most powerful members of the galactic "zoo" - Seyfert galaxies. Seyferts are spiral galaxies with unusually bright, star like cores that fluctuate in brightness. Most are powerful sources of infrared radiation, though some emit intensely in the radio, X ray, and gamma ray regimes too. Around 2% of all spiral galaxies are Seyfert galaxies. Some astronomers believe the heart of our own Milky Way galaxy contains a supermassive (although quiet) black hole. They even suggest that all spirals erupt now and then. We may be living in what was once (or will one day be) a Seyfert galaxy!
There are two kinds of Seyfert galaxies, 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 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.
These differences could be explained by our viewing angle. Some astronomers believe we are viewing Type I Seyferts nearly straight into the jets where we would expect to see broad lines because the gas is very hot and moving rapidly. Type II Seyferts are viewed 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 spectral line.
The point though is that a Seyfert galaxy has huge jets of excited gas shooting from the core, which has all the makings for an excellent painting...
International Association of Astronomical Artists