Seyfert galaxies are quite similar to normal galaxies like our own Milky Way except in one critical respect: their nuclei are fantastically bright -- in some instances as luminous as 100 billion suns. By contrast, the center of the Milky Way shines like about ten million suns. Astronomers think that these huge energies are being generated in Seyfert nuclei not by normal stars, but by material falling on massive black holes. Matter falling towards the vicinity of the black hole heats up due to friction, radiating intense ultraviolet light (along with radiation at other wavelengths). The Milky Way also has a massive black hole at its center, but for reasons that are not well understood our galactic center is not particularly bright compared to its star-filled spiral arms. At the opposite extreme, quasars are galaxies whose stars are very faint - almost undetectable - yet their nuclei glow intensely.
Astronomers suspect that Seyfert galaxies might be an intermediate stage between these two extremes, and are working to understand the conditions in Seyfert galaxies in the hope of resolving whether or not some normal galaxies might one day end up as quasars. Optical observations of Seyfert galaxies disclose that they come in two general types: some with bright lines from excited atoms, and others lacking these lines. One popular explanation is that both kinds of objects are intrinsically the same, but a disk (or torus) of dust around active nuclei obscures visible light if the Seyfert happens to be seen edgewise from the earth. In this interpretation, the different kinds of Seyfert galaxies simply reflect their different orientations in the sky.
SAO astronomer Howard Smith, together with five colleagues, has used the Spitzer Space Telescope to study in the infrared a set of 116 Seyfert galaxies representing all the various known types. The collection is based on a well studied sample of bright, nearby Seyfert galaxies -- but a sample for which there had never been systematic infrared spectra obtained. At infrared wavelengths the extinction effects of the dust are minimized, and therefore if the orientation picture is correct, the infrared results should find minimal differences among the members of the set. Writing in a paper scheduled to appear in the Astrophysical Journal, the team reports not only that the geometrical picture is generally confirmed, they derive from the infrared spectra a set of new diagnostics for the physical conditions in the galaxies. The results help to clarify that Seyfert galaxies indeed represent an intermediate stage in the life of many galaxies.