| Term: | Seyfert galaxies |
| Definition: |
A member of an important class of active galaxies which are characterized by the presence of an intensely bright nucleus in the optical wavelengths (10^9-10^12 L_sun) displaying emission spectral lines. The presence of these emission features, which are not seen in the spectra of normal galaxies, indicates a very high degree of ionization. Moreover, the nucleus radiates non-thermal continuum emission extending over the whole electromagnetic spectrum. It is thought that a massive black hole in the nucleus of a galaxy accretes gas (accretion) from its surrounding environment to power Seyfert galaxies. These galaxies are divided into two types according to the width of their spectral lines. Type 1 Seyfert (Sy 1) galaxies have very broad emission lines (10^3 - 10^4 km s^-1), while Type 2 Seyferts (Sy 2) show relatively narrow lines (several hundred km s^-1). These spectral differences may be the result of viewing the nucleus from different angles. A Type 2 Seyfert galaxy may be a mostly edge-on view of matter spiraling in toward the supermassive black hole, whereas a Type 1 Seyfert provides a more pole-on view, allowing us to see the more turbulent region around the black hole. |