Astrophysics (Index)About

Seyfert galaxy

(visible active galaxy)

The term Seyfert galaxy roughly means an active galaxy nearer than a quasar. The term was coined for resolved galaxies with especially bright centers, with distinct emission lines (now interpreted as signs of an active galactic nucleus), which were noticed before quasars were first observed. Thus point-like distant active galaxies are referred to as quasars, and nearer, resolved ones, Seyfert galaxies, though there are also active galaxies with more obscured AGNs that are generally referred to by other terms.

A common classification of Seyfert galaxies is as Type 1 or Type 2 (aka Sy 1 and Sy 2), the former having broad optical emission lines, the latter with narrower emission lines, the broadness indicating the size of the velocity dispersions of the material producing the lines. The lines of both could be considered broad: it is a matter of degree. The same classification is used for AGNs, as are the terms broad-line AGN (BLAGN) and narrow-line AGN (NLAGN), the latter of which are sometimes used to indicate Type 1 versus Type 2, but the classification of AGNs and galaxies can be broken down further, and the exact relation of the terms depends upon the author. It has been proposed that at least some of this classification actually depends upon the viewing angle: that active galaxies generally have a central broad-line region (BLR) and narrow-line region (NLR) and the BLR can be hidden, given some viewing angles. Such regions are of current study interest.

The term Type 3 (Sy 3) has been used for LINER galaxies.

(galaxy type)
Further reading:

Referenced by pages:
active galactic nucleus (AGN)
Circinus Galaxy (ESO 97-G13)
Fairall Galaxy List (Frl)
galaxy classification
IRAS 13224-3809
Messier 106 (M106)
Palomar-Green Survey (PG)
3C 303
Tonantzintla Surveys (Ton)
X-ray source