Seyfert galaxies were first isolated as a class by Seyfert (1943), although their unusual emission line spectra had been noticed earlier (e.g., Mayall 1934 and references therein). These otherwise normal spiral galaxies drew attention for their uncommonly luminous nuclei, and for their powerful high-excitation emission lines. (The emission lines come from clouds within a few hundred pc of the central source and are ionized by the intense radiation produced by the AGN core, e.g., Osterbrock 1987.)
Two types of Seyfert galaxy were recognised by Khachikyan & Weedman (1971), based on the widths of their emission lines. Both Seyfert types produce powerful narrow forbidden lines from high excitation species like [O III] and [Ne V]. Some Seyfert galaxies also produce broad (~ 10000 km s-1) Balmer lines, whilst in others these lines are narrow (250 to 1000 km s-1). Such systems are called type 1 (Sy1) or type 2 (Sy2) Seyferts respectively. Later, the intermediate types 1.5, 1.8 and 1.9 were recognised, based on the simultaneous presence of both broad and narrow components to the Balmer lines (e.g., Osterbrock & Dahari 1983).
According to the widely accepted picture of Seyfert nuclei, there are two physically distinct line-emitting regions. The broad-line region (BLR) lies close to the central continuum source, within tens of light days (Peterson et al. 1992). It has a mass of ~ 50 M, densities of 1014 to 1018 m-3, temperatures of 4 × 104 K, and cloud speeds of 103 to 104 km s-1 and produces the broad Balmer series. The narrow-line region (NLR) spans tens to hundreds of pc, has a total mass of 105 to 106 M, densities of 109.5±1 m-3, temperatures of 1 × 104 to 5 × 104 K, and cloud speeds of ~ 250 to 1000 km s-1 (Osterbrock 1987; Lawrence 1987; Haniff, Wilson & Ward 1988). Between these two regions no emission lines are seen, for reasons that are poorly understood but may involve dust absorption (Netzer & Laor 1993).
The absence of forbidden lines in the BLR is due to the relatively high density of the BLR plasma. Large density leads to many collisions during the lifetimes of the excited states and collisional de-excitation is likely. In that event, the energy is carried off as kinetic energy and the forbidden transitions do not radiate.
Such spectroscopic properties are an important input to unification schemes. Before reviewing those schemes I will first introduce the extremely luminous far-infrared galaxies (ELFs), which are studied later in this thesis and are related to Seyfert galaxies.