1. INTRODUCTION

Note: The interested reader is referred to the reviews of Begelman, Blandford & Rees (1984); Antonucci (1993); Urry & Padovani (1995) and the books by Blandford, Netzer & Woltjer (1990); Peterson (1997) for comprehensive overviews of active galactic nuclei and their models of unification.

Seyfert galaxies ⁽¹⁾ which account for ~ 10% of all galaxies Maiolino & Rieke 1995; Ho et al. 1997), constitute one of the two main classes of active galactic nuclei (AGNs); the other class being the more radio-loud quasars. Although sometimes associated with enhanced star-burst activity (e.g. Whittle 1992; Gu et al. 1997; Roy et al. 1998; Gu, Huang & Ji 1999), a Seyfert nucleus, by definition, contains an AGN and exhibits this via the presence of a non-stellar nuclear source (e.g. Rigopoulou et al. 1997; Curran, Aalto & Booth 2000; Curran et al. 2000a) ⁽²⁾ Also, unlike star-burst galaxies, the nuclei are always associated with radio or optical jets ⁽³⁾ (Hummerl, van Gorkom & Kotanyi 1983; de Grijp et al. 1985; Cecil, Wilson & Tully 1992). An infra-red (IR) spectrum showing the characteristic AGN spectrum of a Seyfert galaxy ⁽⁴⁾ galaxy in Circinus, we shall often refer to this as a particular example.} is shown in Fig. 1.

Figure 1. The IR spectrum of the Circinus galaxy taken with the Long and Short Wavelength Spectrometers aboard the infra-red Space Observatory. The higher energy (frequency) lines (Ne, S, Mg, O and Si) result from gas ionised by the AGN, whereas the lower lines result from star forming activity (Section 1.2.2). Taken from Morwood (1997), courtesy of Alan Moorwood.

Following further surveys, these galaxies were divided into two main classes according to their optical properties; type 1 and type 2 Seyferts ⁽⁵⁾ (Khachikian & Weedman 1974). The main difference between these classes is that, in addition to the narrow line emission ⁽⁶⁾ evident in both the main classes, type 1 Seyferts also exhibit broad line emission, which covers a wide range of ionisation states (Wilson et al. 1993). The narrow lines (widths ~ 10² km s^-1) are believed to be due to low density (n_e ~ 10³-10⁶ cm^-3) gas clouds relatively far from the core which are ionised either by photoionisation from the central source (Koski 1978; Ferland & Netzer 1983; Stasinska 1984) or by shock excitation from the radio jets emanating from the core (Dopita & Sutherland 1995). The broad lines (widths 10⁴ km s^-1) result from dense (n_e 10⁹ cm^-3) photoionised gas clouds, orbiting close to the central power source (Robson 1996, and references therein).

In order to account for the absence of broad line emission in type 2 Seyferts, current models of AGN unification ⁽⁷⁾ different observational parameters in different classes of AGN are attributed to orientation effects are collectively named ``unified schemes''.} invoke a dusty obscuring torus which surrounds the Seyfert nucleus and the observed properties are solely determined by the orientation of the torus relative to the observer's line-of-sight to the nucleus (Osterbrock 1978; Antonucci & Miller 1985; Miller & Goodrich 1987; Antonucci 1993): In type 1 nuclei (Sy1), the axis of the torus is close to the line-of-sight and one observes a naked AGN directly with its associated broad line region in full view, Fig 2. In type 2 nuclei (Sy2), the orientation of the dusty torus is such that it shields the nucleus from view, and only the more extended narrow line clouds are observed directly. The observed broad line radiation is polarised ( 15%), suggesting that it has been scattered, and is therefore a reflection of the central source (Antonucci & Miller 1985; Krolik & Begelman 1986; Miller, Goodrich & Mathews 1991; Heisler, Lumsden & Bailey 1997).

Figure 2. Schematic diagram of a ``unified'' radio loud AGN]{Schematic diagram of a ``unified'' radio loud AGN (Urry & Padovani 1995). In this figure (which is not to scale) the black hole and accretion disk are shown at the centre (see Section 1.2). The waved lines indicate roughly from which directions the emission is observed in the case of type 1 and type 2 Seyfert nuclei. Courtesy of Meg Urry.

As well as these two classes, there exists intermediate types, the notation of which is based purely upon the optical spectrum: In type 1.5 Seyferts, the broad and narrow components of the H lines are comparable, in type 1.8 Seyferts, the broad components are weak but nonetheless detectable in H and H, and finally, in type 1.9 Seyferts the broad component can only be detected in the H line (Osterbrock 1981). In general H luminosities are an order of magnitude greater in Sy1s than Sy2s (Gu et al. 1997). From a study of the statistical differences between Sy1s and Sy2s, Keel (1980); Maiolino & Rieke (1995) (and to a certain degree, Curran 2000 ⁽⁸⁾ find that intermediate Seyferts of types 1, 1.2 and 1.5 will occur in face-on galaxies while those of type 1.8 and 1.9 will occur in the edge-on cases.

¹ Originally identified in 1943 by Carl Seyfert. Back.
² Research Papers C and E. Back.
³ Due to ionised gas (Storchi-Bergmann, Kinney & Challis 1995) ejected from a compact nucleus (Whittle et al. 1988; Pedlar et al. 1989; Christopoulou et al. 1997; Radovich, Rafanelli & Barbon 1998). See Section 1.4. Back.
⁴ Since much of the work in this thesis is focused on the closest located Seyfert galaxy in Circinus, we shall often refer to this as a particular example. Back.
⁵ Maiolino & Rieke (1995) find that there are around four type 2 nuclei to every type 1 according to their sub-classification scheme (Osterbrock 1981; Meurs & Wilson 1984; Edelson 1987; Osterbrock & Shaw 1988; Schmitt & Kinney 1996; Schmitt et al. 1997). Back.
⁶ Which is (statistically) identical in appearance for both the main classes (Cohen 1983; Whittle 1985). Back.
⁷ Theories on which different observational parameters in different classes of AGN are attributed to orientation effects are collectively named ``unified schemes''. Back.
⁸ Research Paper D. Back.