Introduction

It is now universally believed that Active Galactic Nuclei (AGN) derive their extraordinary luminosities from energy release by matter accreting towards, and falling into, a central supermassive Black Hole (BH). This process releases energy as UV, X-ray, and -ray continua, and material orbiting close to the BH is photoionised, producing the Doppler-broadened emission lines characteristic of the ``broad line region'' (BLR). In the ``radio-loud'' subclasses of AGN, relativistic jets of material are ejected in the poleward direction and may escape into the intergalactic space as the classical double-lobed radio sources. Also lying generally in the poleward directions, but at a distance of up to several kpc are also found the ``narrow line regions'' (NLR) excited either by photoionisation from the UV continuum of the central source (Koski 1978; Ferland and Netzer 1983; Stasinska 1984) or by shock excitation related to the jets (Dopita and Sutherland 1995). In addition, some AGN display low-ionisation nuclear emission line (LINER) nuclei which differ quantitatively from the NLR of Seyfert galaxies, and which, if photoionised, are excited by a much more dilute UV radiation field.

Whilst this gave a general understanding of the components which make up an AGN, the taxonomic relationship between the various sub-classes of AGN remained very unclear and the ``unified model'' was developed in response (Rowan-Robinson 1977; Lawrence and Elvis 1982; Antonucci and Miller 1985; Lawrence 1987). Our current understanding of the unified model has been summarised in two excellent reviews (Antonucci 1993 and Urry and Padovani 1995) which represent essential reading in the field. To summarise, the basic idea behind the unified model is that orientation effects exercise a profound influence on the appearance of the AGN, and therefore determine into which particular sub-class it will be categorised. The principle cause of the orientation effect is believed to be due to a geometrically thick and optically-thick dusty molecular torus of material located in the accretion plane of the AGN. This is capable not only of obscuring the central source from direct view, but may also be optically-thick to the infrared photons produced by heating of the dust within it (Heisler, Lumsden and Bailey 1997).

A second orientation effect is found in the radio-loud categories, for which the unification concept was orginally developed ( Orr & Browne 1982; Barthel 1991). The classes of AGN which are observed close to the polar direction display apparently super-luminal motions, rapid variability, intense power-law continua, flat radio spectra and high linear polarisation all due to relativistic beaming effects associated with the jet. The BL Lac objects, and the optically-violently variable (OVV) or flat-spectrum radio quasars (FSRQs) all seem to fit into this category (Blandford and Rees, 1978; Blandford and Konigl 1979). It is not so clear to which class of radio-loud objects these various beamed objects belong. Urry & Padovani (1995) would argue that BL Lacs belong in the (Fanaroff-Riley) FR 1 class. However, recent work by Wurtz et al (1997) shows that the clustering properties of BL Lac objects is like that of the radio-loud quasars and the (more luminous) FR 2 double lobe sources.

The ``strong'' unification hypothesis, that all AGN classes are fundamentally the same phenomenon seen at different orientations, has not proven successful. However, in its more restricted form; that some classes of object transform into other classes as a function of viewing angle, it now seems secure. In particular, the following relationships between the Seyfert nuclei found in disk galaxy environments; the Quasi-stellar objects QSOs found in more distant systems; and the weaker radio sources found in luminous Elliptical galaxies are almost certainly correct:

Whilst this scheme is satisfying as far as it goes, it offers no way to fully unify the various tribes of AGN. In particular, there is no explanation of the radio-loud / radio-quiet or the Fanaroff-Riley class FR 1 / FR 2 dichotomies. Clearly, a parameter other than orientation must play a role. It has been variously argued that radio loudness is related to host galaxy type (Smith et al. 1986), black hole spin (Blandford 1990; Wilson and Colbert 1995), or to differences in the rate of nuclear feeding (Rees et al. 1982; Baum, Zirbel and O'Dea 1995).

Nuclear feeding is certainly important in the evolution of AGNs. For example, Hutchings and Neff (1992), Boyce et al. (1996) and the work of Bahcall and his group using HST images (Bahcall et al. 1997), show that QSO host galaxies are, in the main, either interacting or photometrically disturbed post-merger systems. The Luminous Infrared Galaxies (LIRGs) discovered by the IRAS satellite seem to represent the link between spiral and elliptical galaxies. It is now clear that they form as a result of collisions and mergers, and most of those objects which are well resolved display both double nuclei and tidal arms (Melnick and Mirabel 1990). The IR emission may amount to 90% of the bolometric emission, and initially results from dust heated by the light from a massive burst of star formation induced by gas dynamical collisions. In the luminosity range , about 70% of all objects show line ratios typical of normal HII regions (Veilleux et. al 1995) and resolved objects display double nuclei with projected separation in the range 5 - 30 kpc. However, later on in the merger a greater and greater portion of the luminosity appears to be derived from energetic outflows powered by accretion to an active nucleus. These objects are found amongst the most luminous LIRGs (Saunders & Mirabel, 1996). For , less than 30% of objects show HII region-like characteristics in the optical, over 40% have some kind of Seyfert activity in the nucleus, and the balance show line ratios characteristic of a mixed excitation (see Armus, Heckman and Miley 1989, 1990; Veilleux et. al 1995; Kim, 1995) . Late starburst and Seyfert-like objects are typically at a late merger stage and may show the development of the law of photometric profiles (Wright et al. 1990), multiple shells, evolved tidal arms, and (where still separate) projected distances between multiple nuclei <1 kpc. These objects often appear to have warmer dust, as measured by 60m/25m and 60m/100m flux ratios. We therefore believe that warm IRAS galaxies represent merger systems in which the starburst is intially triggered by tidal interactions, is powered through the merger event by the direct collision of gas streams from the merging galaxies, and that in the late phases gas is accreted into the central regions to power the active nucleus or nuclei. The question of whether gas-rich mergers may lead to the formation of elliptical galaxies is not fully resolved. Nontheless, van Albada (1982) showed that the law is an inevitable result of violent relaxation, Scoville et al. (1991) demonstrate that the molecular gas in merging systems has reached surface densities б 10. pc which would be necessary to form an Elliptical-like core, and Kormendy and Sanders (1992) show that a number of starbursts lie in the region of the `cooling diagram' (the surface density:velocity dispersion plot) occupied by Ellipticals.

If mergers drive the formation of elliptical-like systems today, then the much higher rate of gas-rich mergers expected in the early universe ( 2-3) would have ensured that the major epoch of elliptical formation occurred at that time. Since statistical studies suggest that this was the epoch of the maximum space density of QSOs, it is tempting to associate QSO formation with this gas-rich merger epoch.

In this paper, I investigate the idea that rate of nuclear feeding plays the dominant role and that, moreover, feeding at super-Eddington rates into the BLR occurring in merger events is the means whereby massive BHs are grown. In particular, I develop a simple parametric characterisation of the growth of a BH during merger events in order to establish the credibility of the hypothesis that the nuclear accretion rate is a key parameter in unification of AGN phenomena. I also present a model for the radiatively driven winds and use this to suggest a model for the ``big blue bump'' in AGN spectra and its relation to the BLR, investigate the radio-loud, radio-quiet dichotomy based on the rate of nuclear feeding, and emphasise the important diagnostic capabilities offered by analyses of the NLR based on shock excitation models.

б© Copyright Astronomical Society of Australia 1997