6.1. The violent early Universe
The relationships between black holes and their host galaxies are increasingly compelling but unanswered questions remain concerning the relationship between star formation, galaxy formation, quasar activity and black hole creation in the early Universe. Observations of faint galaxies in the Hubble Deep Field suggested a peak in star formation history that matches that of the quasar epoch (e.g. Madau et al. 1996) implying a close link between star formation and quasar activity. More recent measurements, however, suggest that the star formation activity may be constant for redshifts greater than 1 with the onset of substantial star formation occurring at even earlier epochs, at redshifts beyond 4.5 (Steidel et al. 1999). An increasing number of new quasars are also being found at redshifts greater than 4 (Fan et al. 2001; Schneider et al. 2002) providing constraints for cosmological models of galaxy formation and continuing the debate on the relationship between quasar activity, star formation and the creation of the first black holes (e.g. Haiman & Loeb 2001).
The life cycle of an AGN involves a mechanism to trigger the infall of gas to create an accretion disc and continued fuelling, or replenishment, of this brightly-shining accretion disc. A number of models have suggested that at intermediate to high redshifts it may be moderately easy to trigger and fuel AGN, where galaxies might be more gas rich, star formation is vigorous and collisions between galaxies are common (Haehnelt & Rees 1993). Kauffman & Haehnelt (2000) suggest a model in which galaxy and quasar evolution at early times was driven by mergers of gas-rich disc galaxies, which drove the formation and fuelling of black holes and created today's elliptical galaxies, thereby tying together host galaxy and black hole properties. As the Universe ages, a decreasing galaxy merger rate and available gas supply and increasing accretion timescales produce the decline in bright quasars.
An alternative hypothesis, linking black hole and bulge growth with quasar activity, involves strong bars in early galaxies (Sellwood 1999); early disc galaxies developed strong bars which were highly efficient at removing angular momentum from disc gas and funnelling it towards the centre to feed and grow a black hole. This represents the bright quasar phase in which the black hole grows rapidly, but on reaching only a few percent of the mass of the host disc, the central mass concentration soon destroys the bar due to an increasing number of stars that follow random and chaotic paths, thereby choking off the fuel supply and quenching the quasar. In addition, the increase in random motion in the disc leads to the creation of a bulge. A disc might be re-built some time later if the galaxy receives a new supply of cold gas, perhaps from a `minor merger' whereby a small gaseous galaxy or gas-cloud falls into the main disc and is consumed by the disc without causing significant disruption, and without significantly affecting the black hole mass. This scenario nicely accounts for the relationship between black hole masses and bulge properties and lack of correlation with disc properties.
An important unknown parameter in these models is the amount of cold gas in progenitor disc galaxies and how it evolves with time; it is expected that the Universe was more gas-rich in the past (Barger et al. 2001), but observations of neutral hydrogen (H I) and molecular gas such as carbon monoxide (CO) with new generation facilities, such as Atacama Large Millimeter Array (ALMA), the Giant Metrewave Radio Telescope (GMRT), the Extended Very Large Array (EVLA) and the proposed Square Kilometre Array (SKA), will offer exciting opportunities to measure the gaseous properties of distant galaxies directly to further our understanding of galaxy formation and evolution and its relationship to quasar and star-formation activity.
6.2. Re-activating dormant black holes in nearby galaxies
While the most luminous AGN might coincide with violent dynamics in the gas-rich universe at the epoch of galaxy formation (Haehnelt & Rees 1993), nuclear activity in nearby galaxies is more problematic since major galaxy mergers, the collision of two equal-mass disc galaxies, are less common and galaxy discs are well established; reactivation of ubiquitous `old' black holes is therefore likely to dominate. Host-galaxy gas represents a reservoir of potential fuel and, given the ubiquity of supermassive black holes, the degree of nuclear activity exhibited by a galaxy must be related to the nature of the fuelling rather than the presence of a black hole (e.g., Shlosman & Noguchi 1993; Sellwood & Moore 1999). Gravitational, or tidal forces exterted when two galaxies pass close to one another may play a role in this process, either directly, when gas from the companion, or outer regions of the host galaxy, is tidally removed and deposited onto the nucleus, or by causing disturbances to stars orbiting in the disc and leading to the growth of structures such as bars, in which stars travel on elliptical paths and drive inflows of galactic gas (e.g. Toomre & Toomre 1972; Simkin, Su & Schwarz 1980; Shlosman, Frank & Begelman 1989; Mundell et al. 1995; Athanassoula 1992; Mundell & Shone 1999).
Numerous optical and IR surveys of Seyfert hosts have been conducted but as yet show no conclusive links between nuclear activity and host galaxy environment. Neutral hydrogen (H I) is an important tracer of galactic structure and dynamics and may be a better probe of environment than the stellar component. H I is often the most spatially extended component of a galaxy's disc so is easily disrupted by passing companions, making it a sensitive tracer of tidal disruption (e.g. Mundell et al., 1995). In addition, because gas can dissipate energy and momentum through shock waves (Mundell & Shone, 1999), whereas collisions between stars are rare, the observable consequences of perturbating the H I in galactic bars are easily detectable. However, despite the diagnostic power of H I, until recently few detailed studies of H I in Seyferts have been performed (Brinks & Mundell 1997; Mundell 1999).
The strength of a galaxy collision, which depends on initial galaxy properties such as mass, concentration, distance and direction of closest approach, ranges from the most violent mergers between equal mass, gas-rich disc galaxies, to the weakest interaction in which a low mass companion, perhaps on a fly-by path, interacts with a massive primary. In this minor-merger case the primary disc is perturbed but not significantly disrupted or destroyed. Indeed, Seyfert nuclei are rare in strongly interacting systems, late-type spirals and elliptical galaxies (Keel et al. 1985; Bushouse 1986) and sometimes show surprisingly undisturbed galactic discs despite the presence of H I tidal features (Mundell et al. 1995). Seyfert activity may therefore involve weaker interactions or minor mergers between a primary galaxy and a smaller companion or satellite galaxy, rather than violent major mergers (e.g. De Robertis, Yee & Hayhoe 1998). A key question is whether the gaseous properties of normal galaxies differ from those with Seyfert nuclei and a deep, systematic H I imaging survey of a sample of Seyfert and normal galaxies is now required.