Most bright galaxies are spirals, like our own, and in the nearby well-studied cases they reveal the beautiful and intricate grand-design spiral patterns that are both familiar and appealing (e.g., Sandage & Tammann 1981). At the centre of a few out of every hundred spirals there is also an extremely bright point of light. This point can be so bright that it can outshine the entire stellar output of the disc and bulge of the galaxy, and this enormous luminosity is produced from an exceedingly small volume indeed. This thesis studies aspects of these active galactic nuclei (AGNs) that inhabit some spiral galaxies.
Most likely, the power is provided by the release of gravitational energy as gas in the galaxy nucleus spirals inward towards a black hole of large mass (~ 108 M; Zel'dovich & Novikov 1965). The gas is continually sheared as inner orbits move faster than outlying ones. Close to the black hole, the viscous dissipation grows strong enough to heat the gas to incandescence at optical and X-ray wavelengths (Shakura & Sunyaev 1973). The disc of accreting material then radiates the enormous optical luminosity that we see (up to ~ 1041 W, H0 = 75 km s-1 Mpc-1; Hagen et al. 1992). Optical synchrotron emission from relativistic electrons is also important in some AGNs.
Although most AGNs seem to share these same sources of power, they present to us a variety of guises. For example, some AGNs produce powerful radio emission whilst others do not. Some produce broad optical emission lines, or high optical polarization, or are particularly variable, or produce powerful X-ray and gamma ray emissions, whilst others do not. AGNs seem to know about the type of host galaxy in which they reside, and take on certain characteristics accordingly (e.g., spiral galaxies host radio-quiet AGNs only, and blazars are found only in elliptical host galaxies). The causes of these diverse appearances are the focus of much current research.
The taxonomy of AGNs groups together objects that share certain combinations of these properties. The zoo of nomenclature is now home to the beasts shown in Fig 1.1. Such naming schemes occasionally prove particularly useful, where they reflect differences in the underlying physics. For example, the distinction between starbursts and all other AGNs provides insight, as starbursts and AGNs are driven by different processes. In contrast, it seems less useful to distinguish between, for example, type 1 Seyfert galaxies and radio-quiet quasars (described later). These seem to be the same process, differing only in optical luminosity.
Figure 1.1 The main active galactic nuclei that are currently recognised. Space densities, compiled from the literature by ALR, are only approximate and depend on survey depths and incompleteness. However, they should serve for the purpose of introduction.
Understanding which of these distinctions reflect fundamental differences and which are interesting illusions is a central challenge. Some AGN classes might be related by evolution (e.g., extremely luminous far-infrared galaxies (ELFs) might evolve into quasars with time). Others may be related by variability (e.g., type 2 Seyferts might be type 1s in which the central source has switched off momentarily). Many may appear different only because of our viewing angle. (AGNs obscured by patchy dust look different viewed end on or side on. Relativistic jets also have aspect-dependent appearances, due to Doppler boosting.) This thesis considers the "Seyfert" and "ELF" classes and, among other things, looks at possible relationships between these and other AGNs.