3.3. Other Models
Other models have been proposed to explain the observed emission lines. Shock-wave excitation is the most likely alternative. This cannot provide a full explanation of the broad line spectrum since the role of photoionization has been clearly established by the line variability observations. The situation is somewhat different in the NLR and some shock-wave models are moderately successful in explaining the narrow line spectrum. This is of greater interest in faint AGNs, in particular in LINERS. Recently, there have been several attempts to combine shock-wave and photoionization into a composite model. This is discussed in chapter 11.
A completely different, perhaps parallel view of activity and line excitation, is found in the so called "Warmers" model of AGN. In this model, the nuclear activity is due to a burst of star formation. The so called "nonstellar continuum" is in fact the spectrum of a young metal rich cluster containing a few, extremely hot, Wolf-Rayet stars. The broad emission lines and the observed variability are due, in this scenario, to supernovae explosions in the nuclear cluster. In this picture there is no need to assume a massive central object to provide the observed continuum. The model is getting to the stage that detailed calculations of the line and continuum spectra are becoming available. It has some very interesting features, as well as difficulties and drawbacks. It will not be discussed here any further.
Fast particles and radio wave heating have been mentioned, in several papers, as possible additional sources of heating and excitation. In particular, radio-loud AGNs are thought to host such an additional energy source. There is too little work, so far, to address this idea in any detail. There are some obvious drawbacks, similar to the ones associated with any model where much of the ionization is due to collisions. There are uncertainties to do with the unknown magnetic fields and the remarkably small observed differences between the emission line spectrum of radio-quiet and radio-loud objects.
The discussion in the following chapters concentrates on a particular AGN model in which the emission line region is made out of a large number of clouds that are photoionized by a central continuum source. The principles involved in this kind of modeling, and the physical processes affecting the spectrum of individual clouds, are discussed in chapter 4. Chapter 5 addresses the question of how to combine the flux emitted by many individual clouds into a composite theoretical spectrum and chapter 6 contains a detailed comparison with AGN observations.
References on AGN photoionization models are given in chapters 4 and 5. References on shock-heated AGN clouds are given in chapters 9 and 11. "Warmers" are discussed in a series of papers by Terlevich, Melnick and collaborators, see Terlevich and Melnick (1988) and Terlevich (1988) for references. Some papers on the role of fast particles are Ferland and Mushotzky (1984, emphasis on FeII emission) and Viegas-Aldrovandi and Gruenvald (1988, NLR models with relativistic electrons).