|| © CAMBRIDGE UNIVERSITY PRESS 1991
The actual initiation of jets is a subject that remains extremely difficult to discuss in a detailed and convincing manner despite the obvious importance of this fundamental topic. There are two main reasons for this. The first is a lack of unequivocal observations. Although VLBI measurements have provided structural information on scales corresponding to 0.1 pc in extragalactic sources, the phenomena that govern the beginnings of jets almost certainly occur on scales at least two to three orders of magnitude smaller. Other observations, especially of X-ray and optical variability, are indubitably important and provide useful constraints on models; however, they do not yield information that is clearly interpretable in a model-independent fashion.
The second generic difficulty has to do with the certainty that the physical processes involved in producing jets are extraordinarily complex. The core of the picture, that accretion onto a super-massive black hole (SMBH) of somewhere between 106 and 1010 M is at the heart of beam generation as well as the other properties of active galactic nuclei (AGN), has been commonly accepted for about a decade. However, in attempting to add details to this picture, astrophysicists find that general relativity, hydrodynamics, plasma physics and radiation transport all form thick blobs on their palettes, and the portraits which emerge from combining them in different proportions are, not surprisingly, rather different. On the basis of current observations and theoretical understanding, choosing between several of these candidate views is extremely hard from a scientific standpoint and such choices might better be deemed art criticism. Furthermore, the probability that several blends of "colours" are involved in different types of AGN is high.
Because this chapter is addressing such a broad and difficult aspect of this subject the approach cannot possibly be as quantitative as that found in several other chapters. The dearth of directly applicable data also forces this discussion to be somewhat more speculative than most other parts of this volume. Section 2 summarizes some characteristics of the most popular models for central engines. Attention will be directed to the ways in which beams are supposedly produced in those models. Models based upon hydrodynamic mechanisms are treated first, and in somewhat more detail, than those dominated by magnetohydrodynamics. The emphasis in treatment is determined more by our current knowledge and the level of development of the scenarios than by their viability probability. In Section 3 some of the relationships between small scale jets and the matter in the central regions of AGN are discussed, with an emphasis different from those in Chapter 4. A few conclusions are given in Section 4. In conformity with the great majority of literature in this area, cgs units are used throughout this chapter.
To close this introduction, it should be pointed out that while the possibility of eventually sorting out all of the difficulties with powerhouse models does exist, the "galactic weather forecasting" analogy of Phinney (1985) must be borne in mind. Although the gross features of our weather are determined by solar insolation and the density of the earth's atmosphere, no one can argue that the full complexities of the weather are properly understood, nor can one possibly claim that additional knowledge of these details will give us much significant information concerning the solar interior. Similarly, many of the observational details of jets and other properties of AGN may be able to shed very little light on the ultimate nature of the powerhouse. We may have to rely on self-consistency and aesthetics in choosing which picture(s) to buy.