| © CAMBRIDGE UNIVERSITY PRESS 1991
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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.