Introduction

The attractive features of the ‘beam’ model for galactic and extragalactic radio sources are predicated on the contention that astrophysical processes permit the formation of high-power collimated flows, and that such flows survive their passage from the generating engine to the outer parts of the source without losing most of their energy. In other words, astrophysical plasma beams must be capable of exceptional stability, although there are sources for which less stability is necessary. This may be seen in the ‘P-D’ diagram of Baldwin (1982) (cf. Chapter 2) – for a given radio power (say 1027 W Hz −1 sr −1), radio sources spanning a wide range of linear sizes (about 1 to 1000 kpc) are found, and hence the beams driving these sources must be stable over distances exceeding 1 Mpc in the largest objects, but need be stable for only 1 kpc in the smallest. The purpose of this chapter is to discuss the physical mechanisms that are effective in stabilising and destabilising beam flows, and to calculate the stability properties of some beams that might be components of extragalactic radio sources. Much of the physics discussed here can be applied to Galactic jets (Chapter 10), with some modification for the difference in physical parameters.

Introduction

Our interferometric images of radio sources reflect the synchrotron emissivity arising from their relativistic electrons and magnetic fields. These trace the underlying plasma flow, albeit imperfectly. The local dynamical evolution of the particles and fields is determined by their transport from the nuclear source, and by their in situ dynamics. This chapter presents the physics necessary for an understanding of current theories of particle acceleration and magnetic field evolution. It describes these theories and attempts to assess whether or not they provide an adequate account of the inferred particle spectra, energetics and magnetic field geometry of extragalactic jets.

It was shown in Chapter 3 that some sources have severe lifetime problems, in that the time for the electrons to be carried out to the lobes (even with a jet speed ~ c) is longer than their radiation lifetime (the upper limit of which is the lifetime to Compton losses on the 3 K background) and that the surface brightness and spectral index distributions do not decay as fast as would be expected in a constant velocity, expanding flow. These problems may be overcome by the local reacceleration of the radiating particles. Further, simple estimates of convection of flux-frozen magnetic field out from the core predict that the convected field decays significantly; however, this is probably offset by in situ amplification of the magnetic field by some dynamo process.

Introduction

The interpretation of the diverse forms of observed radio source structure has always been problematical since this normally involves the use of some form of classification scheme. With the benefit of hindsight, it is clear that this exercise has not always proved to be a total success. Every astronomical object is the product of a unique set of physical circumstances which must, at some level, ultimately preclude the imposition of a generalised classification scheme covering many objects. It remains, however, a necessary basic stage in the process of scientific investigation. Any classification scheme is based upon gross structural features derived from observation. Such observations are of an inhomogeneous set of objects and are limited by sensitivity and imaging techniques. Schemes are therefore subject to strong selection effects and their subdivisions are arbitrary. A scheme can, however, prove useful provided the subdivisions broadly map out differing segments in the parameter space of the physical conditions of radio sources. The problem is, of course, that it is those very same physical conditions that are as yet unknown and that one is attempting to investigate. Thus, any current classification scheme is dominated by the characteristics of the telescopes available to observers at the time, and incorporates the ‘conventional wisdom’ derived from the interpretation of previous work. Such circumstances are profoundly inelegant but probably unavoidable.

Introduction

Flat spectrum nuclei are found at the centre of many types of extragalactic radio source, including the powerful classical doubles, and the ‘isolated’ compact sources (that have comparatively weak extended structure). They turn out (when examined with sufficient resolution) to be the self-absorbed bases of jets that feed the extended structure, whose continuations are often seen on much larger scales. The systematic properties of VLBI jets have been reviewed frequently and can be summarised thus:

1. The jets are nearly always seen on only one side of the nucleus. ‘Counterjets’ have only been seen in a few sources (for example 3C 236, Schilizzi et al. 1988).

2. The jets contain bright ‘knots’ of emission that are often seen to move outwards, away from the base at speeds commonly in the region of 5 to 10 times the speed of light (Ho = 100 km s−1 Mpc−1). Although some apparently stationary knots have been observed, for example the outer component in 4C 39.25 (Shaffer & Marscher 1988), none has ever been observed to move inwards (Marcaide et al. 1985).

3. The speeds and trajectories of the individual knots do not in general vary greatly as the knots move out; it is difficult to place very severe limits on acceleration because of the limited accuracy of positional measurements and the relatively short distances over which the knots are observed to move. Significant changes in speed and direction have been detected in 3C 345 (Biretta et al 1986).

4. […]

Gross morphology

A crucial morphological feature of extended extragalactic radio sources is that (see Chapter 2) there are actually two fundamentally distinct classes of object, in which the weaker, Fanaroff & Riley (FR) class I, sources are characterized by quasi-continuous luminous jets which often become distorted as they interact with the inter-galactic medium (Fig. 2.3), whereas the more powerful, FR II, sources have a simple, linear, double-lobed structure, with the brightest emission occurring in compact hotspots at the edge of each lobe (Fig. 2.8). A central challenge for any theoretical model of extragalactic radio source structure is therefore to reproduce this observed dichotomy, and to identify the factor, or factors, that determine which type of extended structure develops. Furthermore, we might hope that an improved theoretical understanding of the Fanar off & Riley classification would enable us to make improved, dynamical, estimates for the, observationally badly determined, physical parameters of jets in radio sources.

Although a complete model for radio source structure would probably have to involve variations in both the strength and direction of the central engine, and a non-uniform external medium, considerable insight into their gross morphology can be gained from axisymmetric simulations of steady jets in a constant ambient medium. This model, which we shall refer to as the basic model, has the additional virtue of being less computationally expensive than fully three-dimensional simulations, permitting a wider range of jet parameters to be investigated.

Introduction

What are extragalactic radio sources?

We believe that extragalactic radio sources are interactions between large-scale jets and the hot, diffuse gas that surrounds elliptical galaxies. The interactions not only cause the hotspots, bridges, tails etc., but also the radio emission from the jets themselves. Radio sources should be thought of as processes rather than objects: the overall radio structure must change substantially on the shortest possible dynamical time scale, the sound-crossing time. Another way to put this is that, at least in the FR II sources, there is no steady-state description. This is the main reason why radio sources are much harder to understand than, say, main-sequence stars. On the other hand, the lack of equilibrium means that the structure of radio sources reflects their past history, so that in principle it should be much easier to deduce the life-cycle of radio sources than that of stars. At present, the most successful theoretical models concentrate on regions for which a local steady-state description is likely to be appropriate, notably the bases of jets, far from any end-effects, and in the co-moving frame of the hotspots, where the jets terminate.

It is worth emphasizing that in the standard model of FR II sources, and to a lesser extent FR Is, we only see half the story in the radio. As we shall see, it is usually assumed that synchrotron radiation is only emitted by plasma which has entered the system via the jet.

Introduction

Whilst the luminous jets of radio galaxies and quasars are the most powerful examples of collimated outflow in the cosmos, there are many examples of jets and outflows to be found much closer to home, within our own Galaxy. These span a great range of luminosities and collimation factors, from the optically visible jets and “lobes” associated with low-mass young stellar objects, which are morphologically very similar to the classical radio galaxies, to the poorly collimated and much less clearly denned “jets” associated with the Galactic Centre and with various supernova remnants. Galactic jet sources also include the singular object SS 433, which is known to be emitting a two-sided jet at a quarter of the speed of light. This jet is known to be associated with a binary star system, and there is some evidence that other mass-transfer binaries may also have jets.

In many cases the jet material itself is insufficiently excited to dissociate it completely, giving us a variety of spectral lines at optical, infrared and radio frequencies with which to probe the underlying kinematics, while the mere fact that these objects are close gives us greatly enhanced linear resolution. If there is a lesson to be learnt from the wide variety of systems which exhibit collimated mass-loss it is that jets are very easily formed once symmetry is broken through rotation.

Introduction

In Chapters 1 through 4, we saw that the outer radio lobes associated with active galaxies receive their energy from a bulk hydrodynamic flow which emanates from the galactic nucleus. Bisymmetric outflow occurs on a wide range of scales in less energetic objects as well, as will be shown in Chapter 10.

Perhaps the most astonishing feature of cosmic jets is their ability to stay together over a very large range of distance scales. On their way from the black hole in the galactic nucleus to a radio lobe, cosmic jets cover a stupendous factor 109 in length scale. That is as if, exhaling forcefully at my desk in Leiden, I could blow about the papers on the desk of a colleague in Minneapolis. This should be a caution against off-the-cuff comparison between jets in radio galaxies and such comparatively easily understood items as laboratory jets, rocket exhausts, and numerical simulations.

The most natural explanation of the coherence of jets is that they are not jets at all, but gaseous cannonballs with a density that is much higher than that of their surroundings. What we perceive as jets would be a mixture of gas ablated from these “tracer bullets” and surrounding gas set aglow by their passage.

General introduction

History

In a description of an optical image of M 87 (NGC 4486), Curtis (1918) wrote “a curious straight ray … connected with the Nucleus”. By the 1950s the term ‘jet’ was being used to describe this feature which it seemed plausible to associate with ejection of material from the innermost region of the galaxy (Baade & Minkowski 1954), although the concept of a continuous flow was not then envisaged. Baade (1956) measured the optical polarization of the M 87 jet, supporting the idea that the material was a synchrotron emitting plasma akin to that of the Crab supernova remnant.

Shklovskii (1963), in an attempt to explain the double radio sources and M 87's jet, discussed many ideas that play a role in current theories – accretion of matter in the gravitational potential of a galactic nucleus; the consequent heating of a plasma that breaks out along a preferred axis; the flow of this material into intergalactic space and the re-energization of the electrons within the flow. However, the model still did not encompass the idea of a continuous flow, carrying energy in the form of bulk motion. Schmidt (1963) wrote of “a wisp or jet” on the image of the optical counterpart to 3C 273, and by about this time, the term ‘jet’ was in common usage (e.g., Greenstein & Schmidt 1965; Burbidge, Burbidge & Sandage 1965) – but still without a clear recognition that a continuous flow of matter and energy was involved.

More than three decades have passed since our present picture of extragalactic radio sources began to unfold. The latter half of that time has witnessed the ‘mapping’ or ‘imaging’ of jet-like structures in many of these sources, and the realisation that apparently similar phenomena are associated with many Galactic objects. Numerous books have discussed instrumentation and radiation processes; overviewed the physics underlying both extragalactic and Galactic sources, and their intervening media; and attempted to present a coherent picture of the AGN phenomenon. And yet, although some excellent reviews have appeared, no book has addressed the subject of astrophysical jets in a detailed and comprehensive manner. This volume is an attempt to fill that gap.

What makes such a volume particularly timely, is that we are now digesting the first generation of high-resolution observations of extragalactic jets (MERLIN, VLA and VLBI data), the first generation of numerical simulations (mostly two-dimensional and nonmagnetic), and the first generation of theoretical studies, which have given us a quantitative framework for estimating physical properties and energetics, and for discussing jet formation, propagation, and stability. Now is a time to take stock, as we await the first results of the VLB Array, satellite VLBI, three-dimensional and MHD simulations, and more refined theoretical studies. It also seems timely to compare and contrast the bodies of research on extragalactic and Galactic objects.

In order to achieve a detailed, comprehensive and critical text, it has been necessary to adopt a multi-author approach.

Let me organize this report by comparing some of the issues with the achievements described in the contributed papers.

Two important questions of ‘Asymptopia” are: (1) Relations between the sources and the asymptotic field of space-time, and (2) The existence and smoothness property of solutions of the field equations admitting a null infinity in the sense of Penrose. In view of its importance it is regrettable that not a single paper addressed the first question. Apparently it can still only be treated in the context of approximation methods. (Compare the workshops A5, A6.) Concerning the second question there is still no proof or counter example known.

There were however two contributions dealing with existence questions of solutions with certain asymptotic properties. Choquet-Bruhat demonstrated the existence of global solutions of the Yang-Mills Higgs equations of Anti-de Sitter space-time, under the condition that there is no radiation at timelike infinity. Reula showed - via implicit function theorem techniques - that near the Schwarzschild solution there does exist the expected number of stationary solutions of the vacuum field equations, with well defined Geroch-Hansen multipole moments. Up to now this was only known for the Weyl solutions, hence in the axisymmetric case.

The further contributions under this heading consisted mainly in extensions or refinements of already known results or approaches to certain questions. Let me mention two examples: Bičak and Schmidt extended the investigation of the global structure of boost-rotationally symmetric vacuum spacetimes.

Twenty six abstracts were submitted for this workshop, seven of which were selected for oral presentation. The main topics covered were gravitational lensing (9 abstracts), large-scale structure (6 abstracts) and cosmic strings (4 abstracts), all of these topics being represented in the talks. These are not the only areas which have seen important advances recently but they are perhaps the most interesting ones from the perspective of a relativist. In this report I will summarize the contents of the talks, referring to some of the posters where appropriate. Whenever distance scales arise, the Hubble parameter is assumed to be 50 km/s/Mpc.

Gravitational lensing

Blandford's plenary contribution (Chapter 5, this volume) illustrates the increasing usefulness of gravitational lensing as a cosmological tool in recent years and this is reflected in the large number of abstracts on the topic. Besides confirming light-bending itself, gravitational lensing can provide evidence of the existence and distribution of dark matter in galaxies, identify the presence of objects on scales from jupiters to supermassive black holes, probe features of large-scale structure, and perhaps even measure the Hubble constant. The posters of Ho Tenlin and Yakimov illustrated how particular instances of gravitational lensing can provide cosmological information. The talks focussed on more general mathematical issues.

The interpretation of observations of distant objects is complicated by the fact that a beam of light may suffer many weak gravitational encounters rather than a single strong one as it propagates through an inhomogeneous background.

Introduction

The gravitational interaction between waves is a phenomenon in which the richness and the originality of the theory of general relativity are explicitly manifested. It became apparent in 1970-71 when Khan, Penrose and Szekeres found the first exact solutions describing the collision of pure gravitational waves: it was shown that when two plane gravitational waves with collinear polarization, and with a step or an impulsive profile collide, their subsequent interaction culminates in the creation of a curvature singularity, an event unpredicted by any linearized version of the theory of gravity. As we shall see, this is only a particular result, although probably the most remarkable, of the interaction of gravitational waves. Similar behaviors are also manifested when waves of a different nature collide. This is due to the fact that any kind of energy generates a gravitational field. As a consequence, when two arbitrary waves collide, a gravitational interaction will accompany, as a side effect, the interaction which is peculiar to the particular fields considered. These gravitational effects, though negligible to some extent, are nevertheless relevant from a theoretical point of view. In this lecture we shall investigate the main features of the scattering of plane waves in terms of exact solutions of Einstein's equations. Therefore, let us start by explaining what gravitational plane waves are and how to find exact solutions of Einstein's equations describing their interaction.

Though historically, the solar system has been the principal area for testing theories of gravitation, we seem to be at the end of the golden age of solar-system tests (Reasenberg (1987)). The classical effects have now been measured within the limits of today's technology and further significant improvements cannot be expected in the near term. Future space-based experiments such as GPB, Gravity Probe B; LAGOS, Laser Gravitational-Wave Observatory in Space; and POINTS, the (proposed) Astrometric Optical Interferometer, await further technological as well as engineering developments and logistic (launch) support to deliver them to the laboratory of space.

Recent years have seen, however, ground-based gravitational experimentation undergo a resurgence, driven by new experimental capabilities and by new theoretical work. The question that was raised by Fischbach et. al. (1986) of a possible short-range gravity force, dubbed the “Fifth Force,” has been particularly important. Though the experiments that gave rise to this suggestion have, in retrospect, turned out to be less compelling than was originally thought, the gravitational physics community has been forced to recognize the possibility of a short-range gravitational interaction.

This suggestion lent itself to fairly straightforward testing and the experimental community responded with great enthusiasm and ingenuity. Now some five years later, though it appears that this quite plausible theoretical suggestion has been ruled out by experiments at the level which initially was suggested. Nevertheless it gave rise to a rather exciting period in gravitational physics.

Introduction

Although the roots of string theory go back to the late 1960's, the first connection between string theory and gravity was noticed in 1974 independently by Yoneya (1974) and by Scherk and Schwarz (1974). By the early 1980's it became clear that the recently developed superstring theory was an excellent candidate for our first perturbatively finite quantum theory of gravity. One loop calculations were shown to be finite and general arguments suggested that this should hold to all orders. (For a review of what was known at that time see Schwarz (1982).) Since then, an enormous amount of work has been done and progress made in our understanding of string perturbation theory. The evidence for finiteness has grown stronger and stronger (see e.g. D'Hoker and Phong (1988); Atick, Moore and Sen (1988); La and Nelson (1989), and references therein). Although there is always a chance of some unexpected results, no one who works on this subject doubts that it is true.

Conspicuously absent from this brief history is the remarkable explosion of interest in string theory beginning in the fall of 1984 and the almost equally remarkable drop in interest in the past year or so. This mood swing had nothing to do with string theory providing a consistent quantum theory of gravity. Rather, it resulted from the hope that the “uniqueness” of string theory would lead to definite low energy predictions in a simple way. Unfortunately, this hope has not been fulfilled.