A campaign of VLBI observations of SS433 using the European network was begun in January 1980 and since October 1980 has continued at intervals of about two months except for the last two observations which were 6 days apart, see Table 1. The results of the first two epochs have been published in Nature (ref. 1), and those of the following six epochs are being prepared for publication.

Radio observations have clearly demonstrated that the kinematic twin-jet model (Milgrom 1979; Abell and Margon 1979) is the correct description of the general behavior of SS433 and have determined the orientation parameters that could not be obtained from the optical observations (Gilmore and Seaquist 1980; Hjellming and Johnston 1981 (HJ); Niell, Lockhart, and Preston 1981 (NLP)). Figure 1 shows the observed position angle of the radio jet at a distance of approximately from the core during the period 1979 May to 1981 May as determined from our VLBI measurements at 2.3 GHz. The mean position angle is 98°±2° for a 20° half-angle cone of precession about an inclination of 79° to the line of sight. The phase is consistent with the expected propagation time out to (1016cm) at a speed of 0.26c for a distance to SS433 of 5 kiloparsecs (HJ; NLP).

Observations of SS433 are consistent with the view that the Doppler-shifted line emission originates in a pair of oppositely-directed, precessing jets in which a gas outflow is maintained at the remarkably time- and space-invariant speed of 0.26c. A radiative acceleration mechanism is described for the jets and a detailed, numerical, relativistic flow calculation presented which explain this terminal velocity as the result of “line-locking”. The “line-locking” mechanism suggested here for SS433 may be important as well in extra-galactic radio sources in which the radio luminosity is similarly weak compared with the kinetic energy and optical luminosities.

The evidence is now compelling that “jets” delineate the channels along which power is supplied from galactic nuclei into extended radio sources — this accumulating evidence, reported by many speakers, has been one of the major themes of this conference. Jets (often apparently one-sided) have been discovered inside many symmetrical double sources. and M87, familiar optically as a “one-sided jet” for over 60 years, is now found to have weak double radio lobes. The VLA has resolved ∼ 70 jets in extended sources; there are now many instances where small jet-like structures are found on the VLBI scale; indirect arguments (some of which I'll mention) indicate that there is directed outflow on still smaller scales, and that the primary collimation may occur right down in the central “powerhouse” (scales ∼< 1015cm). The length-scales relevant to jet production and propagation thus span 9 orders of magnitude; the physical processes and conditions may vary widely over this vast range of scales. This paper will deal briefly with two aspects of jet physics: firstly, some direct inferences from radio maps; and, secondly, some possible mechanisms in galactic nuclei that could set up a collimated outflow.

The slow decline of surface brightness along many large-scale jets indicates that their internal pressures are determined largely by dissipation. If dissipation arises from a viscous interaction between a jet and its environment, then the observed degree of collimation enables one to constrain the nature of the viscous stress. Simple phenomenological models of the stress account for the frequently observed “gaps” and provide a means of slowing down jets without their becoming decollimated.

Henriksen, Bridle and Chan (1981; HBC) have proposed that the energy for the synchrotron emission of radio jets is derived ultimately from the turbulent hydrodynamic eddy cascade between the Taylor wave number, kT, and the Kolmogorov wave number. This cascade is established during the development of a turbulent jet by the shearing action of large-scale vortical entrainment of ambient material (Brown & Roshko, 1974).

Several authors have suggested that radio jet morphologies resolved in extragalactic sources are the effects of large-scale Kelvin-Helmholtz instabilities in high-speed, pressure-confined fluid beams ejected from parent active galactic nuclei (Ferrari et al. 1978, 1979, 1981; Hardee 1979;Benford et al. 1980). In particular results from studies for cylindrical geometries indicate how to connect the “wiggles” (observed in 3C449, NGC 6251, M87 and Cen A) with helical perturbations and the “knots” (observed in NGC 315, M87, Cen Aetc.) with pinching modes. Correspondingly small scale MHD perturbations, generated by the same instability or nonlinear cascade processes, are efficient in accelerating relativistic electrons via stochastic scatterings (Lacombe 1977; Ferrari et al. 1979). This picture may satisfy both the requirements for in situ re-acceleration and the intrinsic correlation between morphology and emission.

Radio jet morphologies may be caused by large-scale Kelvin-Helmholtz (KH) instabilities. If high-speed, pressure-confined fluid beams lie at the core of these jets, they are susceptible to KH modes, as studied by a number of authors.1–5 The “wiggles” seen, for example, in 3C449, NGC 6251, M87 and Cen A, may arise from helical instabilities.3,4 “Knots” may be radial oscillations (not necessarily unstable) forming sausage-like regions of compression, as in NGC 315, M87, Cen A etc. There seems no reason why such macroscopic modes should not appear. The smaller scale waves, however, cannot be easily resolved by radio astronomy, and can have profound effects, such as particle reacceleration. It would be rather more satisfying if there were observable large-scale implications of the microturbulence.

Many extended radio sources seem to need in situ regeneration of the relativistic electrons. MHD turbulence generated by surface instabilities has been suggested as the reacceleration mechanism. However, Eilek (1981) has shown that short wavelength MHD waves, which are the most effective particle accelerators, are strongly damped in radio sources. This results in the turbulent region being confined to a thin layer on the edge of the source, so that particles accelerated here must propagate into the radio source if this reacceleration mechanism is to account for the internal synchrotron luminosity. Most likely the particles diffuse across tangled field lines; using numerical modelling of the turbulence, Eilek showed that the particles propagate only a small distance (10 pc–1 kpc) in from the edge. This predicts that larger sources should appear limb brightened; but such limb brightening is rare. If short wavelength MHD waves are indeed the source of reacceleration, they must be generated internally.

Various authors have suggested that there is a close connection between jets in radio galaxies and the precessing beams of SS433, and that the underlying mechanism for forming the jets is the same on both scales (Rees, 1981). We examine a possible model for generating gas jets close to the central object in either of these two cases.

A consistent model of extra-galactic double radio sources must evidently involve all aspects of the source from the underlying power source to the production of the radio lobes. Here, we give an overview of our work on the different aspects of a self-consistent model which includes gravitational accretion of fluid with angular momentum as the power source, the production of hydrodynamic or relativistic particle-beam jets, and the formation of expanding radio components.

For technical reasons, infrared studies of active galaxies have lagged far behind optical and radio ones. This is unfortunate, since entirely new aspects of these sources are often revealed in the infrared. The extreme efficiency of dust at degrading ultraviolet photons into cool thermal emission frequently makes the luminosity of an extragalactic source inaccessible to optical and radio astronomers. At the same time, the effects of dust on optical emission line ratios and continuum shapes can be profound. The complete identification of samples of radio sources will require infrared observations to supplement the optical techniques now generally employed, and the extreme properties of the sources bright in the infrared can provide new insights to conditions in extragalactic nonthermal sources. To illustrate these points, I will discuss three cases: 1.) galaxies undergoing a powerful burst of star formation, 2.) intermediate type Seyfert galaxies, and 3.) an extreme infrared identification of an extragalactic radio source.

For more than 20 years it has been known that extragalactic radio sources contain up to 1060–1062 ergs in the form of relativistic electrons and magnetic fields. One arrives at these figures if one assumes that the radio emission is due to the synchrotron process and the source contains an equal amount of energy in electrons and fields (Burbidge 1956). Any deviation from the postulated equipartition increases the energy required to account for the observed luminosities. Some authors believe that the real demands on the energy source may be still higher because of the probable presence of high energy protons. The ratio Ep/Ee is determined by the way in which particles gain and lose energy, and it is impossible to estimate it a priori. Observationally one has two conflicting lines of evidence: (a) in galactic cosmic rays one measures (Ep/Ee) ≃ 102; (b) in the Crab Nebula one infers (Ep/Ee) ≲ 1 (otherwise the dynamical pressure of the proton gas would cause a nebular expansion much faster than observed).

Powerful, extragalactic radio sources might be fuelled by energy release near a massive black hole. In this article we describe some relativistic effects which may be relevant to this process. We use Newtonian language so far as possible and illustrate the effects with “simple” analogies. Specifically, we describe the gravitational field near a black hole, Lens-Thirring and geodetic precession, electromagnetic energy extraction of the spin energy of a black hole and the structure of accretion tori around a black hole.

This paper reviews some of the properties of supercritical accretion which make it a possible model for the quasar optical central source and for producing the jet in radio sources. Some of the problems with this scenario can be remedied with alternative, but related, models.

A schematic model is suggested to describe the various activities in the centers of massive galaxies. Basic assumptions are that (i) a uniform model can account for all the observed phenomena, such as quasars, blazars, radio galaxies, Seyferts, and the centers of normal galaxies (with disks), (ii) activity in galactic centers is repetitive, and (iii) in-situ-acceleration of highly relativistic electrons (other than by adiabatic compression) has an insignificant efficiency, i.e. is ignorable.

The first Einstein observations established that quasars, as a class, are luminous X-ray emitters (Tananbaum et al. 1979). Since then, one of our major programs has been to carry out further X-ray and optical observations to improve our estimate of the contribution of quasars to the 2 keV extragalactic isotropic X-ray background and also to reconcile number counts of discrete X-ray sources with reported optical number counts of quasars (cf. Zamorani et al. 1981 and references therein). This paper is a preliminary report on our observations of the 1.7 square degree field studied originally by Braccesi and his colleagues (Formiggini et al. 1980 and Braccesi et al. 1980). A more detailed presentation of our results is in preparation (Marshall et al., 1981).

Hybrid maps of the nuclei of radio galaxies and quasars show a variety of morphologies. Among compact sources, two structures are common: an asymmetric, “core-jet” morphology (eg, 3C 273), and an “equal double” morphology with two separated, similar components (eg, CTD 93). The nuclei of extended, double radio galaxies generally have a core-jet morphology with the jet directed toward one of the outer components.

We report here preliminary results of observations of the quasar 3C147 which were made at 6 cm with a resolution of about 1 milliarcsecond using a VLB interferometer system with four antennas in the USA and one in Europe. Our observations are shown in Figure 1 along with previously published maps on larger size scales. VLA observations made at 2 cm wavelength (Fig. 1a) show an extended feature lying about 0.5 arc-seconds (3.5 kpc) to the northeast of a bright core (Readhead et al. 1980), while VLBI observations made at 18, 50, and 91 cm (Readhead and Wilkinson 1980, Wilkinson et al. 1977, and Simon et al. 1980) show a jet-like feature extending about 0.2 arcsec (1 kpc) in the opposite direction (Fig. 1b). The 18 cm VLB observations also indicated the presence of a smaller elongated feature extending only 3 milliarcseconds (20 pc) again toward the northeast (Fig. 1c). We have observed 3C147 in March 1978 and again in April 1981 with a resolution of 0.7 and 1.5 milliarcsec respectively. The 1981 data (Fig. 1d) clearly show the double structure of the core as well as a lower surface brightness feature which can also be seen in the 18 cm map. Our 1978 data (Fig. 1e) has better resolution and shows considerable structure in the milliarcsec component, but due to the absence of phase information in these data, the details are not reliable and the orientation is uncertain by 180°. The orientation is, however, specified by reference to the 1981 data.

The extremely powerful compact radio nucleus of NGC1275 is perhaps the most complex structure seen at milliarcsecond scales. Early attempts to determine its structure (Schilizzi et al. 1975) were unsuccessful, although it was evident that the structure consisted of several bright regions whose relative positions remained fixed while their intensities varied. Later observations and analysis (Pauliny-Toth et al. 1976; Preuss et al. 1979) confirmed this inference and revealed a slow expansion in a direction transverse to the primary axis of alignment.