We review attempts to incorporate radio sources within the context of general models of active galactic nuclei. The behaviour of gas accreting onto a massive black hole depends upon its angular momentum and accretion rate. It is argued that radio galaxies, QSR's and QSO's (and Seyfert 1 galaxies) be associated with increasing mass accretion rates The classification of an active galaxy appears to be aspect-dependent. In particular BL Lac objects, OVV quasars and the superluminally expanding compact sources appear to be beamed towards us. We show how the choice of source model can influence the statistics of beaming.

Although VLBI reveals structure in active galaxies on scales as small as a few parsecs, this is still likely to be several orders of magnitude larger than the scales on which jets are accelerated. We must therefore rely on theory, bolstered by indirect evidence, to deduce the behavior of plasma immediately surrounding the “central engine”. One hypothesis which has received much attention recently postulates that jets emerge from thick tori of gas which surround massive black holes. The low optical and X-ray luminosities of radio galaxies imply that tori in these objects would have to be supported by ion pressure, while tori in quasars and Seyferts may be supported by radiation pressure. Rees et al. have proposed that two parameters, α(the ratio of viscous stress to pressure) and (the ratio of the accretion rate to the value associated with the Eddington limit), are sufficient to determine the general characteristics of a torus, independent of the mass of the black hole. While existing observations of radio jets do not contradict the existence of such a homology, systematics involving the broad emission lines in quasars and Seyferts suggest that the homology may fail in these objects, at least insofar as conditions at ≲1 parsec reflect conditions in the torus. I report here on how the onset of nuclear reactions in a radiation-supported torus may break the homology, and regulate the properties of the broad line regions in quasars and Seyferts.

The growth lengths of Kelvin-Helmholtz instabilities are calculated for three vortex-sheet bounded beams with v = 0.9c. These beams survive the action of the instability for distances of several hundred beam radii, but no prediction of a single dominant mode of breakup can be made.

In the framework of continuous beam from small (VLBI region) to large scales (VLA region), we deduce some typical quantities of VLBI-jets like maximum luminosity and time-scales of formation of turbulence in the hypothesis of turbulent cascade triggered by relativistic Kelvin-Helmholtz instabilities.

We discuss wind-type solutions for flows from accretion funnels, and show under what physical conditions such flows can become supersonic and relativistic already very ciose to the stagnation point within the funnel. The acceleration is due to radiation emitted by the funnel walls, while the location of the transonic points is also affected by the geometrical shape of the funnel's cross-section.

In recent years an elegant hypothesis for a unified scheme for compact and extended radio sources, and also radio quiet QSOs and radio loud Quasars has been developed (Scheuer & Readhead 1979, Orr & Brown 1982). In this scheme the relative orientation of (relativistic) beam and observer is thought to explain the observed source morphology. We feel that observations suggest examining compact and extended radio sources from still another “angle”.

In the context of superluminal motions, the statistical difficulties intrinsic to the canonical jet geometry are alleviated if the relativistic flows occur on a plane screen, orthogonal to the source axis. The strongest superluminal effects should be detected when the source lies in the plane of the sky, rather than pointing at the observer. The polarization properties, the simplest deviations from the basic geometry, and the connection with other non-thermal phenomena are also indicated.

We have conducted a series of VLBI observations of the gravitational-lens images of the quasar Q0957+561 (Walsh et al., 1979), utilizing the Mark III VLBI data acquisition system (Rogers et al., 1983). The goals of our observations are to (1) map the milliarcsecond structure of the A and B images, (2) detect the predicted third image of the quasar, and (3) determine the time delay between the images. We will use these results to constrain the mass distribution of the lens and, possibly, cosmological constants.

On 1981 March 17–18 we undertook MkIII VLBI observations of the quasars 1038+528 A, B (Owen et al. 1979; Owen et al. 1980) with an array of 7 telescopes operating simultaneously at λ3.6 and λ13 cm with right circular polarization reception at each wavelength. Because the sources are ~33″ apart they could be observed simultaneously at every telescope. Thus the corrupting contributions of the propagation medium and the instrumentation were approximately the same for each of the quasars, hence allowing us to calibrate the structure phase of B with respect to a reference point chosen in the map of A using the expression where φB and φA are the observed fringe phases, φGB and φGA are the geometric contribution with respect to the reference points chosen in each map and φSA is the structure phase contribution with respect to the reference point chosen in the A map.

The double quasar 00957 + 561 A,B along with bright radio arches and VLBI structures is modelled using a gravitational lens consisting of an elliptical galaxy and a cluster. The effective time-delay between components A and B comes out to be about a year and this enables one to distinguish between intensity fluctuations resulting from intrinsic quasar variations and minilensing by low mass stars.

This review is an attempt to summarize VLBI continuum observations at cm- and dm-wavelengths of active galactic nuclei at distances ≲ 100 Mpc (Ho = 50 km s−1 Mpc−1). ‘Nearby galaxies’, thus defined, are close enough for achieving the highest possible spatial resolution. Galaxies at these distances, however, typically do not show extreme and rare forms of nuclear activity such as powerful radio sources, the cores of which are relatively easy to map with VLBI, and which are therefore the subject of most of the VLBI work done so far (see e.g. Preuss, 1983). Nearby active galaxies show rather more ‘ordinary’ forms of nuclear activity; they include a few of the weaker classical ‘radio galaxies’, but most of them are Seyfert galaxies and mildly active ‘normal galaxies’. Their total radio emission is typically weak (P(5 GHz) ≲ 1031 erg s−1 Hz−1) and so are their compact radio nuclei (if any). The highest available sensitivity is therefore required for their study and the current instrumental performance is just becoming sufficient to tackle the strongest of them in the hope of obtaining maps.

NGC 3894 is a 13th magnitude E/SO galaxy (de Vaucouleurs, de Vaucouleurs & Corwin 1976; Nilson 1973) whose recession velocity (Kelton 1980) implies a distance of 44 Mpc if Ho=75 km s−1Mpc−1. The radio emission from NGC 3894 is compact-core-dominated and, as shown in Fig. 1, this compact core is surrounded at 1.5 GHz by weak diffuse emission with a characteristic size of ~2 arcmin (26 kpc) (Wrobel & Heeschen 1983). If the compact core radiates isotropically, then its radio luminosity is ~1040 erg s−1.

New observations of the radio nucleus of the nearby bright spiral galaxy M81 (NGC 3031) show that the structure of the nucleus is considerably more complex than previously thought. The radio nucleus has a slightly inverted, variable spectrum (de Bruyn et al. 1976; Crane, Guffrida, and Carlson 1976). The variability and VLBI observations (Kellermann et al. 1976; Jones, Sramek, and Terzian 1981) both indicate a linear dimension of ~1500 AU. The recent VLBI observations of Bartel et al. (1982) determined that nearly 100% of the emission originates in an elongated region with linear dimensions of 1000–4000 AU. Peimbert and Torres-Peimbert (1981) have classified the optical nucleus of M81 as Seyfert type 1.5 (the weakest known) with the narrow-emission-line region extending over ~5″ (Münch 1959).

150 nearby bright galaxies have been observed for compact components at 327 MHz by the method of Interplanetary Scintillation (IPS) using the Ooty radio telescope; scintillation was detected in 28 of these galaxies indicating compact components with size < 30 mas to 300 mas having flux density > 100 mJy. Comparison of our data with other results at high frequencies indicates that these compact components are associated with the central regions of the galaxies.

The type 2 Seyfert galaxy Markarian 348 (NGC 262) is a normal early-type disc galaxy (SO/a or Sa) surrounded by a very large HI envelope. The continuum radio source in the galaxy is unresolved by conventional interferometers on scales as small as an arcsecond, is variable on time scales of a few months, and is quite strong by Seyfert standards (300 mJy). Mkn 348 is relatively nearby, allowing one to study very small-scale structure in the galaxy (at the assumed distance of 60 Mpc (Ho = 75 km/sec/Mpc), one light year subtends an angle of about one mas). Both the relative proximity and the radio source strength make it a good candidate for VLBI studies.

The center of our Galaxy contains an extremely compact nonthermal radio source. For the first time, elongation in the source structure has been detected. The long axis is nearly aligned with the minor axis of the Galaxy. Recent high resolution observations of the ionized gas within the central 3 parsecs suggest that matter may be falling in towards the center. This has interesting implications on the processes within our Galactic nucleus.

Mark III VLBI observations of the pulsars PSR 0329+54 and PSR 1133+16 were made at 2.3 GHz using antennas with diameters and locations as follows: 100m, Effelsberg, West Germany (but only for SPR 0329+54); 43m Green Bank, WV, USA; and 40m, Big Pine, CA, USA. The Mark III processor at the Haystack Observatory was “gated” to compute visibility amplitudes and phases as a function of pulsar longitude. This method allowed a) an improvement of the signal to noise ration, by as much as a factor of ten in the case of PSR 1133+16, and b) an interferometric investigation of the pulse structure.

Since 1982 July we have been engaged in a systematic investigation of the milliarcsecond structure of the radio emission from several binary star systems, mainly in the RS CVn class (Hall, 1976). The first few observations utilized the MkII VLBI recording scheme, but no useful data were obtainable because of the relatively low flux levels involved Beginning in 1982 December, we have been using the MkIII system with large telescopes and have successfully detected seven binary systems (UX Arietis, HR 1099, Algol, II Peg, σ Crb, and the distant system LSI 61°303). A summary of the three experiments already completed is given in Table 1.

Cyg X-3 underwent a series of giant radio outbursts beginning on September 28, 1982 (Geldzahler et al. 1983). The flux densities at 2.7 and 8.1 GHz (11.1, 3.71 cm respectively, see Figure 1) were measured with the 2.4 km baseline of the Green Bank interferometer once every three days before October 5, 1982 (= JD 244 5248) and three times daily thereafter.

Sco X-1 was observed in the A configuration of the VLA on 4 Feb. 1981 and 18 Mar. 1982 (Fomalont et al. 1983). The radio map at 1.5 GHz, made with the 1982 data, is shown in Figure 1. The central component is nearly coincident with the binary and has an angular size of less than 0″.1. A 0″.5-resolution map of the SW component of Sco X-1, shown in Figure 2, displays only the brightest peaks in the two emission regions. The brightest part of the emission is located at the extreme edge of the component, and its location and shape are reminiscent of hot spots in radio lobes associated with luminous extragalactic radio sources. Although the NE component is only slightly resolved at this resolution, it is extended in approximately the same position angle as that of the entire source and that of the SW component.