On 1980 February 20 we conducted an 8-station intercontental VLBI experiment in order to study the nucleus and jet of M87 at 1666.6 MHz in right circular polarization. Our array was sensitive to structures from 0.001 to 0.1 arcsec. We made a hybrid map of the nucleus of M87, and also searched for compact structures within the knots of the jet. The map (Figure 1) shows that the nucleus of M87 contains a one-sided jet. This morphology is similar to that observed in many compact extragalactic sources. The position angle of the nuclear jet is 290.5 (±1) degrees, which precisely matches that of the 20 arcsec jet. No bending of the jet through an angle greater than about 2 degrees is observed. The nucleus also contains a large component (>0.1 arcsec) which is elongated along the same position angle as the jet and has a flux density of roughly 1 Jy. This component is fully resolved by the vast majority of our (u, v) points, and we could not map it with standard techniques.

Compact radio sources are both interesting from an astrophysical point of view and useful for geophysics. They propose some very interesting astrophysical questions as to their energy generation and transport. Their small size allows the measurement of their positions to unprecedented accuracy extending astrometric measurements of parallax and proper motions to further distances. These celestial “beacons” may be used to establish an almost inertial reference frame against which positions on the earth and the earth's motions such as Universal Time (UT1) and polar motion may be measured to centimeter scale accuracy. Finally these sources when used in conjunction with radio interferometry can allow precise time synchronization over global distances at subnanosecond accuracy.

Possible mechanisms for producing the observed broad radio spectra of compact extragalactic radio sources are discussed. The explanations considered are: (a) superposition of the spectra of sub-components, (b) inhomogeneous synchrotron sources, and (c) synchrotron radiation from “non-standard” energetic electron spectra. These three models have been compared with results of spectral and VLBI observations. These comparisons indicate: (1) if the “superposition” hypothesis is correct, then the subcomponents themselves must be inhomogeneous synchrotron sources, (2) there is apparently no general inhomogeneous synchrotron source which characterizes compact extra-galactic sources, and (3) many compact sources have spectra which resemble synchrotron radiation from a relativistic Maxwellian electron spectrum, but the inferred electron energies and magnetic fields have a wide range of values. VLBI observations of a selected sample of sources generally favor a single component model, but cannot distinguish between models (b) and (c).

The polarization structure of synchrotron sources provides information about the magnetic field structure and thermal particle distributions near the source. Such information is critical to a full understanding of compact sources in the nuclei of galaxies and quasars. VLBI maps of these objects are frequently interpreted as showing jets even though the maps are dominated by a few bright regions. If these sources do indeed contain jets then the jet should have a relatively well-ordered component of the magnetic field. The presence of this ordered magnetic field, as well as the presence of thermal plasma in and around the jet, should be revealed by the polarized radiation. We present below the results of the first successful VLBI measurements of the polarized emission from an extragalactic compact radio source.

During the past decade many kinds of optical surveys have discovered hundreds of quasars most of which (∼90 %) are radio quiet (<10 mJy at 5 GHz). We have observed two samples of quasars brighter than 17m.6 found by their emission lines and by their ultraviolet excess. We have also selected quasars with redshifts known to be greater than 3.00. A brief description of the observing technique is given by Sherwood et al. (1981b). We have compared our millimetre photometry of flat radio spectrum quasars with that of Ennis and Werner and find excellent agreement. For four of the 8 sources in common the data have been published: Kreysa et al. (1980) and Jones et al. (1981). In addition, analysis of our “noise” shows it to be white, gaussian distributed about zero. The three samples are summarized as follows.

Observations of NGC 1275 at ∼ 1396 MHz with the NRAO line interferometer in 1974 and 1976 suggest the presence of a very broad, shallow HI absorption feature centered at ∼ 5300 km sec−1. These observations were repeated in 1981 June with the Very Large Array using a greater bandwidth to determine a satisfactory baseline.

Many searches have been made in the last few years for 21 cm emission from neutral hydrogen in elliptical galaxies. Emission has been detected in several galaxies, which have 108 to 109 M⊙ of HI (for H○ = 100 km s−1Mpc−1). Upper limits between 106 and 109 M⊙ have been set for the HI mass in about 40 other galaxies. Why most E galaxies have so little gas, and why some few have detectable gas, remains a matter of great interest. Two of the galaxies with large HI mass, NGC 1052 and 4278, are known to have powerful nuclear continuum radio sources (P2380 ∼ 1022 WHz−1). Since both of these attributes are fairly rare among elliptical galaxies, their coexistence in these galaxies is not likely to have occurred by chance. We have therefore observed twelve other elliptical galaxies with nuclear radio power P2380 > 1022 WHz−1 at Arecibo Observatory, to determine whether a large mass of HI is a necessary auxillary to nuclear continuum emission.

The BL Lac object AO 0235+164 is an extreme example of a QSO containing a compact, non-thermal energy source which is violently variable. In 1975 it brightened simultaneously at optical and radio frequencies, with a hundred-fold increase in optical brightness on a time scale of a few weeks (Rieke et al. 1976; Ledden, Aller and Dent 1976). During the outburst 0235 was among the most luminous objects in the universe. It exhibits longer term variations at meter wavelengths (Condon and Dennison 1978), and has experienced repeated optical and short wavelength radio outbursts since 1975 (Pica, Smith and Pollock 1979). Very long baseline interferometry shows it to be the most compact known extragalactic radio source.

The variability of the highly redshifted 21 cm absorption spectrum in AO 0235+164 presents us with two essential facts. First, rapid changes occur in the line depths of foreground clouds with redshift z ≃ 0.524. Second, the background radio source is among the most violently variable objects known. Thus it is reasonable to suppose that changes in the lines stem from activity in the background BL Lac object. The question is how?

Flux variations are a common feature of flat spectrum compact extra-galactic radio sources. Detailed analysis and quantitative comparisons with theoretical models (e.g. van der Laan, 1966) are difficult due to the complex characteristics of the flux variations, which generally appear to consist of different outbursts blended together in time. Nevertheless, the general consensus is that the basic process has been correctly identified and consists in an expansion of a synchrotron radiating plasma cloud of relativistic electrons and magnetic field partially opaque to its own radiation. The main differences between data and predictions of the theory are that the variations propagate too fast and with too large amplitude toward lower frequencies. This behaviour however may be indicative of continuous energy supply and consequent accelerated expansion.

Attempts to use interstellar scintillations (ISS) to study the fine angular structure of quasars and galactic nuclei have not yet met with any success. The main reason for this lies in the comparatively large angular size of the nuclear structure which has been investigated. If dependencies on wavelength for the scintillation index, flux and angular size of a source are taken into account (Ozernoy and Shishov, 1980), it appears that centimeter wave band rather than decimeter wave range used in previous work is most appropriate for observing ISS.

Compact sources (compactness evidenced by flat/complex spectra) display a “flicker” in their intrinsic centimeter wavelength radiation, with an amplitude of about 2% and a characteristic time scale of a few days.

Progress is reported on a program to study the polarization, spectral and variability properties of compact sources at centimeter and millimeter wavelengths. Source characteristics divide according to the broadband shapes of their spectra.

Figure 1 shows observations for four sample sources from the Parkes 5 GHz polarization monitoring programme. Interesting features illustrated include

1. • Sudden changes of the position angle of the linear polarization by ≳ 70° in PKS 0537-441 and 1253-055 (3C279).

2. • A linear increase in the position angle of the polarization of PKS 2134+004 through 70° over 3/12; years.

3. • Distinct bursts of circular polarization in PKS 0430+052, 0537-441 and 1253-055. In PKS 0430+052 (3C120) such a burst coincides with the possible superluminal expansion (Walker et al., 1981). In PKS 1253-055 (3C279) a burst of circular polarization is currently occurring at a time of very low linear polarization.

Cm-wavelength observations of 15 BL Lac objects are presented. The degree of radio-wavelength variability is compared with the strength of the emission at optical and X-ray wavelengths.

Four sources have now been found by the Michigan variability program which exhibit large amplitude rotations in polarization position angle with time. The most straightforward explanation for the phenomenon is a physical rotation in the radio emitting region.

In the last decade correlations of the depolarization parameter λd with several parameters, i.e., the radio luminosity L, redshift z, and source size D have been investigated by several authors, where λd is the wavelength at which the percentage polarization drops to its half maximum. Kronberg ET AL. (1972) first pointed out that λd decreases with increasing z, while Morris and Tabara (1973) showed that λd increases with L and suggested that the depolarization is due to the internal Faraday rotation within radio sources. Conway ET AL.(1974), on the other hand, suggested that the λd-(1+z) relation is primary. Recently, Cohen (1979) showed that the λd-z relation may be the remnant of the physically meaningful relation λd-L, though the former is statistically real.

Recent optical polarization studies indicate that there are two distinct types of QSOs — “normal” QSOs with P ≲ 1%, and highly polarized QSOs (HPQs) with P > 3%. The HPQs are very similar to BL Lac objects, yet still share some properties of normal QSO's (e.g. strong emission lines). Our results generally support the relativistic beaming model for QSOs; however, certain key predictions of this model are not observed.

Several compact radio sources studied with VLBI show apparent transverse velocities much greater than the speed of light. This review is a summary of VLBI observations of these “superluminal” sources as of August 1981. The physical models proposed to explain this phenomenon are also discussed.

Figure 1 shows hybrid maps of the core of 3C 273B at five epochs, made with arrays of 4 or 5 VLBI antennas. The maps span a period of 3.5 years. They all show a bright eastern peak and a lower-brightness extension to the west. There is a local maximum in the western extension between 6 and 8 milliarcsec from the main peak. This “blob” moves steadily further away from the main peak along a roughly straight line in PA −116° ± 2°. Compare this with the position angle of the 25-arcsec optical jet, −137°. The maps show a slight curvature to the south with increasing separation from the main peak. Lower-resolution VLBI maps at lower frequencies show that this curvature continues at greater separations, suggesting a smooth connection between the milli-arcsecond position angle and the position angle of the optical jet. In our latest map (1981.09) the blob is no longer detectable with the limited dynamic range of the VLBI network (about 20:1).