BL Lac shows tightly correlated events in flux density and polarization. When the time scale slows enough for VLBI observations to track them, moving features which mimic piston-driven shocks seem to carry the disturbances. Evidence for BL Lac's shocks may ameliorate the difficulties posed by the excessive incidence of superluminal motion among core objects; superluminal motion may arise from any number of conditions combining shocks and optical depth, not just real jet or real component motions.

Due to sensitivity problems only about a dozen of the powerful double-lobed radio galaxies have so far been mapped with VLBI. Even less is known about the time dependence of the small scale structure in these objects (Preuss and Alef, 1987). We have recently reported the first results of our monitoring program of classical double sources. We observed strong changes of the pc-scale structure in 3C111 (Götz et al., 1987) and 3C390.3 (Alef et al., 1987) implying “superluminal behaviour” in both sources. This is the first time that such a phenomenon has been found in lobe-dominated radio galaxies. Both objects are Broad Line Radio Galaxies of type N, and their radio emission from m to cm wavelengths is dominated by their outer lobes which are ∼300 kpc apart (H0 = 50 km/s/Mpc).

There have been recent suggestions and suspicions that the earlier unified scheme, which largely involved quasars, could be enlarged upon to include galaxies as well. In this scheme, the extended radio galaxies are believed to be close to the plane of the sky while the lobe and core-dominated quasars are seen at progressively smaller angles to the line-of-sight. In this paper, we study the correlations of the degree of core prominence with optical properties as well as radio structures on VLBI and larger scales for the both radio galaxies and quasars, in order to investigate whether they are consistent with the enlarged unified scheme. From earlier statistical studies of quasar samples, the degree of core prominence has been shown to be a reasonable statistical indicator of source orientation (cf., Saikia 1985 and references therein). Saikia (1984) also showed that jets in quasars are more easily detected in the ones with more prominent cores, consistent with the unified scheme. In the present paper, we present the results of our observations of 30 low-luminosity radio galaxies and investigate whether such a result holds for galaxies as well.

A good correlation is known to exist between the X-ray and radio luminosity of flat-spectrum, core-dominated radio sources (e.g., Owen, Helfand and Spangler 1981). Worrall (1987) presents a logarithmic plot of spectral luminosity in the source frame at 2 keV versus that at 5 GHz for a variety of QSOs, Highly Polarized QSOs (HPQs), and BL Lac Objects. Friedmann cosmology with Ho = 100h km s−1 Mpc−1, qo = 0 is assumed. Exclusion of objects which are optically or X-ray selected, or in which the radio emission is not dominated by a flat-spectrum compact core, gives a sub-sample consisting of 50 QSOs, 20 HPQs, and 10 BL Lacs, of which 5,4,3, respectively, are known superluminals. The dispersion of these data about the log-log correlation (assuming a Gaussian distribution), is σobs = 0.44 ± 0.06 (90% confidence errors for one interesting parameter).

Compact Steep Spectrum (CSS) radio sources are defined as having straight steep spectra typical of extended double-lobed radio sources and structures unresolved by conventional interferometers (Peacock and Wall 1982; van Breugel et al., 1984; Fanti et al., 1985). Their spectra remain straight up to quite high frequencies, indicating that no dominant core is present. Sometimes the spectrum bends at frequencies lower than 1 GHz. The angular sizes imply projected linear sizes generally < 10 kpc.

A large fraction of sources in flux limited radio samples consists of compact steep spectrum (CSS) sources (Fanti et al. 1987). These objects have their dominant emission on kiloparsec scales and are found in a variety of active galaxies, including Seyfert II's, peculiar galaxies and quasars. Observations of very nearby CSS sources have shown good correlations between their complex radio structures and optical emission-line gas suggesting violent interactions between jets and dense gas in their ambient interstellar media (van Breugel et al. 1984). Similar jet/gas interactions may occur in more distant and more powerful CSS sources.

Fifteen sources known to be varying at low frequencies have been observed at six epochs during 1983–84 with a global VLBI array. Some of the sources show structural variations similar to the superluminals. Beaming effect may therefore play an important role at low frequencies as well as at higher.

The compact steep spectrum radio sources, 3C119, 3C287 and 3C343, are identified with QSO'S of redshift 0.46, 1.055 and 0.988 and visual magnitude 20.0, 17.7 and 20.6 respectively. The overall physical extent of each source is −500 pc (Fanti et al. 1985, 1986). Here we present the results of new global VLBI 18 cm observations which have been combined with MERLIN data to improve the U-V coverage. The data were mapped using the CALTECH and AIPS software packages and also using OLAF software system developed in Jodrell Bank for 3C119 and 3C343.

We have made VLBI observations of a mixed sample of bright galaxies and quasars at 327 MHz using the Ooty Radio Telescope in India and the telescopes at Jodrell Bank, Westerbork, Torun, and Crimea in December 1983 and March 1986. MK II recording was used and the data were processed at MPI, Bonn. The preliminary results of the 1983 analysis (Ananthakrishnan and Kulkarni 1986) showed most of the nearby galaxies to be resolved over the longest baselines of 8 million wavelengths. The only seyfert galaxies for which good S/N ratio was available at the longest baselines were 3C84 and 3C120. Although UV coverage was poor we attempted to make maps of these objects. The radio source 1148-001 was used for gain calibration of the telescopes.

In recent years it has become clear that the common practice of dividing radio sources into two classes, compact flat-spectrum sources and extended steep-spectrum sources is too simplistic and that compact steep-spectrum sources (CSS) sources which seem to form a separate class in themselves, need to be taken into consideration. A majority of flat-spectrum sources are known to show asymmetric or D2 structures. In contrast to this many CSS sources when observed with subarcsecond resolution show symmetric structures typical of extended steep-spectrum sources. However, further high resolution observations of CSS objects are clearly needed to establish the structural differences, if any, between the two types of compact objects and to understand the interrelationship between the three classes of objects. In this connection we have undertaken a program to observe a sample of 18 CSS sources using the European VLBI network. The sample was selected from the compilation by Peacock and Wall (1982), according to the following criteria: (1) flux density > 1.8 Jy; (2) largest angular size < 1 arc sec; and (3) steep high frequency spectral index (> 0.5) and high degree of low frequency spectral curvature. Our aim was to find out the structures common to these sources and their relation to more commonly observed “centimetre excess” compact objects. Here we will present the preliminary results.

The observed variability at low frequency of the radio sources can be explained within the framework of the generally accepted models either extrinsic (refractive scintillation in the interstellar medium) or intrinsic (bulk relativistic motion along direction near the line of sight) for variability. Both explanations require a large fraction of the source flux density to be contained in a small high brightness component, of tens of m.a.s. in size. Radio sources with steep straight spectral index are usually tens of Kpc sized, with weak central components and they do not generally show low frequency variability.

3C236 is the largest radio source known. It has a 39 arcmin double structure (3.0 Mpc, Ho=75 kms−1 Mpc−1) in which the SE lobe is narrow and edge-brightened and the NW lobe more diffuse and centre-brightened (Barthel et al., 1985). About half the flux density comes from a steep spectrum (α = −0.7) radio core of overall size 1.3 arcsec (2.2 kpc) located in the centre of a 17m elliptical galaxy with redshift 0.0988.

The steep, small angular size QSO 3C286 (z = 0.846, mv = 17m) (Burbidge and Burbidge 1969) is a bright object in the radio band, which has a steep spectrum (α = −0.65, S ∼ fα) from 1 GHz, peak in spectrum at about 100 MHz, and self-absorption below 50 MHz. 3C286 is a member of a well established class of radio sources (the CSSQSO) and is similar to 3C147, 3C309.1, and 3C380 in general.

The well known jet in M87 (Virgo A) has been extensively studied in the radio regime using conventional arrays with resultant resolutions in the range 0.22–1 arcsecond. We present here the results of a VLBI experiment to map the jet over its entire length (30 arcseconds) to high resolution. The observations were made in April 1984 at 1.67 GHz with a ‘World Array’ containing 18 VLBI telescopes. In addition, simultaneous observations were performed with the 6 station MERLIN array.

A critical test of the simple relativistic beaming models is provided by the search for structural changes in the weak cores of lobe-dominated quasars. In these sources, which are expected to be close to the plane of the sky, superluminal motion should be comparatively rare. We are engaged in the study of a complete sample of 30 extended quasars (Barthel et al. 1984), ranging in projected size from 40h−1 to 200h−1 kpc (Ho = 100h, qo = 0.05). High resolution multi-epoch observations have been obtained for two objects, the quasars 1721+343 and 0742+318, at 5 GHz (1982.3, 1983.3, 1986.5) and 10.7 GHz (1986.2, 1986.5). Superluminal expansion with transverse velocities of 3.1h−1 c has been detected in the core of 1721+343, which is the largest quasar known (Barthel et al., these proceedings). No superluminal motion was found in 0742+318. The large and small scale structure of 1721+343 are shown in fig.1a,b. The spectra of the individual components point to component A as the core

The radio source 3C205 is identified with a quasar of redshift 1.53, and is, at first glance, a standard, powerful double radio source with hotspots. Upon closer examination (MERLIN, VLA A-array), the southern lobe is seen to consist of two distinct hotspots plus some more diffuse emission, some 50 kiloparsecs from the quasar (Ho ∼ 75). The arcsecond-scale morphology and polarization structure of this hotspot complex strongly suggests that the larger and weaker off-axis secondary hotspot has been formed by outflow of material from the more compact primary hotspot (Lonsdale and Barthel 1984,6). This interpretation implies a collision between the presumed energy supply beam and something capable of deflecting the flow energy towards the secondary. Because the flow is so energetic, this obstacle would have to be both massive and dense. The main constraint on the density of the obstacle comes from measurements of the internal energy density of the compact primary hotspot coupled with model-dependent estimates for the maximum advance velocity of this feature, thus yielding a balance with the ram-pressure exerted by the obstacle medium.

VLBI observations of the nucleus of Centaurus A were made in April, 1982 at two frequencies with an array of five Australian radio antennas as part of the Southern Hemisphere VLBI Experiment (SHEVE). Observations were undertaken at 2.29 GHz with all five antennas, while only two were operational at 8.42 GHz. The 2.29 GHz data yielded significant information on the structure of the nuclear jet. At 8.42 GHz a compact unresolved core was detected as well.

Six radio telescopes were operated as the first southern hemisphere VLBI array in April and May 1982. Observations were made at 2.3 and 8.4 Ghz. This array produced VLBI images of 28 southern hemisphere radio sources, high accuracy VLBI geodesy between southern hemisphere sites, and subarcsecond radio astrometry of celestial sources south of declination −45 degrees. This paper discusses only the astrophysical aspects of the experiment.

On 8–12 October 1981 a 5-station MkII VLBI experiment at 1660 MHz was performed using antennas at Westford, Maryland Point, Green Bank, Fort Davis, and Owens Valley. Fifteen extragalactic sources (Table 1) were observed with an average synthesized beam of roughly 10 mas. The data were processed using the NRAO correlator, globally fringe-fit, and mapped with the AIPS package. In addition to the usual self-calibration techiques, corrections were made to eliminate baseline-dependent calibration errors. Some of the sources, for example 1641+399 (3C345), display extended components not before observed at the dynamic range of the maps, which on the average is roughly 200:1.

We review the polarization sensitive VLBI observations of compact extragalactic radio sources that have been made to date, and discuss some preliminary physical results suggested by the data.