Results of instantaneous (with accuracy of a minute) spectra observations for a sample of 113 compact objects, selected from the VLBI survey, are presented. Measurements have been made in 1989.9 at the RATAN–600 at 7 wavelengths of 31, 13, 8.2, 7.6, 3.9, 2.7 and 1.4 cm as a set of our spectra monitoring program.

Here are analyzed the results of instantaneous multifrequency observations for a complete sample of 113 extragalactic radio sources (see the paper by Yu.A. Kovalev et al. in these Proceedings) with declinations −30° ÷ 43° from the VLBI–survey. It is tested the hypothesis that quasars and BL Lacs have different nature. Statistical distributions for the turnover flux, turnover frequency, spectral indexes, and other parameters are analyzed.

The Swedish-ESO Submillimetre Telescope (SEST) has been used for the high radio frequency observations of our group's AGN monitoring projects since the end of 1987.

Our SEST results from October 1987 until June 1994 will be published in A&AS (in press); the data will be available electronically. The data set consists of 155 sources with the signal-to-noise -ratio of at least one observation (at 90 or 230 GHz) ≥ 4.

Quasars have been monitored with the Metsähovi Radio Telescope since 1980 at 22 GHz and 37 GHz. During these years we have recorded numerous outbursts in our sample. Some of the outbursts are simple in nature, while others also seem to have fine structure. Can the outbursts be modelled in a simple way?

Time variability is commonly observed in the most compact extragalactic radio sources. Low-frequency variability (LFV)—at frequencies <1 GHz—is thought to arise through two different mechanisms, intrinsic and extrinsic. The former is just an extension of the often rapid high-frequency variations, delayed and reduced in amplitude. The latter is usually attributed to refractive interstellar scintillation (RISS; Rickett et al. 1984), whereby the variations in intensity are the result of wavefront distortions caused by transverse gradients in electron density. If RISS arises predominantly along the signal path through our Galaxy, we might expect to find evidence for a dependence on Galactic coordinates.

In approximately half the systems currently recognized as strongly gravitationally lensed, the background object is an extragalactic radio source. Radio observations have played an important role in the identification of lensed systems, and the properties of radio sources allow some of the astrophysical applications of lensing to be realized. High redshift galaxies can be studied through lens modeling and by observing more than one ray path through the lens. The morphological, spectral, and polarization information of high resolution radio images provides strong constraints on the mass distribution in the lensing galaxies. On cosmological scales, radio variability has been applied to the time delay measurement of angular diameter distance.

There are now several examples of distant radio sources being gravitationally lensed by foreground galaxies (e.g. PKS1830–211, Jauncey et. al. 1991). In such cases we know that the line of sight must be passing through a substantial amount of ionized material, so it is pertinent to ask whether these free electrons influence the observed properties of the source in any significant way.

In the Ooty lunar occultation survey of about 711 sources at 327 MHz over 200 sources were classified as unresolved with upper limits of about 4″ (Kapahi(1986)). In order to study the nature of these sources I have observed a sub-sample of 71 sources with the VLA in the A-Configuration at 6 cm. The angular resolution ranged from 0.4 to 0.8 arc second. My observations show that of the above 71 sources 56% are well resolved doubles and triples resembling FRII sources with angular separations of 0″.4 to 8″. The flux density ratios of well separated doubles and triples range from 1 to about 8. However the LAS of sources with ratios of less than 2 is found to increase with decreasing flux density. Since most of the low flux density sources are in blank fields in the PSS prints no redshift information is available at present. Being FRII sources they are likely to be very distant sources at comologically interesting ranges of redshift. In order to test whether the above observed increase in the angular size is of cosmological significance we need optical identification and redshift information. These are being planned.

The gravitational lens system B0218+357 comprises 2 image components (A and B) and a radio ‘Einstein Ring’ (Patnaik et al, 1993). The redshift of the lens galaxy is 0.6847 (Browne et al, 1994) and that of the imaged source 0.96 (preliminary result; Lawrence et al, 1995). The separation of A and B, which are both flat-spectrum radio sources, is only 0.335 arcsec, leading to the hope that the lens is a single galaxy with a relatively simple mass distribution. Refsdal pointed out (1964) that a model of such a distribution, and a measurement of the time difference along the two image paths, leads to an estimate of the Hubble constant, independent of the usual steps in the distance ladder. B0218+357 is one of only a few lensed systems well suited for such measurements. A preliminary value of 12 days has been measured for the A-B time delay, derived from a comparison of the percentage polarisation variations of the images at 15GHz, using the VLA (Corbett et al, 1995).

PKS 1117+146 is a high power radio source (L327MHz =5.4 × 1026 W/Hz) identified with a galaxy of 20.1 red magnitude at z=0.362 (de Vries et al. 1995). At this redshift 1 mas ≃ 2.9 pc (H0 = 100 km/s–1Mpc–1). Based on the properties of the radio spectra, PKS 1117+146 is classified as a GigaHertz Peaked Spectrum source (GPS) (Stanghellini et al. 1990). The GPS are powerful but physically small (sub-galactic sizes) radio sources with turnovers in their radio spectra at v ≃ 1 GHz. They are supposed to be isotropically emitting radio sources confined by exceptional dense circumnuclear gas (O'Dea et al. 1991) or still relatively young (Fanti et al. 1990). PKS 1117+146 is also a low frequency variable (LFV) with no sign of variability at v > 1 GHz (Padrielli et al. 1987, Mitchell et al. 1994). The low frequency variability is caused by propagation effects in the interstellar medium of our Galaxy (Mantovani et al. 1990, Spangler et al. 1993). PKS 1117+146 was observed with VLBI global arrays at 608 MHz (Padrielli et al. 1991), at 327 MHz (Altschuler et al. 1995), and at 1667 MHz (Bondi et al. 1996). All the maps are in agreement showing a compact double structure with components separated by about 70 mas. Flux densities and separation of the two components derived from VLBI and MERLIN (see below) maps are listed in Table 1. The flux ratios of the two components from the VLBI observations are very similar, and the spectral index is relatively flat (α ≃ 0.3–0.4), even if the strong low frequency variability can introduce uncertainties. The similarity of the VLBI morphology and spectral properties of the two components suggested that 1117+146 could be a possible gravitational lens candidate prompting for higher frequency observations. We observed PKS 1117+146 with MERLIN at 22 GHz in March 1993. MERLIN observations reveal for the first time a weak central component with a total flux density of about 20 mJy (Fig.1). From Table 1 we can note that the P.A. between the components is constant at all the frequencies while the separation between the peak flux densities significantly increases at higher frequencies. This is the expected behaviour if the 2 components are 2 lobes with hot-spot at the outer edges. The MERLIN map at 22 GHz seems to rule out the possibility that the morphology of PKS 1117+146 is caused by gravitational lensing.

The technique of differential astrometry using the phase-delay VLBI observable promises fractional precisions of ≃2 × 10–9 in the determination of the separation of sources 5° or 6° apart on the sky (Guirado et al. 1995a; Lara et al. 1996). In our present research we seek further improvement in this technique through using triplets of radio sources, which provide a closure constraint in the determination of relative angular positions. This constraint not only eases the resolution of the phase-cycle ambiguities (a major problem in the least-squares approach to astrometry with phase delays), but it also strongly constrains the space of allowable parameter values.

Machalski and Engels (1994) have drawn attention to the recent outburst in the extragalactic radio source 1308+328. We observed it with VLBI at 2.3 and 8.4 GHz in February 1995, using the EVN, and including antennas at Seshan, Kashima, Hartebeesthoek and Ny-Alesund, all of which provided long baselines. During these observations we switched every few minutes between 1308+328 and the nearby compact source 1308+326, 14 arcmin away. This allowed us to determine a precise relative separation between these sources at both frequencies using phase referencing techniques, and provided a useful comparison between source sizes. We found that the target source, 1308+328, is considerably more compact than the reference source 1308+326, the latter being resolved (correlated flux decreased by 50 %) at 8.4 GHz at 250 million wavelengths. In contrast, 1308+328 appears completely unresolved at this resolution (Fig 1), corresponding to a source size < 0.5 mas.

The Fanaroff-Riley type I radio galaxies (Fanaroff & Riley, 1974) presented in this paper belong to the complete sample of low-intermediate luminosity radio galaxies published in Giovannini, Feretti & Comoretto (1990). This sample includes radio galaxies with different morphologies on the arcsecond scale, such as compact sources, core-halos, FRIs and FRIIs.

We shortly discuss here the parsec scale information presently available on the extended low power radio galaxy (FR I, Fanaroff and Riley, 1974) NGC6251 (Jones and Wehrle, 1994) and on 11 radio galaxies from the sample presented in Giovannini et al. (1990).

Here we will describe briefly some of the VLBI observations we are making of low-redshift, compact radio sources in the southern hemisphere, using the Southern Hemisphere VLBI Experiment (SHEVE) array of telescopes (Jauncey et al., 1994).

The elliptical galaxy M84 (NGC 4374, UGC 07494) hosts an FR-I radio continuum source (Laing & Bridle 1987, MNRAS, 228, 557) and a dusty, warped optical emission line “disk” (Baum et al. 1988, ApJS, 68, 643; Goudfrooij et al. 1994, A&ApS, 105, 341). HST imaging shows that the inner dust distribution is not relaxed, but filamentary and complex (Jaffe et al. 1994, AJ, 108, 1567). M84 is a member of the Virgo Cluster, at which distance 1 arcsec = 73 pc independent of Hubble's constant (Jacoby et al. 1990, ApJ, 356, 332). Our VLA imaging, at 6 cm with a resolution of 500 mas (36 pc) FWHM, shows that the two large-scale jets in M84 are initially asymmetric: the ratio RI of the intensity of the northern main jet to the southern counterjet exceeds unity. However, these jets symmetrize (RI ≃ 1) by a projected nuclear offset rp ≃ 13 arcsec (950 pc). Is this VLA symmetrization trend consistent with Doppler boosting in a decelerating, but initially relativistic, jet? To test this, we obtained preliminary VLBA images of M84 at 18 cm and 4 cm, with resolutions of 11 mas (0.80 pc) and 1.9 mas (0.14 pc), respectively, and used them to evaluate RI on pc scales for comparison with RI on 100-pc (VLA) scales.

Sensitive, high-resolution VLBA observations of the nuclear region of Hydra A are presented at 1.3, 5 and 15 GHz. Hydra A (3C218) is an outstanding example of a high-luminosity FRI radio galaxy embedded within a cooling flow cluster. VLA observations by Taylor & Perley (1993) have demonstrated extremely high (>5000 radians m–2) Faraday rotation measures (RMs) and a striking RM and depolarization asymmetry between the northern and southern radio lobes. In view of this asymmetry on the kpc-scale Hydra A appears remarkably symmetric on the pc-scale in the radio continuum. Hydra A is also unusual in that the 21 cm atomic Hydrogen line is seen in absorption against the nucleus.

This review talk will deal almost exclusively with the radio aspects of low luminosity radio galaxies, as the other wavelength bands (optical, X, IR) are covered by others during this conference. By low radio luminosity we mean a radio power at 20 cm in the range 1019 to 1024,5 W Hz–1 (Hubble constant of 100, as throughout this talk). At the upper limit occurs the “break” in the radio luminosity (RLF) (Auriemma et al 1977) as well as the transition from Fanaroff -Riley type I to type II (Fanaroff & Riley, 1974). This limitation to weak radio powers does not necessarily imply that such radio sources constitute a homogeneous class of objects; it has been shown by Wrobel & Heeschen (1991) that a number of radio galaxies of very low luminosity have a ratio between far-infrared and radio emission similar to spiral galaxies and this might suggest that these objects are powered by star formation phenomena. Long baseline interferometry is able to pinpoint compact radio nuclei and therefore can distinguish starburst related radio sources from those powered by nuclear activity (see Slee et al 1994).

Sources flagged as extended or multiple in the Molonglo Reference Catalogue (MRC, Large et al. 1981, 1991), south of δ = −30°, were observed with the Molonglo Observatory Synthesis Telescope (MOST) with a resolution of 44 arcsec at 843 MHz (Jones and McAdam 1992) to give a sample of 193 southern extended sources. Optical identifications were made using the UKST bJ sky survey. We are now using the Australia Telescope Compact Array (ATCA) near Narrabri in Australia to study a subsample of Fanaroff-Riley class I radio galaxies and fit models to the jets.

All the information on the morphology, spectral index and polarization properties of low and intermediate luminosity radio galaxies were collected in an intermediate frequency range, and some of the basic questions to be elucidated with such measurements are still unanswered. We therefore decided to extend the study of their characteristics towards higher frequencies. From the B2 and 4C catalogue we selected 26 radio galaxies for which VLA or WSRT data are available. These sources were observed at 10.6 GHz using the Effelsberg 100-m telescope (Gregorini & al. 1992; Mack & al. 1994; Klein & al. 1995).