This paper examines some of the consequences of the hypothesis that jets in all radio galaxies and quasars are relativistic on small scales, in the sense that the flow velocity >0.5c. This idea is suggested by a number of lines of evidence. Firstly, Unified Models (Urry & Padovani, 1995) imply that the relativistic motion required in core-dominated objects must also occur in a larger parent population consisting of most, if not all, extended sources. Secondly, superluminal motion is detected in the nuclei of extended sources and in the kpc-scale jet of M 87 (Hough, 1994; Biretta, Zhou & Owen, 1995). Thirdly, jets are one-sided in the same sense on pc and kpc scales; at all luminosities, the radio emission tends to become more symmetrical on larger scales, as expected if an initially relativistic flow decelerates (Bridle & Perley, 1984; Bridle et al., 1994a; Parma et al., 1994). Finally, depolarization asymmetry occurs in both low (Parma, de Ruiter & Fanti, 1996) and high (Laing, 1988; Garrington et al., 1988) luminosity sources: the implication is that the brighter jet is on the near side of the source. It is likely that the key difference between radio sources in the two morphological classes defined by Fanaroff & Riley (1974) are that relativistic flow persists to the extremities of FRII sources, but that FRI jets decelerate smoothly on intermediate scales (Laing, 1993; Bicknell, 1995). On kiloparsec scales, we can identify structures which we propose should be called fast jets. These are well-collimated and generally one-sided (in the sense that the jet/counterjet ratio >4:1). They also have longitudinal apparent magnetic field (B ||). They occur both in FRII sources, and at the bases of FRI jets (Bridle & Perley, 1984). We suggest that they are relativistic flows, and that this fact is crucial to an understanding of their evolution. A framework for the understanding of the variety of extended structures in extragalactic radio sources in this context is illustrated in Figure 1, which is an improved version of the diagram presented by Laing (1993). A fast jet appears to be able to: decelerate and recollimate to form a slow jet with β << 1 (therefore two-sided unless external effects dominate); disrupt, as in wide-angle tail sources, or hit the external medium and form a hot-spot. Slow jets are probably formed only when a decelerating fast jet can be recollimated by the external pressure gradient (Phinney, 1983; Bowman, Leahy & Komissarov, 1995). This may not be possible for more powerful sources in flatter pressure gradients and it is likely that wide-angle tail sources are formed when a fast jet decelerates rapidly but cannot recollimate. Deceleration by entrainment is efficient when the jet is transonic, and Bicknell (1994) showed that this corresponds to β ≈0.3 − 0.7 for a relativistic jet. If the jet does not slow down sufficiently (e.g. by mass loading; Komissarov 1994), then the flow will remain supersonic until it impacts on the external medium, and an FRII source will result. The radio morphology is therefore determined by a combination of initial jet speed and thrust and the effects of the environment, via the rate of stellar mass loss and the pressure gradient. On the largest scales, a bridge(backflow) or tail (outflow) will be formed. If the jet remains supersonic as far as the end of the lobe (as in an FRII source), then it is inevitable that a backflow (bridge) will be generated. As emphasised by Parma, de Ruiter & Fanti (1996), the majority of FRI sources also show bridges: the residual momentum of the jets, their density contrast with the external medium and the external pressure gradient are all likely to be important in determining their large-scale morphologies.

If we believe that jets trace the energy transport in radio sources, we might in principle expect to see them in every classical double radio galaxy with compact hot spots. Until recently the detection rate of jets in FRII radio galaxies has been low, although most FRII quasars have bright one-sided jets. However, it seems likely that this is due to lack of sensitivity. Black et al. (1992) found jets in up to 70% of a sample of FRII galaxies with z < 0.15. We have observed the FRII sources in Laing et al. (1983) with 0.15 < z < 0.3 and discuss results from the combined samples.

We present preliminary results from imaging and spectroscopy of a complete sample of radio quasars selected from the B2 survey. The optical data, which provide us information on the environment around the quasars, as well as the redshifts, allow us to address several questions related to the origin and evolution of the radio quasars population. Using VLA observations of the sources we investigate the radio morphology and distortions of these low-luminosity radio quasars. We compare these properties with those of powerful radio quasars.

Our Atlas of Extragalactic Radio Sources will present high-quality images of the nearer half of “3CRR”, the sample defined by Laing, Riley & Longair (1983). This is the best-studied complete sample of extragalactic radio sources. All 173 members have secure redshifts and most have been imaged in the radio at high resolution. There is also copious information on their optical line emission, and many have been detected in the sub-mm, FIR, and in X-rays. 3CRR is widely used as a baseline against which fainter, higher-redshift samples can be compared to define the evolution of the population (e.g. Neeser et al. 1995; Law-Green, this conference).

Slysh (1966) first pointed out the bizarre properties of the polarized emission from Cygnus A: a strong wavelength dependence on fractional polarization, and a large difference in rotation measures (RMs) for the (integrated) emission from each lobe. These properties were confirmed, but not resolved, in the work of Mitton (1971) and Alexander et al., (1984).

We present some interesting results from a radio study of 17 radio galaxies at z >2, selected from a complete sample of Molonglo sources with S 408 > 0.95 Jy for which optical identifications and spectroscopic redshifts are being obtained in a major observational programme (Kapahi et al. 1995).

Observations and results : The 17 galaxies were mapped using the VLA at 1.4, 4.8 and 8.3 GHz with the highest resolution of ≃ 0.3″ at 8.3 GHz. The integrated spectra are generally quite steep and convex in many sources, steepening to α ≥ 1.2 (S ∝ v –α) at the highest frequency. Most of the sources have a double structure with sizes ranging up to 22″. Unresolved radio cores, coincident with the optical galaxies, are detected at the mJy or sub-mJy level in most cases. Several sources also show one-sided jetlike features. Surprisingly, most of the cores appear to have steep spectra between 5 and 8.3 GHz!

The international workshop on Cygnus A was held in Green Bank WV, USA and was attended by 45 participants who reported on radio, infrared, optical, UV, X-ray, and theoretical results. Rather than attempt to cover all of the papers presented at the workshop, we have selected 8 topics. We apologize to those whose work is not mentioned here. Because of the limitations of length, we have foregone the luxury of figures and we refer to articles in the Cygnus A book (to be published early in 1996 by Cambridge University Press) by first author only. All references to page and figure numbers refer to the Cygnus A book.

An intriguing question about extragalactic jets is why they are so few being seen at optical wavelengths, or equivalently, why the cutoff frequency of the synchrotron radiation is generally not in the optical, but rather in the infrared or even in the sub- millimeter domain. The answer is undoubtedly related to the efficiency of the acceleration of the relativistic electrons responsible for the synchrotron emission. The presence of a break at low frequency somewhere in the synchrotron spectrum is another feature that constrains the model parameters, but its precise location is unknown for most jets, because of the lack of photometry in the millimeter domain. It was thus necessary to fill the gap between radio and optical wavelengths in the synchrotron spectrum of optical jets. The required observation had to be of high sensitivity and high spatial resolution (of the order of 1″). Another reason for observing at millimeter wavelengths is that molecular lines and thermal emission from cold dust are detectable in this frequency range.

Markarian 6, a Seyfert Galaxy of type 1.5, is one of several Seyfert nuclei to be observed with MERLIN at 6 and 18 cm as part of a project to investigate the radio structures of these objects on sub–arcsecond scales (corresponding to scales of a few tens of parsecs at typical distances). The angular resolution of MERLIN at 6 cm is equivalent to that of the HST, making the radio images ideal for comparison with HST images of the optical Narrow–Line Region (NLR). In this paper we briefly discuss the results of our MERLIN observations of Markarian 6, along with a 6–cm Westerbork Synthesis Radio Telescope (WSRT) map of the arcsecond (kpc) scale radio emission. The data is discussed in more detail by Kukula et al. (1996) and Baum et al. (1993).

We present new VLA observations of IRAS 0421+0400 at 1.4 and 5 GHz, providing dual frequency information and higher resolution than previously available. We find extremely accurate alignment of the central double with the closest (though not brightest) feature in the southern kiloparsec hotspot region. There is a tight relation between the symmetric radio structure and emission line gas in the hotspot regions of this Seyfert 2 type galaxy.

Interstellar H 2 O masers in the Galaxy occur in active star forming regions. The spectrum often shows multiple distinct features. The VLBI maps reveal that each spectral feature corresponds to emission of spatially distinct maser sources (maser spots), whose sizes are ≈ 1013 cm. The maser in M33 is associated with the HII region complex IC133, which has been studied optically by Boulesteix et al. (1974) and Kiwitter and Aller (1981). This maser is the nearest (≤ 1Mpc) extragalactic H 2 O source visible in the northern sky, although it is not the strongest. It is known to have persisted for over a decade, and its spectra consistently show peak flux densities of ≈ 1.5 Jy and at least 10 features spread over ≈ 50 km s–1 (Huchtmeier, Eckart and Zensus 1988). Using VLBI Greehill et al. (1990) have obtained the positions of 14 H 2 O maser spots.

In the past decade it has become clear that many classical, double radio galaxies have hosts with extensive ionized gas inside the body of the optical galaxy (Hansen et al., 1987) (Baum et al., 1988). We have observed three such sources (PKS349-27, PKS634-20, and Pictor A) in both Hα and HI (with the VLA).

1821+643 is exceptionally bright at optical wavelengths with M B ≈ −27 (Hutchings & Neff 1991), at a redshift of only 0.297. It is however, a radio-quiet quasar (RQQ) as can be demonstrated by considering e.g., the ratio of the luminosity in [Oiii]λ5007 to the radio luminosity (Lacy, Rawlings, & Hill 1992). In contrast with other radio-quiet quasars however, E 1821+643 is located in a very rich cluster (Lacy et al. 1992) and is purported to reside in a gE host galaxy (Hutchings & Neff 1991). The full-resolution 8 GHz map is seen in Fig. 1. A bright unresolved core is seen with extended emission to the north-west (feature B). Features A and C are accurately diametrically opposed on either side of the core and therefore suggestive of jets; this is further supported by the alignment of features C, D and E.

The study of the optical counterparts to the radio jets has been the subject of a number of observing programs with the Hubble Space Telescope (HST). We know that these jets play a fundamental role in transporting energy from the central source to the extended radio lobes. Observations at optical and ultraviolet wavelengths with the HST are essential to obtain spatial resolutions similar to, or better than, those achieved in the radio band and, thus, provide the possibility of directly comparing the sites and mechanisms responsible for the emission at these different wavelengths.

The Hubble Space Telescope has proved to be remarkably useful for discovering and for studying the optical counterparts of radio lobes and radio jets. Since much of the structure seen in the radio is found on subarcsecond scales, it is not surprising that HST, with it's improved resolution relative to ground based observations, would be a major contributor to this field of research. This paper reports briefly on some of these successes, and refers to more detailed descriptions in the literature.

The jet of 3C273 is the only quasar jet visible at both radio and optical wavelengths. Investigations using ground-based observations suggest that the optical emission, like the radio, is produced by the synchrotron process. We present here a new radio map at 18 cm wavelength made with the improved MERLIN array (Fig. 1b) and compare it with a new optical image obtained with WFPC2 on the Hubble Space Telescope (Fig. 1a) This work has been carried out in collaboration with colleagues from Princeton, Pennsylvania State University and Caltech and is being published by Bancall et al. (1995).

In this paper, I review some of the evidence for gaseous circumnuclear disks in radio-quiet active galaxies (Seyfert galaxies and LINER's). Results from observations of Fe Kα lines, H2O megamasers, extended structures seen in near infrared observations and ionization cones are discussed.

Observations of the angular distribution of the water masers in the nucleus of NGC4258 reveal the presence of a thin molecular disk in nearly perfect Keplerian orbit (Miyoshi et al., 1995; Moran et al., 1995). About 300 galaxies have been searched for nuclear water masers to a limiting sensitivity of about 0.1 Jy (e.g., Braatz, 1996); and 16 masers have been detected. The maser imaged by Miyoshi et al. (1995) offers the best example of disk structure. VLA data of NGC1068 shows evidence of disk structure (Gallimore et al., 1996; Greenhill & Gwinn, 1996) and NGC2639 has drifting features, which may be due to centripetal acceleration (Wilson et al., 1995).

I report here centimeter–wavelength observations carried out at the Very Large Array (VLA) to help resolve two questions. First, what is the source of the far infrared (FIR) emission in infrared-luminous IRAS galaxies, active nuclei or more widely distributed star formation? And what physics underlies the tight correlation (Helou et al., 1985) between FIR and radio flux? To test potential answers to these questions, we believe it is important to study the most luminous IRAS galaxies. We selected 39 for study from the ultraluminous catalog of Strauss et al.(1990 and 1992). All sources had FIR luminosity ≥ 1011.4 L⊙. Radio wavelength observations of these systems provide several advantages. First, in the radio there is no obscuration, so we can “see” the active galactic nuclei, if present. Radio spectral indices can distinguish between synchrotron and thermal emission. And finally, observations at the VLA provide sub–kpc resolution. We observed these sources with the VLA in its C configuration. At 1460 MHz, the effective resolution was ≃ 15″; and ≃ 4″ at 4860 MHz. We made follow-up observations on 24 sources in the A configuration with resolution at 4860 MHz of ≃ 0″.5 (or 300–800 h–1 pc for these sources).