Using the Video Camera (e.g. Butcher et al. 1980) and the High Gain Video Spectrometer (e.g. Heckman et al. 1981) we are carrying out a program at Kitt Peak to search for optical line emission associated with the jets and lobes of radio galaxies. Several sources have been found in which extended optical line emission is clearly related to the non-thermal radio emission. Some general information on these and a few other sources is summarized in Table 1.

In whatever physical state the gas is found, observations of the interstellar medium in elliptical galaxies are of considerable interest. This is particularly true in the case of radio galaxies where we believe that the gas is an indespensable part of the cause of nuclear activity and plays a role in the origin and the evolution of the radio galaxy phenomenon. In a few cases we are fortunate to find some of the gas to be ionized with a temperature of about 104 K where optical spectroscopy allows us to deduce something about the excitation/ionization mechanism, about its chemical composition and about its state of motion. Here I wish to summarize observations of three Southern radio galaxies which show optical emission lines from regions tens of kiloparsecs in extent.

In addition to the occurrence of emission line regions coexistent with extended radio sources which have been discussed at this Symposium, this phenomenon has been observed earlier in 3C277.3 (Miley et al. 1981) and in Centaurus A (Graham and Price 1981). This gas has been detected only in the Fanaroff and Riley “Class I” radio sources. Data concerning this class of object suggest that outflow from the nucleus is proceeding at transonic or subsonic speeds and this correlation has led to the suggestion (De Young 1981) that the origin of the emission line gas arises from entrainment of the interstellar medium into the ejected material.

In this contribution the reported detection by Saslaw, Tyson and Crane (1978) of weak optical emission in the lobes of three 3C sources — 3C 265, 3C 285 and 3C 390.3 — is reappraised in the framework of three optical emission mechanisms — synchrotron (SYN), synchrotron inverse Compton (SIC) and blackbody inverse Compton (BIC). This effort has been motivated partly by the knowledge that the contribution to the synchrotron inverse Compton emission in a radio source component is likely to become significant for very compact and bright radio components (see e.g. Okoye 1972), and partly by the recent availability of high resolution radio frequency structural data on the sources in question. Another incentive arose from the demands of a separate investigation into high energy particles interactions in radio sources (reported by Okoye and Okeke in this volume) involving very high energy protons thus making it necessary to ascertain whether such highly energetic particles exist in radio source components.

The majority of extragalactic radio sources are known to consist of two extended components straddling an optical galaxy or quasar with each component being maintained from the nucleus of the associated optical object through a beam or jet of relativistic plasma and magnetic fields. Hitherto, the energetics of radio source components have been considered essentially from the point of view of the cooling of the relativistic electrons through their interaction with ambient magnetic fields (synchrotron radiation) and with low energy photons (inverse Compton emission). Here we consider a hitherto neglected problem involving the mutual interactions between the fast particles themselves. (The results of a detailed investigation into these interactions will be reported elsewhere — see Okoye and Okeke, 1982.)

From a comparison of X-ray and radio data for 20 clusters which contain tailed radio galaxies, we find evidence for the effects of buoyant forces on the low brightness parts of radio tails. In several cases, the width of the tail increases markedly in low gas density regions, strengthening the case for thermal gas confinement of radio tails. Three examples of enhanced X-ray emission around radio galaxies which are further than 2 Mpc from their cluster centers are found.

Study of the properties of radio galaxies in clusters is beset with problems of selection and identification. To reduce these problems we have selected a complete sample of Abell clusters and are using optical spectra of identified galaxies to determine their cluster membership. The sample is in a volume of space defined by the limits of coverage and sensitivity of the two Sydney University radiotelescopes and the Anglo-Australian Telescope; it comprises the 42 clusters with δ≤-8° and distance class 4 or closer. Radio maps have been prepared using archive 408 MHz data from the Molonglo Cross. Possible optical identifications have been selected from the Palomar Sky Survey and low-dispersion optical spectra have been taken of these and sometimes other galaxies or stellar objects within the cluster area. When needed, maps of the cluster areas have been produced at 1415 MHz using the Fleurs Synthesis Telescope. Some details of our early results have been published (Mills et al. 1978, 1979) and here a brief summary of the overall statistics is presented. Observations on five clusters remain to be completed but should not greatly affect the present conclusions.

We present here analyses of a radio survey of Abell clusters at 1400 MHz using the NRA0 91-m telescope. Details will appear in a paper to be submitted to the Astronomical Journal where we present two lists. The first contains sources within 0.5 of an Abell radius (hereafter Ra, 3 Mpc if H=50) of the center of an Abell cluster. The second contains those clusters for which there were no sources within that limit. The flux limit is 100 mJy, the beam size ∼10 1/2 arcmin, and the declination limit −19°30′. For consistency we use Corwin's (1974) m10 − z calibration throughout. The errors in m10 and therefore z and Ra, combined with a beam large compared to galaxy and cluster size (preventing identifications) preclude all but the simplest analyses which we present here.

We observed the cluster of galaxies A1367 to map the structure of the cluster and that of the radio galaxy 3C264. We report on 3C264 and on the peculiar galaxy UGC6697.

We have observed with the Westerbork Synthesis Radio Telescope (WSRT) at 1.4 GHz the Abell clusters included in the HEAO-2 satellite observing program, for which radio information was not available. In practice we excluded the clusters south of 15°, in order to ensure an adequate angular resolution in declination; moreover we did not observe distance class (d.c.) 6 clusters for which better resolution and sensitivity would be necessary. The final list includes 61 clusters. These have been observed to a limiting peak flux density of about 5 mJy, corresponding to average minimum radio powers ranging from ∼5×1021 to ∼2×1023 W/Hz for d.c. 1 to 5. By adding the present data to those already available in the literature, we have radio information about all the clusters of d.c. 1 and 2, north of 15°, except A1185. The sample of d.c. 3 clusters contains 60% of the clusters of this class, but seems unbiased both for richness criteria and for radio characteristics; therefore it is useful for statistical studies. The observed clusters of d.c. 4 and 5, instead, are richer than average. The list of the observed clusters is presented in table 1: the Abell name is given in column 1, the d.c. in column 2, the richness class in column 3, the number of radio detected cluster galaxies in column 4 (in parentheses the number of galaxies for which the membership of the cluster is doubtful or the radio identification is not certain is given). The identification of the radio sources with the cluster galaxies of d.c. 5 is still in progress. While a more complete discussion of the properties of radio sources in clusters will be performed later, using the data about all the clusters of the sample, here we summarize the results of the discussion on d.c. 1 and 2 clusters.

Gravitational interaction is a straightforward interpretation of some of the peculiar optical morphologies shown by galaxies. There have also been attempts to study the effects of a gravitational interaction on the radio continuum emission. Statistically, the central radio sources (inner 1 kpc) in interacting spiral galaxies are about three times stronger than in isolated spirals; on the other hand, the intensity of the extended emission does not seem to be affected (Stocke, 1978; Hummel, 1981). Peculiar radio morphologies are not a general property of interacting galaxies, since in the complete sample studied by Hummel (1981) of spirals with a probability ≥0.8 of being physically related to their companion, less than 5% have a peculiar radio morphology.

VLA observations at 1465 MHz of the Stephan's Quintet region reveal that the arc-shaped area of emission discussed by Allen and Hartsuiker (1972) breaks up into several components. The idea that NGC 7318b is a recent interloper in the group and that the interaction resulting from this event causes the enhanced activity at the east side of NGC 7318b is adopted as still the most reasonable explanation. The results are discussed in more detail in another paper (van der Hulst and Rots, 1981).

The earliest Uhuru observations showed that cluster X-ray sources were not associated with single individual galaxies but were extended sources (Gursky et al. 1971, Kellogg et al. 1972, and Forman et al. 1972). The detection of iron line emission from X-ray spectroscopic observations (Mitchell et al. 1976 and Serlemitsos et al. 1977) showed both that the dominant X-ray emission process was thermal bremsstrahlung and that the gas had been processed through stellar systems before being injected into the intracluster medium.

Recent studies of the nearby radio galaxy Centaurus A with the Very Large Array and the Einstein X-Ray Observatory reveal complex radio and X-ray structures. A prominent one-sided jet comprised of resolved knots located 0.2–6 kpc from the nucleus is seen in both radio and X-rays. The X-ray emission is probably synchrotron, requiring in situ reacceleration up to Γ ≃ 107. Inverse Compton emission is not a likely explanation though a thermal model in which the nucleus ejects dense 105M0 clouds cannot be excluded. An elongated X-ray region is also found near the “middle” radio lobe and optical HII regions ∼ 30 kpc NE of the nucleus. Conditions around the active nucleus, the absence of X-rays from the inner radio lobes, and X-ray evidence for a hot interstellar medium are briefly discussed.

Centaurus A, at an estimated distance of five megaparsecs, is the closest radio galaxy. It presents the best opportunity to examine in detail the physical mechanisms and resulting structures that are to be found in radio galaxies. Centaurus was first studied in detail at radio wavelengths by Cooper, Price and Cole (1965), hence CPC. Many of the comments, interpretations, and conclusions recorded in that paper remain valid today and provide the broader framework in which the more detailed studies using today's more powerful instrumentation can be understood. Historically, it is also interesting to note that Centaurus A was the first extragalactic radio source in which linear polarization and Faraday rotation were discovered and extensively studied.

Observations of 3–12 keV X-ray emission from NGC 5128 (Cen A) were made by Vela spacecraft over the period 1969–1979. These data are in good agreement with previously reported data, but are much more complete. Numerous peaks of X-ray intensity occurred during the period 1973–1975, characterized by rapid increases and equally rapid decreases (in less than 10 days). Thus it seems probable that most of the X-ray flux from the nucleus of Cen A came from a single source of small size.

VLBI observations of the nucleus of Centaurus A have been made at three southern hemisphere observatories. Since Centaurus A is the nearest active galaxy, VLBI investigations are important because the physical processes in the nucleus can be studied in greater linear detail than in other similar galaxies. Previous VLBI observations of Centaurus A have been hampered by its southerly declination (−43°) and the sparsity of VLBI capability in the southern hemisphere, leading to only scattered single point u, v coverage. This paper presents results from the early stages of development of a southern hemisphere VLBI network.

Radio jets occur in sources with a wide range of radio luminosities, and in 70% to 80% of nearby radio galaxies. There may be two basic types of large-scale (>1 kpc) jet — B‖-dominated one-sided jets in sources with luminous radio cores, and B⊥-dominated two-sided jets in sources with weak radio cores. The large-scale jets that have been observed at high linear resolution are well collimated within a few kpc of their cores, then flare and recollimate further out. Their brightness-radius evolution is often “subadiabatic”.

The first big radio jet to be found in a quasar was in 4C32.69 (z=.659) (Potash and Wardle, 1980). A new higher resolution map of this jet, made with the VLA at 5 GHz, is shown in Fig. 1. Observationally the important points are 1) it is very luminous, 2) it is very well collimated, 3) the magnetic field is parallel to the jet over its entire length, 4) it bends at least three times along its path to the outer radio lobe. Physically this is interesting because 1) it is a very lossy pipeline to the outer lobe, 2) the high degree of collimation suggests either the jet is confined by external pressure or it is highly supersonic. But the minimum internal energy density is very high (>3×10–10 erg cm–3), so if it is confined the external pressure must also be very high. On the other hand, if the jet is expanding freely, it is easy to show that the momentum flux is enormous (>4×1038 dynes). Such a jet is very rigid. It is difficult to stop and difficult to bend, but evidently both of these things happen. A detailed discussion of these problems is given in Potash & Wardle (1980), who concluded that the jet cannot be expanding freely. They suggested that the jet might be confined either by the thermal pressure of external hot gas, or by a helical component of magnetic field due to currents in the jet.

The radio source 4C 18.68, identified with the 16.5 m QSO 2305+187 (z = 0.313), was observed at 20 cm and 6 cm with the VLA in its highest-resolution configuration (A) on April 25, 1981, as part of a programme to map the radio structure of quasars for which optical structure has been mapped using the Canada-France-Hawaii telescope. The radio structure is remarkable, showing complex, curved structure at 6 cm resolution embedded in a halo ∼ 60 kpc across (HO = 100 km/sec/Mpc, qO = 0) seen most clearly in the 20 cm map. The polarisation is low (∼ 8% maximum at 6 cm) and follows the curved structure quite closely, showing the magnetic field direction to be along the curved structure. The central source has a flat spectrum, while the spectrum of the structure generally steepens with distance from the centre.