In order to gather a sample of intermediate redshifts (z ≃ 0.1 ÷ 0.2) clusters avoiding evolutionary effects or biases induced by limited sensitivity of instruments and optical plates that affect samples selected through inspection of optical plates, color diagrams or X-ray emission properties, we plan to use radio galaxies as suitable tracers of dense environments (e.g. Allington-Smith et al., 1993). This would allow us to effectively test different environments and population properties, and would also give valuable information on the effect of environment on the radioemission phenomena. Moreover, it would not impact on the X–ray or optical properties of clusters, since there is no significant correlation between the radio properties of galaxies within a cluster with its LX (Feigelson, Maccacaro and Zamorani, 1982), or with richness of the cluster (Zhao, Burns, and Owen, 1989).

The radio morphology of thirty radio-loud quasars at z ≃ 1/2 is studied as a function of cluster galaxy density using data available in the literature. While correlations between several morphological parameters of the radio emission are consistent with interactions with an intracluster medium (ICM), we do not see the correlation between galaxy density and quasar radio morphology expected from ICM interaction. Therefore, distorted radio morphology is not an indicator for the presence of a distant cluster as previously suggested. Because such correlations are found at z ≃ 0, these results are suggestive that the rich clusters containing quasars at z ≃ 1/2 have not yet developed a dense ICM. An inelastic collision with neighboring galaxies and intergalactic clouds can account for the morphology of several of the sources in this sample and is a viable alternative explanation.

We have VLA snapshots at 6cm and 20cm of ≃50 QSOs of redshift ≤0.7 with a range of known optical galaxy environments. The radio sources were characterised by measures of flux, size, and shape. The cluster density of the QSOs is given by the Bgq number from galaxy companion data largely published by Yee and Ellingson (e.g. ApJ 411, 43, 1993).

The centers of dominant cluster galaxies in cooling flows are often unusually blue, they have spatially extended nebular line emission and bright, FR I radio sources (Fabian 1994). As a class, they are the most rapidly evolving giant elliptical galaxies known. Among the most interesting of these objects, the Abell 1795 (z = 0.06) and Abell 2597 (z = 0.08) central cluster galaxies have very blue, lobe-like structures that are located along their FR I radio lobes (McNamara & O'Connell 1993). This discovery was surprising because correlations between the radio source and blue optical continuum were thought to occur exclusively in powerful, FR II radio galaxies at redshifts z > 0.6 that show the alignment effect. By analogy with the distant radio galaxies, the blue lobes are thought to be regions of star formation that were triggered by the passage of the radio source (De Young 1995), or scattered light from an obscured, anisotropically radiating active nucleus that is beaming its light obliquely to the line of sight (Sarazin & Wise 1993; Crawford & Fabian 1993; Sarazin et al. 1995). Scattered light is usually polarized. Therefore, polarization measurements of the aligned optical continuum should provide a strong test of the scattering hypothesis.

McNamara et al. (1995) have obtained U-band polarimetry of the blue lobes in the Abell 1795 cluster central galaxy. They found an upper limit to the degree of polarization of the light emitted from the lobes of < 7%. The accuracy of this measurement is limited by the presence of diluting background starlight. This limit is inconsistent with the lobes being scattered light that originated in an obscured, anisotropically radiating nucleus, unless the radiation is beamed and is viewed at an angle < 22° to the line of sight, which is unlikely. The absence of a detailed correspondence between the radio lobes and optical lobes and the absence of a polarized signal is also inconsistent with synchrotron light.

The blue optical lobes are probably regions of vigorous star formation. If a burst of star formation were triggered by the expanding radio lobes, the age of the burst population should be ≃ 107 yr. The star formation rate in both lobes, assuming the Local IMF, would then be ≃ 20 M⊙ yr–1 and the stellar mass of the lobes would be ≃ 108 M⊙. The large cooling flow in A1795 may be fueling the star formation and the radio source or the fuel may have originated from one or more gaseous cluster galaxies that recently fell into the cluster's core. This result strongly suggests that the radio sources in central cluster galaxies may be a significant factor driving the evolution of their stellar populations.

Diffuse radio sources in clusters remain a poorly understood phenomenon. They are very extended sources (0.4-0.6 Mpc), of low surface brightness and steep spectrum, which cannot be identified with any active radio galaxy. They are a rare phenomenon, as they have been found so far in few clusters of galaxies. This paper reviews the current findings about this kind of sources, and the suggestions about their formation and evolution.

The inner regions of the Coma cluster of galaxies contain a source of diffuse synchrotron emission (‘Coma C’) which has a linear dimension of at least 500 kpc. There has been interest in the question of where the relativistic electrons responsible for this emission originate, and how they are transported through the intra-cluster medium (Tribble, 1993). It is widely thought that one or more of the radio galaxies in the centre of the cluster provides a likely source for particles which then diffuse out into the halo (Giovannini et al., 1993); a process which depends critically on the structure of this field in the intra-cluster medium. Recent observations of the emission from NGC4869 (Feretti et al., 1995), which occupies a central position in the Coma cluster, indicate the presence of a magnetic field which is both stronger (B ≈ 8μ G) and tangled on much shorter scales ≲ 1 kpc than had been thought previously (Kim et al., 1990). These new results suggest not only a shorter cooling time for energetic electrons, but also a slower rate of diffusive transport. In this paper we show the constraints that the new observations place on transport theories of the relativistic electrons.

The Coma cluster is situated near to the North Pole and its galactic coordinates are: l≃ 50° and b≃87°. The distance to the cluster is about 138 Mpc. Observations in soft X-rays (see e.g. Briel, 1992) discovered there the thermal emission from the diffuse hot intracluster gas whose density and temperature are: n≃ 3 · 10–3 cm–3 and T≃ 108K. Measurements of hard X-ray flux from Coma showed controversial detection in different experiments (see Bazzano et al. 1990 and Rephaeli et al. 1994).

In the framework of our multi-frequency study of Virgo A we have performed observations of Vir A at 10.55 GHz with the Effelsberg 100-m telescope. Using our improved CLEAN procedure for single dish data we have increased the dynamic range to some 40 dB.

By applying our newly developed polarization CLEANing technique we are able to diminish instrumental polarization effects. Since Faraday rotation is negligible at λ2.8 cm the measured linear polarization is a direct trace of the projected magnetic field in Vir A. In combination with low-frequency data obtained with the VLA it is possible to determine parameters like spectral indices, break frequencies, and spectral ages.

The double radio source B2 0924+30, associated with the luminous E/S0 galaxy IC2476, may be considered a prototypical genuine relic of a ‘dead’ radio galaxy as it seems to perfectly fulfill the following criteria: It has a rather steep overall radio spectrum (Ekers et al., 1981; Cordey, 1987). Its core luminosity is by far the lowest known so far (Giovannini et al., 1988). No coherent jet structure or other signs of activity are visible. Since only four possibly genuine relics of radio galaxies are known so far (Harris et al., 1993), a study of the archetypical source B2 0924+30 is of eminent importance for the understanding of this rare species of radio galaxies.

We have performed sensitive observations of three classical head-tail radio galaxies at λ11.1, 6.3, and 2.8 cm using the Effelsberg 100-m telescope (Zech, 1994). Complete maps of the sources 3C129, NGC1265, and 3C465 were obtained, including the distributions of the linearly polarized intensity. Together with the low-frequency interferometric maps these allow a comprehensive study of their radio spectra and, based on models of particle losses, the derivations of particle ages across these sources. The highest frequency involved allows an unambiguous derivation of the projected magnetic field structure, unimpeded by Faraday effects. Here we focus on NGC1265, which is located in the Perseus Cluster.

We have begun VLA observations of straight-angle tailed radio sources (SATs) at 1.5 and 4.8 GHz (L and C band) to achieve one arcsecond resolution at each frequency. This will provide a SAT data set similar to the O'Donoghue, Owen, and Eilek (1990) WAT data set with both total intensity and spectral index information. We will use these data to examine morphological and dynamical properties of straight-tailed radio sources in clusters of galaxies.

The bright radio galaxy Hercules A (z = 0.154), has power P178MHz = 8 × 1026 W Hz–1sr–1 and although this puts it well above the FR break, its structure is intermediate between Fanaroff and Riley classes I and II. With a linear size of 334 kpc (for H0 = 100 kms–1Mpc–1), Hercules A possesses an unusual jet-dominated morphology and no compact hotspots. The western jet shows partial or full ring-like features that form a linear sequence heading from the core to the lobes and follow an inner jet.

We are engaged in a radio–IR-optical–X-ray study of two flux-limited samples of ROSAT clusters of galaxies south of declination −20° (Pierre et al., 1994a). One sample covers an area of 1750 deg2 in Hydra (Pierre et al., 1994b) and includes some distant (z ≈ 0.3) and X-ray luminous (LX ≈ 1045 ergs–1) clusters. The other sample derives from a volume-limited subset (nominally z ≤ 0.1) of southern X-ray clusters, which are the focus of an ESO Key Program (Guzzo et al., 1995).

We observed the cluster of galaxies A3556 (< v > = 14300 km/sec), belonging to the supercluster of galaxies known as the Shapley Concentration (Bardelli et al., 1994, MNRAS 267, 255, and references therein), with the Australia Telescope Compact Array. Our observations took place in the continuum band at 20 cm with the configurations 1.5D and 6C, and at 20 cm and 13 cm in the configuration 6A, for a total of 12×3 hours, with a resolution ranging from ≃ 6″ to ≃ 40″. We observed a sky region of ≃ 2° × 1° around the cluster center taking advance of the mosaicing technique developed ATNF.

Clusters of galaxies are the largest aggregates of matter that have decoupled from the universal expansion and have approximately evolved to a proper dynamical equilibrium configuration. While in the optical they are just observed as dense concentrations of galaxies, they are seen in X-rays as continuously connected entities through the emission of the X-ray luminous intracluster plasma (e.g. Sarazin 1986). Relativistic particles and magnetic fields are observed in clusters in radio halos, in regions around radio galaxies and through Faraday rotation of the radiation from background radio sources (see contribution by L. Feretti and e.g. Kronberg, 1994). Thus Clusters of galaxies are the largest, well characterized astrophysical laboratories in which plasma physical processes of diffuse media can be studied. In this brief summary the bulk properties of the intracluster medium (ICM) are discussed and two studies of the interaction of galactic radio lobes with the intracluster plasma are presented (related topics are discussed by F. Owen and L. Feretti in this volume).

A ROSAT PSPC image of the galaxy cluster Abell 2241 has been obtained, showing X-ray emission from the intracluster medium and from individual objects. The brightness distribution of the cluster gas is used to assess the physical conditions at the location of two tailed radio galaxies (in A2241E and A2241W, at different redshifts). Together with radio and X-ray information on the two galaxies themselves the results are of relevance to the question of energy equipartition in radio sources.

X-ray emissions from clusters are most likely originated from a thin hot plasma in a collisional ionization equilibrium. The optical depth of continuum component is order of 10–3, whereas that of emission lines is around unity. Present emission models used for spectral fitting can not estimate this effect, so that the determination of elemental abundances seems to include large uncertainty. The high resolution spectroscopy with ASCA gives a clue to investigate the physical state of hot intracluster gas and a impact to reconsider the basic atomic processes. This is important issue to deeply understand the structure, formation and evolution of clusters, and the origin of intracluster gas.

The most widely discussed class of unified schemes for radio-loud extragalactic sources attempts to interpret their seemingly disparate types as the same objects seen from different directions. The orientation dependence is attributed to relativistic beaming of the nonthermal jet and, possibly, anisotropic obscuration/re-radiation of the nuclear emission by a circum-nuclear distribution of dusty material with polar openings, possibly a torus. Although alternative approaches have been mooted for unifying radio galaxies (RGs) and quasars (QSRs) by incorporating a strong jet-environment interaction, or temporally decaying nuclear prominence, the orientation based unified scheme, thanks to its rich predictive potential, has been subjected to a multitude of observational tests and its pros and cons have been discussed extensively in recent reviews. Here we briefly address some recent developments, including the claim that the radio size measurements of powerful RGs and QSRs are incompatible with orientation being the primary distinction between them. On balance, it seems that while the basic orientation picture can broadly explain the bulk of the observations, its viability could be much enhanced by taking into consideration the (inevitable) temporal evolution of radio sources.

In a recent review paper we summarized the current status of unification of radio-loud AGN (Urry & Padovani 1995 PASP 107, 803), connecting high-luminosity (FR II) radio galaxies with quasars, and low-luminosity (FR I) radio galaxies with BL Lac objects. Unified schemes are motivated by the knowledge that AGN appearance depends strongly on orientation (Fig. 1): optical/UV light from the centers of many AGN is obscured by circumnuclear matter, and in radio-loud AGN, bipolar relativistic jets beam light along the jet axes. Understanding these radiation anisotropics allows us to unify apparently distinct classes of AGN that differ primarily because of orientation.

Our review described the classification and general properties of AGN and summarized the evidence for anisotropic emission caused by circumnuclear obscuration and relativistic beaming. We outlined the evidence, both observed isotropic properties and statistical arguments, for connecting FR IIs with quasars and FR Is with BL Lacs. The population statistics (with beaming) are in accordance with available data and suggest γ ≃ 5 for low-luminosity AGN and γ ≃ 10 for high-luminosity AGN. The distinctions between X-ray-selected and radio-selected BL Lac objects, and between BL Lacs and flat-spectrum variable quasars, still not understood, provide clues to the underlying physics of blazars. Our review discussed several possible problems and complications, and concluded with a list of the ten questions we believe are the most pressing in this field.