Many of the most important discoveries in the study of extragalactic radio sources have resulted from investigations of the relationships between optical and radio properties. The optical/radio connections include: correlations between optical emission line luminosity and radio power; alignments between optical/UV and radio structures; and UV excesses in the spectral energy distributions of radio galaxies compared with normal early-type galaxies.

3C 265 is a high-redshift (z=0.811) radio galaxy showing extended emission line regions (EELR) to 50 kpc from the nucleus (McCarthy et al 1995). However, it does not show the alignment effect (McCarthy 1993) that is common in distant galaxies: the EELR is not extended along the radio axis.

Extended emission line regions aligned with the radio axis are a common feature of powerful radio galaxies and there is much interest in the origin of the extended gas and excitation mechanism. One model that can produce this alignment is photoionization by anisotropic nuclear continuum radiation. However, strong evidence exists, especially in high redshift radio galaxies, for powerful interactions between the relativistic radio jets and the ISM/IGM. Here we present the results of our study of the southern radio galaxy PKS 2250–41 (z = 0.308). This object is the most spectacular found in a sample of southern radio sources studied by Tadhunter et al. (1993) and it displays particularly clear evidence for such an interaction (Tadhunter et al. 1994; Dickson et al. 1995).

Seven Seyfert galaxies, Mrk 3, Mrk 78, Mrk 348, Mrk 6, Mrk 573, NGC 3393, IRAS 04210+0400 and IRAS 11058-1131 (all Seyfert galaxies type 2 expect Mrk 6 which is a Seyfert 1.5) were observed with HST. For the first five objects images were taken with the FOC with filters centered on the emission lines [O II] and [O III]. For the remaining three galaxies WFPC2 observations were obtained in the [O III] and Hα lines.

The extended ionized gas in Seyfert and Radio-Galaxies is characterized by large values of the ratio He ii/Hβ, which exceeds the value predicted by the standard photoionization model in which the ionizing continuum consists of a power-law. This has lead to the suggestion of considering a matter-bounded (MB) component for explaining such extreme values. We now find that it is also possible to resolve the temperature problem if the thickness and the ionization parameter of the MB is appropriately selected. Adopting a canonical power law (α= −1.3) and solar abundances (Z=l), we can account for the observed trends in excitation (represented for example by the ratio [0 ii]/[0 iii] in Fig. 1)) by varying the relative number of MB clouds (which emit the high excitation lines Civ, [Ne v], He ii… and most of [O iii]) versus the number of ionization-bounded (IB) clouds (which emit [N ii],[S ii] [O ii], [O i]…). We obtain a one-parameter sequence (solid line) which is function of the weight AM/I of the MB component relative to the IB component. This AM/I–sequence successfully reproduces the observed range in He ii/Hβ. Note the failure of the traditional U–sequence (long dashed line). Fig. 2 indicates that we can also reproduce the ratio ROIII= [O III]λ4363/[O iii]λ5007 and therefore resolve the temperature problem. Interestingly, our model indicates a temperature difference of 5 000 K between the IB component ([N ii] temperature ≃ 10 000 K) and the MB component ([O iii] temperature ≃ 15 000 K) while the traditional U–sequence predicts a difference of only 1 000 K. Such difference of 5 000 K has been reported in the extended gas of Cygnus A.

When a FRII radio galaxy at z > 0.7 is observed in the optical, its continuum appears extended and aligned with the radio axis. This phenomenon occurs actually when the optical bands start to sample the rest-frame UV, and it was called alignment effect (McCarthy et al. 1987). The UV continuum of high z radio galaxies shows also strong linear polarization due to scattering of anisotropic radiation escaping from the obscured quasar nucleus (di Serego Alighieri, Cimatti & Fosbury 1994). However, the observations of the UV continuum have been concentrated mostly on high z radio galaxies, leaving open a main question : is the alignment effect an evolutionary phenomenon present only at high z, or is it simply a selection effect due to the K-correction ? In order to investigate the origin and evolution of the UV alignment effect, we have started a ground-based imaging and polarimetric survey for studying the UV continuum in FR II radio galaxies with 0.1 < z < 0.5. Depending on the redshift, the filters U and B can be used to sample the UV continuum free from strong emission lines, and in the same rest-frame spectral region observed in radio galaxies at higher redshift. The survey is in progress and here we present only our first results.

Previous studies show that: a) radio galaxies and radio-loud quasars have emission-line gas (ELG) which is extended on scales of tenths of kiloparsecs; b) there is convincing evidence that the kinematics and excitation of the very extended emission-line gas is governed by its interaction with the outflowing radio plasma; c) the evidence for an interaction is weaker in some radio galaxies. It is argued that the ionization of the ELG may be predominantly produced by the nuclear ultraviolet continuum and the kinematics of the gas due to the gravitational potential of the host galaxy, but it is not yet known whether there is a physical relationship between the ELG and the extended radio jets.

From the VLA 20cm survey of ≃ 500 Abell clusters reported by (Ledlow and Owen, 1995), we have obtained optical R-Band CCD observations and optical spectra for 265 radio galaxies. The survey is complete for 20cm flux density greater than 10 mJy within 0.3 corrected Abell radii of the cluster center. All Abell clusters with measured z < 0.09 were surveyed. This statistically complete sample was supplemented by ≃ 200 clusters with 0.09 < z < 0.25 including sources with flux density > 200 mJy. Only 6% of the sample consists of FR II radio sources, the remainder are twin-jets, tailed, or compact sources associated with the FR I class.

We present a large program of CCD imaging of low-luminosity radio galaxies selected from the B2 survey of radio sources. We aim to study their optical properties: brightness profiles, isophotal distortions, morphological peculiarities. As the sample contains jet and no-jet sources, this investigation will allow us to compare both populations of low-luminosity radio galaxies. In particular, we will study the local environment and test whether the galaxy density around radio sources plays an important role on the presence of jets or not. The analysis of morphological distortions on the host galaxies will provide us with detailed information on past or recent galaxy encounters.

The continua of BL Lacertae objects and other “blazars” are dominated by nonthermal emission that is variable and highly polarized at UV–radio wavelengths (Angel and Stockman 1980; Kollgaard 1994; Allen et al. 1993). It is believed that this non-thermal emission is associated with the relativistic jets that are known to exist in these sources, but details of the jet structure and physics are still very uncertain. It is usually expected that the polarization behavior at optical and radio wavelengths should show little or no correlation, even if genuinely simultaneous measurements are compared. It is typically thought that the emission in these two wavebands originates in vastly different parts of the source, where the magnetic field geometries are likely to be quite different. In some inhomogeneous synchrotron source models for blazars, however, depending on the model parameters considered, the radio and UV-optical-IR (UVOIR) emission may be co-spatial (Ghisellini, Maraschi & Treves (1985)). It is thus of interest to search for correlations between the emission of blazars in the UVOIR and radio, to test such models. Our approach to doing this has been to compare simultaneous measurements of the optical and VLBI polarization characteristics of compact AGN. The polarization of the radiation is effectively used as a probe of the magnetic field structures in the regions where the emission at the two wavelengths arises.

The problem of BL Lac classification is a long standing one and it is mainly due to the subjectiveness of selection criteria used in the definition of BL Lac samples. For instance, an object will undoubtedly be classified as a BL Lac if it shows flat radio spectrum, high optical and radio polarization, featureless optical continuum with weak or absent emission lines, and variable flux and polarization. However, the problem arises when the object shows some but not all of these properties. In face of this difficulty, different authors (Stickel et al. 1991, Stocke et al. 1991) have tried to make a systematic analysis of the data and it has been common to classify as BL Lacs those objects whose strongest emission lines have equivalent width (EW) ≤ 5 Å. Another common criterion is to require the 4000 Å break contrast to be ≤ 0.25. Nevertheless, both of these criteria are rather arbitrary and more directly related to practical observational considerations, than they are to any physical distinction between objects. What is proposed here is a slightly different approach; it is proposed that we take a step back from common classification and that instead of imposing strict selection criteria, we create a multi-observational parameter space to investigate any breaks in the distribution of observed properties that will help clarify the distinction between BL Lacs and other flat radio spectrum sources.

BL Lac objects, characterized by their high variability across the electromagnetic spectrum, strong and variable polarization in the radio and optical domain and a (nearly) featureless continuum can in general be divided into the radio-selected (RBL) and X-ray selected BL Lac objects (XBL) according to their αro – αox spectral indices (Stocke et al., 1985). Attempts to unify both classes within a single model have been suggested e.g. by Ghisellini et al. (1993) or Giommi et al. (1994).

A clear understanding of both the differences and similarities between the host galaxies of the three main classes of powerful active galaxy – radio-quiet quasars (RQQs), radio-loud quasars (RLQs) and radio galaxies (RGs) – is vital in any attempt to unify or relate the various manifestations of the AGN phenomenon. The unification of RLQs and RGs via orientation effects requires that the hosts of the two types be derived from the same population of galaxies. Meanwhile, the correlation between radio power and host morphology in nearby AGN, with radio-quiet objects (Seyferts) occurring in disc systems and radio-loud sources in ellipticals, is generally assumed to persist at higher redshifts and nuclear luminosities. However, in both cases the evidence remains ambiguous and, moreover, many previous studies have been based on poorly selected samples.

We report on some striking differences between radio loud and quiet emitters that we found in a comparative analysis of the high and low ionization lines for 52 low redshift AGN (31 loud; 21 quiet).

The broad components of Civλ1549 and Hβ were chosen as representative of the high and low ionization lines respectively. CIVλ1549 observations were obtained with the Faint Object Spectrograph on the HST. They were retrieved from the HST data archive and matching optical spectra for the region of Hβ were obtained at several ground based observatories. Details on observations, narrow/broad component deconvolution and profile cleaning from satellite lines (especially FeII) can be found in Marziani et al. (1995). The rest frame for each quasar was determined from the radial velocity of strong narrow lines, typically [Oiii]λλ4959,5007.

The relationship between the radio and optical emission of quasars was first studied using radio-selected objects, which generally had high radio luminosities due to the relatively low sensitivity limits of the surveys. More recently, radio follow-up observations have been made of optical surveys surveys in other wavebands. Taken together, the two survey methods have detected quasars with a range of over 6 orders of magnitude in radio luminosity. We report here new observations with the Very Large Array (VLA) of 103 quasars from the Large Bright Quasar Survey (LBQS), with absolute magnitudes in the range −25 < M B < −23 and redshifts in the range 0.2 < z < 1.1. This can be combined with our previous LBQS observations (Paper I, Paper II) to yield 359 quasars, the largest sample of sensitive radio observations of optically-selected quasars in a single survey.

A small number of radio galaxies have two sets of classical double radio lobes with the radio axes aligned in different directions. Furthermore differences in the properties of the radio lobes such as the surface brightness and spectral index indicate that the two sets of double radio structure have different ages. The radio ejection axis has therefore changed direction with time. In the first two known radio galaxies of this type, 3C 315 and B2 0055+26, the host galaxy is a member of a close pair of ellipticals in a common optical envelope suggesting that the complex radio structure is caused by gravitational interaction.

The X-ray radiation from inverse Compton scattering of CMB photons by the relativistic electrons in ‘radio’ lobes provides a direct measure of their column density at a known energy, unlike synchrotron radiation which also depends on the unknown magnetic field. Thus by combining inverse Compton and radio data we can separately determine the particle energies and field strengths, rather than having to rely on uncertain estimates like minimum energy. The predicted flux is and strong IC signal requires high radio flux and low magnetic field, properties of giant radio galaxies. On the other hand the minimum detectable count rate, Imin , increases with the target size due to the larger background contribution. As a result the detectability of IC X-rays for ROSAT PSPC B measurements is roughly, assuming a spectral index of 0.75. After making detailed prediction of SIC for a number of objects of the 3CR sample, the best candidates were 3C 236, 3C 326, and 4C 73.08.

Complementary information over a large energy range appears to be the best way to study the class properties of Active Galactic Nuclei (AGN). While radio data provide superior spatial resolution required for the analysis of source structures and their morphology, optical data are a necessary ingredient for the classification of the objects and for the determination of their redshifts. Finally, X-ray (and more recently γ-ray) observations give vital information about the energetics of the central engines and on the physical conditions of the ‘heart of the machine’.

We investigate the X–ray properties of a complete sample of 88 radio sources derived from the Wall & Peacock 2–Jy sample. We find that Lx correlates well with core radio luminosity for all object classes, whereas the Lx – Ltotal is probably introduced by sample selection effects. Further, evidence for an anisotropic X–ray component in broad line radio galaxies is reported. A full description of the results will be given elsewhere (Siebert et al. 1996).

Our sample comprises all BL Lac objects listed in the catalogue of Véron-Cetty & Véron (1993) and which are detected in a ROSAT PSPC observation with at least 50 source counts: 74 objects in total. We reduced the data from the ROSAT archives at MPE and GSFC and fitted single power-law models with photoelectric absorption to the spectra. We calculated the broad band spectral indices αrx, αro, and αox from the ROSAT 1 keV fluxes, 5 GHz radio, and optical V band fluxes (Véron-Cetty & Véron 1993).