Activity in galaxies takes on a bewildering array of guises. Not all of this diversity comes from fundamentally diverse physics, but if we can understand the relationships between different types of galaxy then we may be able to perceive the underlying physics. This taxonomic approach aims to determine which properties are common to several types of galaxy, and which properties differ. The danger in this process is that it is easy to invent spurious relationships between similar, but quite distinct, types of object.

This contribution is divided into three parts: 1. A summary of mostly published work that describes the new type of BL Lac Objects discovered with X-ray satellites, “X-ray Bright BL Lacs (XBLs)”, and their relationship to the previously known “radio bright BL Lacs (RBLs).” Most (but not all?) of the differences between XBLs and RBLs are explicable using a “viewing angle” model in which the soft X-ray emission emanates into a much wider cone (23-30°) than the radio emission (8-10°). 2. New ROSAT PSPC soft X-ray observations are presented for complete samples of XBLs and RBLs which may explain the absence of luminous optical emission lines in BL Lacs. 3. A deep, subarcsec optical imaging survey of a large (50) sample of both XBLs and RBLs conducted at the Canada-France-Hawaii 3.6m Telescope (CFHT), whose purpose was to characterize the host galaxies and clustering environment of BL Lacs. This work defines more precisely the “parent population” of BL Lacs and identifies a small number of discrepant objects which may not be “beamed FR 1s”.

The evidence for misdirected quasars in the nuclei of powerful radio galaxies (PRG) is now solid: 1. strong perpendicular polarization of the UV continuum, produced by the scattering of light from the hidden nucleus, has been observed in 10 (out of 11) PRG with z > 0.7 (Cimatti et al. 1993); 2. polarized broad MgII2800 has been seen in 6 PRG (di Serego Alighieri et al. 1994, Antonucci et al. 1994), showing that the hidden nucleus is indeed a quasar. These findings are receiving authoritative confirmation from observations with the Keck telescope (Cimatti, Cohen and van Breugel in these Proceedings). Here I would like to concentrate on recent developments of our understanding of the unification between quasars and PRG.

The evolutionary unified scheme (Vagnetti et al., 1991; Vagnetti and Spera, 1994) is the first attempt to describe the radio source populations accounting at the same time for their cosmic evolution and their anisotropic phenomena. It achieves an interconnection between anisotropy and evolution through a changing balance between two optical components, an isotropic “thermal” one and a relativistic beam. It predicts an evolution of the radio-optical ratio, interpreted as a slow increase of the Lorentz factor with the cosmic time (Vagnetti et al., 1991). It also predicts a link between the high and low-power unified schemes, and in particular between high and low-redshift blazars, and between high and low-redshift radio galaxies.

The viewing angle of ≃45° (between the jet axis and the line of sight) dividing quasars and radio galaxies in the orientation-based unified scheme (Barthel 1989) is based largely on the observation that in the redshift range of 0.5 < z < 1.0 in the low-frequency 3CRR sample, there are about twice as many radio galaxies than quasars and the median linear size of galaxies is about twice that of the quasars. The relative numbers and sizes of quasars in other redshift ranges in the 3CRR are however in conflict with the simple unified scheme even if the opening angle is considered to evolve with epoch or radio luminosity (Kapahi 1990; Singal 1993). Larger sources samples, preferably selected at low radio frequencies, are clearly important in such statistical studies.

In recent years a number of correlations have been observed between the large-scale properties of FRII radio galaxies and quasars. Some correlations find their simplest explanations in terms of environmental effects, others in terms of beaming/orientation effects. The problem, however, is that there is an implied correlation between these different types of effects. This is a report on work underway to investigate this directly and exclude the confusion introduced by use of different samples.

In powerful FRII radio galaxies and quasars the polarization of the two radio lobes is often very asymmetric and strongly correlated with jet sidedness. These sources usually have one-sided jets and the lobe on the jet side depolarizes more slowly than the one on the counter-jet side (Laing 1988; Garrington et al. 1988). This correlation can be interpreted in terms of depolarization of the receding lobe by a halo of hot gas surrounding the radio source and is in the expected sense if the jet asymmetry is due to Doppler boosting.

What causes the differences in the central engines of AGN? Are they intrinsically different or are the differences entirely due to environment and orientation? We have postulated that there is basically only one engine, consisting of a black hole and a closely coupled (even symbiotic) jet/disk system with very similar parameters in all AGN. This has lead to interesting results concerning the modelling of the UV/radio correlations of quasars and the difference between FRI and FRII radio galaxies.

High redshift radio galaxies (HzRGs) are observable up to cosmological distances competitive with the most distant quasars. However, before using them as probes of galaxy evolution, it is crucial to separate the stellar and non-stellar components. One of the most striking properties of HzRGs is the alignment of the UV continuum with the axis of the radio source (alignment effect; McCarthy et al. 1987). However, the relative importance of the stellar and non-stellar radiation to the alignment effect is still unknown, although a significant fraction is recognized to come from scattering of anisotropic radiation emitted by the obscured nucleus, as expected in the unified model of powerful radio sources (di Serego Alighieri, Cimatti & Fosbury 1994). Spectropolarimetry is the most powerful technique to observe at the same time different radiation components, but the 4m class telescopes can reach a sufficient S/N ratio only on the few brightest objects. Therefore, in order to investigate the origin of the alignment effect and to test the validity of the unified model of powerful radio-loud AGN, we have started a program of optical spectropolarimetry of HzRGs with the W.M. Keck 10m telescope equipped with the Low Resolution Imaging Spectrometer (LRIS) in polarimetric mode.

Primarily to illustrate the effects of orientation on the optical properties of radio-loud quasars, we have created a set of composite optical spectra (see also Baker & Hunstead 1995). Optical spectra drawn from the 408-MHz Molonglo Quasar Sample (MQS) have been coadded in four sets according to R, the ratio of radio core-to-lobe flux, which is used as an orientation indicator (eg. Orr & Browne 1982). Compact steep-spectrum (CSS) quasars (see review by Fanti, this volume) have been combined separately, revealing for the first time many distinguishing features in their average spectra.

There is a growing evidence that radio loud quasars and powerful FR II radio galaxies belong to the same population. While X-ray observations of low redshift radio galaxies generally support the unified scheme relating the FR II radio galaxies to the radio quasars, nevertheless detailed studies of the X-ray properties of distant radio galaxies are made difficult due to both the low sensitivity of X-ray-satellites and to the emission of the hot intracluster gas in which they are normally embedded. We point out that significant fluxes of X-rays are produced in the strong radio galaxies by the Inverse Compton (IC) process. In the framework of the unified scheme the radio galaxies are pervaded both by an intense radiation flux from the misdirected hidden quasar and the cosmic microwave background radiation flux (CMB). From the standpoint of IC computation the far and near-IR emissions of the hidden quasar are of particular importance.

More than 50 sources have been detected by EGRET in 4 years of operations (e.g. von Montigny et al. 1995). Almost all of them show the violent characteristics typical of blazars, such as superluminal motions, strong radio emission mainly produced in a flat spectrum core and large amplitude variability at all frequencies. Interestingly, optical polarization does not seem to be required, since more than 1/3 of the detected sources are less than 3% polarized.

We compare the results of a spherical, steady-state cooling-flow model with gas loss with the observed X-ray emission profiles and temperatures for five bright elliptical galaxies in Virgo and Fornax. We adopt a “thermal-instability” mass-loss prescription (following Sarazin & Ashe 1989) and refer to galaxy models constrained by radially extended stellar dynamical data (Saglia et al. 1992). Our cooling-flow models are specified by three free parameters: the mass accretion rate and the pressure pext at some external radius rext , and a dimensionless constant q, which regulates the mass deposition rate along the flow. We find that models with confining pressures of pext ≃ 4 ÷ 14 × 103K cm–3 and significant accretion rates of external material, up to 4M⊙/yr, provide emission and temperature profiles in good agreement with present-day data. The trend shown in the right frame of the lower panel in the Figure suggests a possible correlation between LX/LB and the iron abundance in the gas inside ellipticals. A full account of this work, together with all the relevant references, is given in Bertin & Toniazzo (1995); here below, we add a comparison of the models with some recent ASCA data (Loewenstein et al. 1994; Mushotzky et al. 1994).

This work was partially supported by ASI (under contract 94RS94-202/3 FAE) of Italy.

A model has been developed for impulsive VLBI jet formation and gamma ray outbursts of Blazars. Propagation of newly expelled matter in the old channel of a jet is calculated supposing that the main driving force is the electromagnetic field. The new outflowing matter overtakes the old matter and forms double, fast or slow magnetosonic shock fronts. In the region of the fronts, the number of particles and their energy increase continuously with propagation time from the central object (Romanova and Lovelace, 1995). Accelerated electrons and positrons in the front interact with a diffuse field of UV photons (inverse Compton scattering), with the magnetic field (synchrotron radiation), and with synchrotron photons (SSC processes), thus creating radiation in a very wide range of bands. The self-consistent relativistic equations for the number of particles, the momentum, energy, and magnetic flux in the front are derived and solved numerically (Lovelace and Romanova, 1995). The time-dependent apparent luminosities in the radio to gamma ray bands are calculated taking into account the Doppler boost of the photons. The model predicts a short outburst of radiation in gamma rays (weeks or so) connected with Compton processes, a sharp (less than a day) outburst in the X-rays with a smooth decrease of the luminosity connected with SSC processes, and synchrotron radiation changing from infrared to radio bands (Fig. 1A). The lepton distribution function was taken as fl = K 1/γ2 in the main energy containing range, γ1 ≤ γ ≤ γ2, steeper distribution fl = K2/γ3 for γ2 ≤ γ ≤ γ3, and even steeper for γ ≥ γ3. For γ < γ1, fl is assumed negligible as a result of synchrotron self-absorption. The lowest frequency f(syn 1), determined by self-absorption, corresponds initially to the infrared band, and later - to the radio band. From Fig.1B, one can see that radio at 3 mm may start to appear after 2 weeks after outburst. But its maximum may correspond to much later times (months), because f(syn 1) decreases slowly with time. The appearance of the new VLBI component in QSO 0528+134, which approximately coincides with the strong gamma-ray flash and with the beginning of the strong mm radio outburst (Krichbaum, et al. 1995; Pohl, et al. 1995), supports the proposed model.

Both authors were supported in part by NSF grant AST-9320068. MMR is grateful to RFBR and Organizers of the Symposium for the partial support.

We explain the overall continuous Grazar (Gamma Ray Blazar) spectrum from the synchrotron turnover to the EGRET GeV detections by means of Comptonization in the parsec scale jet's substructures.

While making use of the constraints on the synchrotron spectrum and other measurable quantities, no exotic particle acceleration is needed to achieve the high energy output.

We show, that the “Lighthouse Model” of blobs of relativistic electrons, travelling with the jet plasma at relativistic speeds, produce both, correct timescales and shapes for the lightcurve, and correct ratios and slopes of the synchrotron, X-ray and γ-ray branches.

Nonthermal radio sources near the Galactic Center with flat or weakly inverted spectra (S((ω) ∝ ωα with α ≳ 0) are attributed to optically thin synchrotron emission from a hard electron energy spectrum, N(ε) ∝ ε–α with a = 1 – 2α ≲ 1, produced by Fermi acceleration or diffusive shock acceleration at multiple shocks combined with a synchrotron pile up. This basic mechanism is also plausible for flat-spectrum AGN.

A workshop on the dynamics, emission and morphology associated with the highly collimated outflows seen in radio galaxies and quasars was held at The University of Alabama in Tuscaloosa in September of 1995. Complete workshop proceedings will be published in the ASP Conference Series. This workshop review covers the theoretical developments and problems discussed at the workshop.

We have used massively parallel supercomputer simulations to perform an extensive study of a plausible mechanism for producing the jets seen in extragalactic radio sources - acceleration and collimation by coronal magnetic fields in an accretion disk orbiting a central black hole. We find that such disks can propel jets for a wide range of coronal conditions. The terminal jet velocity is a strong function of the magnetic field equatorial component. Acceleration and collimation are produced by a tight azimuthal field coil generated in the corona, rather than by a stiff poloidal field extending to large distances. The jets are pressure-confined when the external medium pressure is high, but magnetically-confined when it is low.

Numerical simulations of supersonic jets are able to explain the structures observed in many VLA images of radio sources. The improvements achieved in classical simulations (see Hardee, these proceedings) are in contrast with the almost complete lack of relativistic simulations the reason being that numerical difficulties arise from the highly relativistic flows typical of extragalactic jets. For our study, we have developed a two-dimensional code which is based on (i) an explicit conservative differencing of the special relativistic hydrodynamics (SRH) equations and (ii) the use of an approximate Riemann solver (see Martí et al. 1995a,b and references therein).