An MHD computation of a restarting jet is presented and compared with observations of classical double radio sources. Assuming ideal MHD and an adiabatic equation of state, it is found that a “partial” jet exists for about 25% of the time, and that the terminal hotspot dissipates quickly compared to other physical time scales once the outflow of the jet is terminated.

In the framework of the diffusive shock acceleration of relativistic electrons in extragalactic jets, we show that it is possible to derive the speed of the jet. For this purpose, we transform continuity relations through an oblique shock front into the reference frame of the observer. We apply these calculations to knot A of the M87 jet. Measuring the deviation of the fluid through the shock front from high-resolution radio maps and the deviation of the magnetic field from optical polarization maps (Fraix-Burnet et al., 1989), we derive speeds of about 0.01 c (Fraix-Burnet and Biermann, in preparation). The compression ratio is most probably 4 and the magnetic field is nearly parallel to the shock front both upstream and downstream.

We exploite the detailed calculations and observations of the earth bow-shock and suggest that similar collective plasma processes can account for the dissipation in hot spots of extragalactic radiosources. On the base of magnetohydrodynamic models for the origin and propagation of current carrying jets we show that in the terminal shock in hot spots lower hybrid-turbulence is excited. This turbulence driven by drifting electrons in the shock provides the anomalous conductivity required for current closure. Lower-hybrid turbulence is also generated by ion reflection in the upstream plasma. The kinetic energy of these ion beams is partially transformed into lower hybrid wave energy. These waves also determine the ratio of energies taken for plasma heating and particle acceleration. This is due to the fact that lower hybrid waves propagate almost perpendicular to the magnetic fields (large (small) wavenumbers perpendicular (parallel) to the magnetic field), their phase velocity parallel to the B-field are much larger than perpendicular to the field. Therefore electrons and ions are differently coupled to these waves. The resonant interactions of the upstream plasma with these waves leads to strong ion heating and electron acceleration. The ions reach a temperature (Ti ≃ 51011K). The electrons are accelerated up to relativistic energies (γ ≃ 1300).

Using a quasi–one-dimensional MHD model for narrow jets (which we develop by projecting the system of the ideal MHD equations onto the jet axis) we examine the relative importance of the combined effects of rotation and magnetic fields upon the acceleration of matter in jets. We also examine the relative importance of the gas pressure and the gravitational field of the source on the flow and the position of the three critical points (Alfvén, slow and fast magnetosonic). The model, being semi-analytic, allows us to easily extract the relevant physical parameters, explore extreme ranges of these parameters, find the parameter range over which outflows are possible, and identify the physical consequences of different assumptions.

Two aspects of the MHD stationary equilibrium model developed by Chiuderi et al.(1989) to describe extragalactic jets are analyzed and compared with the observational constraints: the global energy flux convected by the cylindrical jet and the ranges of the equilibrium parameters allowed by the stability analysis. In particular, the results obtained from the temporal stability analysis are converted into a spatial point of view. In this context, it is easier to find essentially “stable” equilibrium configurations for shorter jets. In conclusion, the fundamental hypotheses of this model (like thermal confinement and substantial equipartition among the various forms of energy considered) are such that the model turns out to be suitable for the description of class I jets, associated with rather low-power radio sources.

Jets are known to be emitted from the centers of active galaxies and quasars. In two recent papers (Rose, 1987, 1989) calculations of magnetic fields generated in jets because electrons drift with respect to protons have been described. Electron drift velocities can result from interaction with the cosmic background radiation or with ambient gas. We describe how magnetic fields can be generated in jets and radio galaxies.

We propose a model for extragalactic radio jets in which two different flows of particles are taken into account, (i) a beam of relativistic electrons and positrons extracted from the funnel of accretion disc and responsible for the observed superluminal motion, (ii) a classical or mildly relativistic wind of electrons and protons coming out from all parts of the disc (Sol et al., 1989). Studying the mutual interaction of the two flows, we show that the configuration is not destroyed by the plasma-beam instability as long as the magnetic field, assumed longitudinal, is strong enough, with an electron gyrofrequency ωc = eB/mec greater than the ambient plasma frequency ωp = (4πnpe2)1/2 (Pelletier et al., 1988). When ωc < ωp, the relativistic beam loses its energy and its momentum mainly through the development of strong Langmuir turbulence in the wind, and disappears quietly after some relaxation zone where heating and entrainment of the wind occur. This emphasizes one aspect of the important role likely played by the magnetic field in the dynamics of extragalactic jets and provides one example in which the magnetic field, acting on the microscopic scale of an interaction, induces strong effects on large–scale structures. Detailed data on the closest known superluminal radio source 3C120 (Walker et al., 1987, 1988; Benson et al., 1988) allow a check on the likelihood of our model. Observational estimates of the variation along the jet of the magnetic field and of the ambient plasma density np suggest that the magnetic field reaches its critical value (corresponding to ωc = ωp) at a minimal distance of about 1.4 kpc from the central engine. This is amazingly close to the location of the 4′–radio knot, a “rather curious structure” described by Walker et al. (1987), which we interpret as the beam relaxation zone in the context of our two–flow model (Sol et al., 1989).

We apply the theory of the Kelvin-Helmholtz instabilities to trace the jet's pattern. A certain level of noise is introduced.

The existence of non–steady phenomena, namely activity in the form of radial motions (outflow) of matter from the nuclei of galaxies is well established at present. Active Galactic Nuclei (AGN) constitute a topic of great interest and are intensively studied by all existing observational techniques. Conventionally objects classified as AGN span a range from quasars, radio galaxies to Seyferts 1 and 2. It appears, however, that there exist galaxies which exhibit somewhat milder activity which does not qualify their inclusion in the AGN group. The designation of MAGN (M for mildly) was suggested in the past (Pismis, 1986) to cover the less energetic nuclei. It may be reasonable to consider that active nuclei form a sequence, the difference along it being due to the energetics of the nuclei, from the most active quasars and radio galaxies down to the mildest ones like M31 or our Galaxy. The phenomenon underlying the activity may thus be universal, subject to the intrinsic energetics of the nuclei (Pismis, 1987).

We presented new observations in various wavebands of those extragalactic radio sources for which the emission of optical synchrotron light is established by polarimetric investigations (jets of M 87 and 3C 273, hot spots in the lobes of 3C 20, 3C 33 and Pictor A). In addition to optical spectral index and polarization maps, we collected flux measurements at radio, millimetre, and near-infrared frequencies. Our observations result in accurate synchrotron spectra between 109 and 1015 Hz. For some sources a detailed comparison between radio and optical polarization is possible.

Although optical synchrotron light from an extragalactic source - the jet in M 87 - has been discovered more than 30 years ago (Baade 1956) there are still only a handful extended radio sources with established optical synchrotron emission (see Röser 1989 and references therein, also Meisenheimer et al. 1989b). We outline the relevance of optical studies and summarize some results concerning the magnetic fields in extragalactic radio sources.

We present radio polarisation observations of the quasar 4C49.22. It shows no evidence of depolarisation on the counter–jet side, while exhibiting strong variation of fractional polarisation along the jet, as well as transverse offset of maximum fractional polarisation from each knot in the jet.

We have measured the distribution of polarized emission from the radio galaxies NGC315, NGC6251, DA240, 3C236 and 3C326 at 327 MHz (92 cm wavelength) with the Westerbork Synthesis Radio Telescope (WSRT). These radio galaxies were all selected for observation because they exhibited degrees of polarization as high as 60 or 70 percent at 610 MHz (49 cm wavelength) and because they all have angular sizes greater than about 20 arcmin. Therefore they are still well-resolved by the WSRT at 92 cm wavelength. All sources in the sample exhibit significant linear polarization at 92 cm wavelength. Some depolarization does occur in NGC315 and 3C326 but parts of 3C236, DA240 and NGC6251 appear not to depolarize to 92 cm. Assuming an ordered, equipartition magnetic field, we find that the density of thermal depolarizing material in these radio galaxies is very low, typically about 1 × 10−5 cm−3 or less. Therefore these radio galaxies may represent distributions of pure magnetic fields and relativistic particles on scales of several hundred kiloparsecs or more.

It has been recently shown (Bodo et al 1989) that filamentary structures observed in some extended radio sources (e.g. Cygnus A and M 87) can be related with thermal instabilities arising in the gas with a synchrotron radiating relativistic component. In particular is has been shown that, with the typical parameters of radio sources, two kind of unstable modes can develop on time scale comparable with the synchrotron cooling time: condensation stationary modes and travelling slow MHD waves. Both these kind of perturbations should lead to the formation of structures basically parallel to the magnetic field direction, as observed, however their general evolution, in particular if they will lead to the formation of dark or bright structures, cannot be extrapolated from the linear perturbative analysis.

Faraday depolarization estimates of thermal densities within the components of double radio sources agree well with estimates from X-ray observations of hot halos around early-type galaxies, provided magnetic field strengths are close to their equipartition values. Internal Faraday dispersion is the main cause of the depolarization observed.

Faraday rotation measurements of the luminous radio galaxies 3C295, Cygnus A, and Hydra A, all of which are associated with cD galaxies located in massive cooling flow clusters, show very high rotation measures – typically a few thousand radm−2, exceeding 20,000 radm−2 for 3C295. The RMs vary over large ranges and with different scales. The RMs are both positive and negative, implying reversals of the magnetic field – the variations cannot be due to density changes alone. The RMs originate in magnetized cluster gas, but it is unclear what fraction is due to a coccoon surrounding the radio source. If from the cluster as a whole, the magnetic field is , where N is the number of cells along the line of sight. A good case can be made for an enhanced magnetic field localized at or near the surface of the radio source, accounting for a significant fraction of the observed RM.

New VLA observations have revealed sharp gradients in both the depolarization and rotation measure in the lobes of Cen A. We attribute these jumps to a foreground screen in the southern lobe, and to the passage of the jet through a shock in a Malin shell in the northern lobe.

Intrinsic large rotation measures (RM) were searched for 128 extragalactic sources based on polarization data obtained at the NRO 45–m telescope. The number of sources with RM > 500 rad m−2 is only seven.

The asymmetric depolarization seen in powerful extragalactic double sources with one sided jets may be explained in terms of differential Faraday effects in a halo of gas surrounding the source, provided the halo has a core-radius ~30–100 kpc, and contains a magnetic field of order >1 μGauss tangled on a scale <5 kpc.

It is proposed that the large rotation measure gradients observed in the lobes of some extragalactic radio sources are produced by non–linear Kelvin–Helmholtz surface waves. Related small scale turbulence is responsible for a “diffuse spray” of relativistic plasma and magnetic field into the surrounding medium. The mixture of magnetic field and thermal plasma causes large rotation measure variations on the scale of the Kelvin–Helmholtz waves. An order of magnitude prediction of the effect is in good agreement with the observations of Cygnus A and PKS 2104–25 N.