Polarization data of strong double-lobed radio sources (Garrington et al., 1988a; Laing, 1988) in many cases show that one side is more depolarized than the other. Since a jet is seen only on the less depolarized side it can be concluded that this radio lobe is nearer to us, if the one-sidedness of the jet is interpreted by bulk relativistic motion. The effect is then due to random Faraday rotation where the RMS-rotation angle is larger than about π/2 for the longer wavelength. This suggests an intervening magnetized plasma which may be the hot gas in the halos of the (elliptical) galaxies or in the cluster. Comparing the effects of both, the intracluster medium (ICM) probably is the dominating component. Garrington (1988b) comes to the same conclusion. Considering the transport of polarized radio waves in a turbulent Faraday screen (cells of size l0) we further find that the coherence length of the magnetic field is of the order of l0 = 1–4 kpc. From EINSTEIN X-ray data (for 3C9, 4C01.11, 3C270.1, 3C275.1, 3C208) we find luminosities in the range Lx = 0.6–7 × 1045erg s−1, which can only be due to the cluster gas or an active galactic nucleus. If we assume that the total X-ray flux is produced by the ICM the electron core densities are n0 = 2–7 × 10−3 cm−3. Combining this with the values for l0 gives upper limits to the ratio of thermal to magnetic pressure (plasma-beta) of βp = 50–370 and lower limits to the core magnetic field strength of B0 = 3–9 μG. If the AGN contributes substantially to the X-ray emission the given limits would be even stronger, in the direction of equipartition of energy in the hot gas and in the magnetic field, since B0 has to be larger if n0 is smaller to account for the same dispersion in Faraday rotation. We plan to separate the diffuse and the pointlike emission by ROSAT observations. A more detailed version of this paper will be presented elsewhere (Crusius–Wätzel et al., 1989).

A new series of detailed observations of the Coma cluster of galaxies has been undertaken with the aim of better specifying the intracluster gas component throgh high dynamic range, combined-telescope observations of the radio halo emission from the cluster which detect both the cluster-scale and galaxy-scale emission. By combining these multi-frequency maps with a Faraday rotation probe experiment using cluster and background sources, and the published X-ray data, we have been able to estimate the intracluster magnetic field strength independently of the unusal assumption of equipartition. The result is approximately 2 microgauss and the tangling of the rms field occurs on an optical galaxy scale.

We present 22 and 49 GHz interferometric observations of Hyd A (3C218). The source was found to have a very large Faraday rotation measure (RM) (Kato et al. 1987), and to be a dominant member of a luminous X-ray cluster with a large cooling flow (David et al. 1988). These characteristics are very similar to those of Cyg A which is suggested to produce a large RM within a dense sheath around the radio lobes as a result of somehow an interaction between dense intracluster medium (ICM) and radio jets and/or lobes (Dreher et al. 1987). Hyd A is the second example of Cyg A type source. In case of Cyg A, hot spots are the place where the interaction between jets and ICM occurs (Carilli et al. 1988). We then expect in Hyd A that similar interaction also occurs to form hot spots, and consequently that high frequency observations reveal structures of the interaction.

The combination of the observed diffuse radio and hard X-ray emission from clusters of galaxies is used to estimate lower limits on the intracluster magnetic field strength or upper limits on the energy density of the cosmic infrared background radiation.

The Coma cluster is a rich cluster of galaxies nested in an even larger supercluster of galaxies. The plane of the supercluster appears to be defined by the Coma cluster itself and another galaxy cluster, Abell 1367, that lies about 40 Mpc (H0 = 75 Mpc km−1s−1 (≡h75)) farther west (Tifft and Gregory 1976).

Abell 115 is an intermediate–redshift cluster (z=0.1971) well studied at different wavelengths (see Giovannini et al., 1987 and references therein).

We consider a current carrying extragalactic jet and the corresponding return current closing the circuit. The Lorentz force for the return path is expansive. Assuming that the expansion occurs at the local Alfvén velocity, we calculate numerically the radial distance from the jet axis which the return circuit reaches due to this electrodynamic expansion. The calculations are made explicity for parameter values observed for the sources Cygnus A, NGC 6251, 3C465 and M84. The results obtained can explain several magnetic features observed in these sources. The possible importance of previous generations of extragalactic jets in the large scale magnetization of the intergalactic medium is also discussed.

The turbulence of intracluster gas in galaxy clusters and interstellar gas in galaxies can act as a dynamo generating chaotic magnetic fields. These fields are concentrated in ropes with the doubled radius of curvature of about the turbulent correlation length l and thickness where Rm is the magnetic Reynolds number. The field strength within the ropes is close to equipartition with turbulent kinetic energy. These results favorably agree with high—resolution observations of the galaxy cluster around Cyg A and correlation analysis of the Galactic nonthermal background. Ropy magnetic fields in interstellar gas lead to observable variations of, e.g., the Faraday depth at time scale of 1 month.

The influence of the hypothetical extragalactic magnetic field upon the peripherical morphological features of spiral galaxies is examined- When the angle between and the rotation axis is neither 0° nor 90°, warps are expected to be produced. In such a case the spatial distribution of warps should not be random. This fact seems to be confirmed by observations. From the distribution it is possible to obtain the direction of B in the 20Mpc-neighborhood of the Milky Way.

A cosmic magnetic field may play a significant role in the formation of galaxies and large scale structure. In particular, a fossil field of present strength ~ 10−9 Gauss is an essential ingredient in the superconducting string model of galaxy formation (Ostriker, Thompson and Witten 1986 (OTW); Thompson 1988a). We discuss the mechanism by which a current is induced on a superconducting string, including recent work on the reconnection of magnetic field lines near the string (Kulsrud and Thompson 1989). A substantial amount of baryonic plasma is trapped on the magnetic field lines which close around the string. The current on a loop almost certainly does not undergo exponential dynamo amplification; an oscillating superconducting loop emits a relativistic MHD wind (Thompson 1988a). Decaying superconducting loops fill most of the intergalactic medium with a relativistic, magnetized fluid. In this model, the gas between galaxies is highly clumped and strongly magnetized, the field strength approaching 1 μG. The maximum energy of cosmic ray protons accelerated at string-driven shocks is ~ 1020 eV (Madau and Thompson 1989).

This paper reviews early attempts to deduce important constraints on the magnetic fields of high-redshift objects through a study of the rotation measure (RM) of QSOs. We have combined detailed studies of RM and optical spectra of 48 QSOs and have statistically analysed RM data on 116 objects and conclude that the objects which give rise to QSO metal-line absorption are also responsible for the observed extragalactic component of RM. This allows us to estimate their average magneto-ionic properties.

One of the models constructed (Wanas, 1985) within the framework of the generalized field theory (Mikhail and Wanas, 1977) is found to give results in favour of Blackett's speculation concerning rotation and the origin of magnetic fields. The formula giving the surface polar magnetic field of a spherical body of mass M, radius R, and uniform rotational velocity w is given by (Mikhail and Wanas, 1989) In case of a typical galaxy, the model gives a magnetic field of the order of 10-5 Gauss.

It has become quite clear that dynamically significant magnetic fields of order 10−6 Gauss are present in galactic halos (Perry, these Proceedings) at redshifts up to z ≥ 2. Comparable fields have been detected on cluster and even on supercluster scales (Giovannini, these Proceedings). The existence of such high fields is surprising because it is highly unlikely that primeval magnetic fields could have been much stronger than 10–18 Gauss. These observations require that there be an epoch of very vigorous dynamo action that can amplify this seed primordial field up to the observed values. Since strong fields are already in place by redshifts of z ≃ 2, this amplification process must operate very quickly. This argues against the idea that such fields could be generated on huge intercluster scales since the dynamical times are far too long. A much more promising site for the dynamo process is in the dark matter halos of young galaxies during the epoch of galaxy formation. The fundamental point is that vigorous dynamo action can take place in shocks that are expected to occur throughout the dark matter potential wells of protogalaxies (Pudritz and Silk 1989 (≡ PS), Pudritz 1989).

If the galactic field arose from flux imbedded in the protogalacticgalactic medium, then one would expect this field to be strongly wound up by the differential galactic rotation once it had formed into a disk. This process is balanced by the buoyancy forces lifting the flux out of the disk. The resulting field turns out to be only slightly wound up. Further, its magnitude depends only on properties of the ISM provided that the primordial field exceeds 10–13 gauss.

In the past investigations of cosmological magnetic fields, Harrison (1970) assumed primordial turbulence with nonzero vorticity. More recently this idea lost favor with most cosmologists, primarily because vortices decay during the cosmic expansion (Rees, 1987). In contrast to these works we resort to no assumption as for the primordial condition but for the thermal equilibrium in the following. A plasma with temperature T in the early universe sustains fluctuations of electromagnetic fields and density even if it is in a thermal equilibrium. The level of fluctuations in the plasma for a given wavelength of electromagnetic fields, for example, can be rigorously computed by the fluctuation-dissipation theorem (Geary et al. 1986; Kubo 1957; Sitenko 1967). In particular, without assuming any turbulence we show that very low (or ~ zero) frequency magnetic fluctuations can also be excited and the level of these is computedr where summation on k is over all the available wavenumbers V the volume of the Universe, and ωp the plasma frequency (4πe2n/m)1/2. The level of fluctuation is given approximately by the equipartition value of T/2 for kωωp/c and much less than that for k>ωp/c. These fields at the early radiation epoch t=10°sec (we call the radiation epoch in which the radiation effects dominate that of gravity in the universe as the plasma epoch: tε10−2 − 1013 sec) can act as seed fields and can evolve during the plasma epoch. The result is shown in Fig. 1. We show that magnetic fields with the size of λ≤108cm can be amplified by the dynamo effect and that the field strength corresponding to this size is greater than 10–19 Gauss.

Fifty years ago at a conference on the structure of galaxies almost everything would have been ascribed to gravity. A decade later turbulence became the new thing - partly to solve angular momentum problems. Again a decade later magnetic fields followed: synchrotron radiation was observed, angular momentum could be efficiently transported and a variety of bizarre extragalactic shapes was thought to be related to magnetic fields. In the sixties, however, the situation was reversed when it turned out that spiral structure could perhaps be understood on a gravitational picture with some simple hydrodynamics and that some of the most intriguing shapes could be reproduced in simulations of the gravitational interaction between galaxies. As a consequence magnetic fields again diminished in importance. The next decade saw a much increased role for the thermodynamics of the interstellar medium. At the end of this meeting it is clear that while gravity, hydrodynamics and thermodynamics are important, magnetic fields cannot be neglected and should be integrated into the overall picture.