After a brief historical summary of radio observations of spiral galaxies I review the methods of analyzing radio polarization data in view of the magnetic field. Special attention is drawn to the Faraday rotation and depolarization effects and to the identification of the large-scale magnetic field structure. The present observational results and open questions are discussed in terms of the predictions of the dynamo theory and prospects on future work are given.

Multi-frequency radio polarization observations of external galaxies are now available. The resolution is such that we can sample many positions over the galaxy, and different scale sizes. Faraday rotation measures, initial polarization angles, and depolarization can be used as indicators of magnetic field morphology in the source. In order to interpret the variation of polarization characteristics with frequency, a numerical model of Faraday rotation in galaxies is being developed. Preliminary results show that a wide range of situations can be investigated with the model.

A three-dimensional, arc-like structure in the magnetic field was found coming out of the plane of M31. This structure may be the first Parker-Jeans instability observed in an external galaxy.

Gradients in rotation measure and dispersion in rotation measure, both across the telescope beam, depolarize the radio emission from the SW arm of M31. Faraday effects along the line of sight appear to be negligible.

A region with strong radio polarization in the SW of M31 was observed in the 12CO(1–0) line. The results are compared with the radio continuum, HI and Ha. The CO emission appears to be correlated with the total radio emission, but anticorrelated with the polarized intensity.

We present new results from a number of deep radio polarization surveys of the Magellanic Clouds at 2.3 GHz, 4.75 GHz and 8.55 GHz. Extended linearly polarized radio emission has been found at 2.3 and 4.75 GHz from both galaxies.

Measurements at five radio wavelengths between 2.8 and 21.1 cm (Buczilowski and Beck, 1987) with the Effelsberg 100-m telescope make M33 the best studied nearby galaxy in linear polarization so far. The polarized emission is concentrated to the northwestern quadrant of the galaxy. Almost no polarization is detectable around the 35 known supernova remnant candidates. The explosions may have disturbed the interstellar magnetic field on scale sizes smaller than our antenna beams.

The polarized radio continuum emission of M51 is found to be strongest outside the optical spiral arms. The rotation measure varies double-sinusoidally with azimuthal angle which for the first time clearly shows a dominating bisymmetric spiral (BSS) structure of the large-scale magnetic field in M51. The pitch angle of the magnetic spiral is similar to that of the optical spiral arms. The lowest (axisymmetric) dynamo mode is possibly suppressed by interaction with the companion galaxy.

New VLA radio observations of NGC 6946 reveal a complicated structure of the magnetic field with possibly radial changes in the foreground Faraday rotation, the strength of the uniform field and the particular dynamo mode in operation. The uniform field is strongest in the interarm regions, but shows no deducible large-scale structure in the radius range of the optical spiral arms. The high star formation rate in NGC 6946 apparently increases the turbulence of the interstellar magnetic field.

Recent VLA 20 cm radio continuum observations of the southern face-on barred spiral M83 reveal that the magnetic field is very highly aligned at the outer regions (~12 kpc radius) and totally disrupted in the inner regions (<6 kpc) of the galaxy. The RM variation suggests an axisymmetric morphology for the magnetic field. VLA 6 cm continuum polarization observations of the edge-on spiral NGC 891 reveal ordered magnetic fields at large Z-distances (~3 kpc) from the galactic plane, probably emanating from the disk through instabilities.

We observed the linearly polarized emission of the edge-on galaxies NGC891 and NGC4631 in order to determine the magnetic field structure.

A general correlation between the radio and optical continuum surface brightness has been found in the edge-on galaxy NGC 891. This suggests that the relativistic electrons are produced/accelerated in the vicinity of evolved stars or stellar remnants which have a spatial distribution similar to old disk stars. In this picture, the magnetic field required for the synchrotron emission is carried up from the disk to high Z through instabilities and star-forming activity in the plane.

We have begun a long-range project to study southern galaxies using the radio telescopes at Parkes and Molonglo, the Siding Spring optical facilities and soon, the Australia Telescope. Here we present the results of polarization mapping at two wavelengths of the galaxies NGC 55, 253,4945, M 83 and the Circinus Galaxy.

Evolution of a vertical magnetic field which may have existed in the primeval galaxy is discussed based on a primordial-origin hypothesis for the bisymmetric spiral configuration of magnetic fields in spiral galaxies. The vertical field is accumulated toward the nucleus and forms a strong poloidal field, which may trigger activities like jets. Quasar jets are suggested to be the result of such strong vertical fields in the cores of primeval galaxies.

From the literature we collected radio and magnetic field data for the ANS spiral galaxies. We suggest that the groups of objects, as revealed in the UV range, do not differ in magnetic field strength, although statistics of the sample are very poor.

The use of equipartition calculations in estimating magnetic field strengths and energetics of extragalactic radio sources is widespread and well known. Since it is one of the few ways in which to calculate radio source parameters, it is important to determine how reasonable the approach generally is. Since this assumption is approximately a minimum energy criterion one expects that deviations from equipartition are limited at some level by independently determined constraints on the total energy. In this regard we have analyzed radio images of nearby spiral galaxies in order to determine equipartition magnetic fields and relativistic gas energies and to explore their possible nonequipartition configurations.

The remarkably tight global correlation between integrated far-infrared and radio continuum emission from spiral galaxies has recently stimulated interest in determining whether the relation holds spatially within galaxies (Wainscoat et al. 1987; Beck and Golla 1988; Bicay et al. 1989, hereafter Paper I). We report here on a detailed comparison of the distribution of 60μm infrared and 20cm radio continuum emission within 25 galaxies, mostly disk spirals. Local maxima in the thermal infrared and predominantly nonthermal radio maps are found to be spatially coincident on scales <0.3h−1 kpc in nearby galaxies. Superimposed on this broad correlation, we observe in the disks of most sample galaxies a slow decrease in the 60μm-to-20cm ratio Q60 with increasing radius. Values of Q60 within the central regions are often enhanced by a factor of 3 or more compared to the outer disks, whereas the corresponding enhancement in radio surface brightness is greater by at least an order of magnitude. The radial gradient in Q60 is most easily identified in nearby, face-on galaxies (e.g. NGC 5236, NGC 6946) due to the limited IRAS angular resolution. However, the gradient is also observed along the major axis of highly inclined systems (e.g. NGC 55).

We propose a physical model for understanding the tight correlation between far-infrared and non-thermal radio luminosities in star-forming galaxies. This approach suggests that the only constraint implied by the correlation is a universal relation whereby magnetic field strength scales with gas density to a power 1/3≤β≤<2/3.

We have calculated equipartition magnetic fields for a complete, optically-selected sample of 165 spiral galaxies. The magnetic field distribution (fig. 1) is type independent, and shows remarkably little spread in values, around 1 decade in B. This is not due to selection effects because of the nature of the sample and the 95 percent detection rate.

It is shown that the orientation of the dust grain body relative to its angular momentum J by the Barnett relaxation effect and the orientation of J relative to the interstellar magnetic field, by action of anisotropic gaseous fluxes, which always exist in space, is more effective than the orientation of the Davis-Greenstein mechanism. The linear interstellar polarization, which arises by scattering of radiation on oriented grains, is directed along the galactic magnetic field.