Abstract. We performed a high-sensitivity search for galaxy-scale magnetic fields by radio polarimetry at 10.45 GHz and 4.85 GHz with the Effelsberg 100 m radio telescope, accompanied by Hα imaging, for the two Local Group irregular galaxies IC 10 and NGC 6822. Their star-forming bodies are small and rotate slowly. IC 10 is known to have a very high star-forming activity, resembling blue compact dwarfs, while NGC 6822 has a low overall star-formation level. Despite very different current star formation rates, our Hα imaging revealed a large web of diffuse Hα filaments and shells in both IC 10 and NGC 6822. Some of them extend far away from the galaxy's main body. The total power emission of both objects shows bright peaks either at the positions of optically strong star-forming clumps (IC 10) or individual H  regions or supernova remnants (NGC 6822).However, in both cases we detect a smoothly distributed, extended component. In IC 10 we found clear evidence for the presence of a diffuse, mostly random magnetic field of 14 µG strength, probably generated by a fluctuation dynamo. One of the Hα-emitting filaments appears to be associated with enhanced magnetic fields. We also rediscuss the reddening of IC 10 and its implications for its distance. In the case of NGC 6822 we found only very weak evidence for nonthermal emission, except perhaps for some regions associated with local gas compression. We detect in both galaxies small spots of polarized emission, indicative of regular fields ( 3 µG), at least partly associated with local compressional phenomena.Key words. polarization -galaxies: irregular -galaxies: magnetic fields, galaxies: individual: IC10, NGC 6822 -radio continuum: galaxies IntroductionIrregular galaxies are low-mass objects exhibiting a variety of rotational properties with a subclass of them rotating very slowly (rotational speeds V rot ≤ 30 km s −1 ) and often chaotically (e.g. Lo et al. 1993). They constitute important laboratories for large-scale interactions of stars with the interstellar medium: the low gravitational potential and relatively small size increase the probability that superbubbles, forming close to star-forming regions, may break out of the galaxy (e.g. Mac Low & Ferrara 1998). Many irregular galaxies exhibit giant arcs or filaments of ionized gas (e.g. Sabbadin & Bianchini 1979;Hunter et al. 1993;Bomans et al. 1997). The role of magnetic fields in the origin and confinement of these ionized structures (e.g. Hunter & Gallagher 1990) is still a matter of debate.In spiral galaxies magnetic fields, which are sufficiently strong to trigger star formation via magnetic instabilities (Blitz & Shu 1980) or to influence the superbubble expansion (Ferriere et al. 1991), are probably generated by the mean-field dynamo (see . This requires strong Coriolis Send offprint requests to: K. Chyży, e-mail: chris@oa.uj.edu.pl forces (hence a rapid rotation) to give the turbulent motions a preferred sense of twisting. A sufficient size of the ionized gas envelope is also required. Large irregulars...

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Context. The origin of large-scale magnetic fields in spiral galaxies is still a theoretical riddle and better observational constraints are required to make further progress. Aims. In order to better determine the large-scale 3D-structure of magnetic fields in spiral galaxies we present a Faraday rotation analysis of the edge-on spiral galaxy NGC 5775. Methods. Deep radio-continuum observations in total power and linear polarization were performed at 8.46 GHz with the VLA and the 100-m Effelsberg telescope. They were analyzed together with archival 4.86 and 1.49 GHz VLA-data. We thus can derive rotation measures from a comparison of three frequencies and determine the intrinsic magnetic field structure. Results. A very extended halo is detected in NGC 5775, with magnetic field lines forming an X-shaped structure. Close to the galactic disk the magnetic field is plane-parallel. The scaleheights of the radio emission esimated for NGC 5775 are comaprable with other galaxies. The rotation measure distribution varies smoothly on both sides along the major axis from positive to negative values. Conclusions. From the derived distribution of rotation measures and the plane-parallel intrinsic magnetic field orientation along the galactic midplane we conclude that NGC 5775 has an even axisymmetric large-scale magnetic field configuration in the disk as generated by an αΩ-dynamo which is accompanied by a quadrupolar poloidal field. The magnetic field lines of the plane-parallel component are pointing outwards. The observed X-shaped halo magnetic field, however, cannot be explained by the action of the disk's mean-field dynamo alone. It is probably due to the influence of the galactic wind together with the dynamo action.

Abstract. By observing the interacting galaxy NGC 3627 in radio polarization we try to answer the question; to which degree does the magnetic field follow the galactic gas flow. We obtained total power and polarized intensity maps at 8.46 GHz and 4.85 GHz using the VLA in its compact D-configuration. In order to overcome the zero-spacing problems, the interferometric data were combined with single-dish measurements obtained with the Effelsberg 100-m radio telescope. The observed magnetic field structure in NGC 3627 suggests that two field components are superposed. One component smoothly fills the interarm space and shows up also in the outermost disk regions, the other component follows a symmetric S-shaped structure. In the western disk the latter component is well aligned with an optical dust lane, following a bend which is possibly caused by external interactions. However, in the SE disk the magnetic field crosses a heavy dust lane segment, apparently being insensitive to strong density-wave effects. We suggest that the magnetic field is decoupled from the gas by high turbulent diffusion, in agreement with the large H i line width in this region. We discuss in detail the possible influence of compression effects and non-axisymmetric gas flows on the general magnetic field asymmetries in NGC 3627. On the basis of the Faraday rotation distribution we also suggest the existence of a large ionized halo around this galaxy.