6 Conclusions

We observed a sample of 20 barred galaxies with the VLA and ATCA radio telescopes. Polarized radio emission was detected in 17 galaxies.

The flux densities in the radio continuum and the far-infrared spectral ranges are closely correlated in our sample. The average radio/far-infrared flux density ratio and equipartition strength of the total magnetic field are similar to those in non-barred galaxies. These properties are apparently connected to the star formation rate and possibly controlled by the density of cool gas. Radio surface brightness and present star formation activity are highest for galaxies with a high content of molecular gas and long bars where the velocity field is distorted over a large volume. The radio surface brightness is correlated with a newly introduced measure of bar strength proportional to the quadrupole moment of the gravitational potential. However, a few galaxies with strong bars are not radio bright, possibly because their molecular gas has been depleted in a star formation burst.

In barred galaxies with low or moderate radio surface brightness, the regular field (traced by the polarized radio emission) is strongest between the optical spiral arms (e.g. NGC 3359 and 4535) or has a diffuse distribution (e.g. NGC 3059 and 3953). In radio-bright galaxies, the pattern of the regular field can, however, be significantly different: the regular magnetic field may have a broad local maximum in the bar region upstream of the dust lanes, and the field lines are oriented at large angles with respect to the bar (NGC 1097, 1365, 1672 and 7552). We propose that shear in the velocity field around a large bar may enhance dynamo action and explain the observed strong regular fields. Strong bar forcing induces shear in the velocity field and enhancements in the regular magnetic field, and polarized emission traces such shear motions.

The southern galaxies NGC 986, 1559, 1672 and 7552 show strong polarization and are promising candidates for further studies with high resolution. Circumnuclear rings are already known to exist in NGC 1097, NGC 1365, NGC 2442 and NGC 7552 and should be searched for in NGC 986, 1559 and 1672.

Acknowledgements

The authors would like to express special thanks to Dr. Elly Berkhuijsen for critical reading of the manuscript and many useful suggestions. Useful discussions with Drs. Peter Englmaier and David Moss are gratefully acknowledged. ME is grateful to the ATNF for providing support and facilities. His work in Australia was funded through grant No. Eh 154/1-1 from the Deutsche Forschungsgemeinschaft. VS acknowledges financial support from the RFBR/DFG programme No. 96-02-00094G and from the Royal Society. DDS acknowledges financial support from the RFBR programme No. 01-02-16158. JIH acknowledges the Australian Academy of Science for financial support under their Scientific Visits to Europe Program and the Alexander von Humboldt Foundation for its support. VS, JIH, AS and DS are grateful to the MPIfR for support and hospitality. This work was supported by the PPARC Grant PPA/G/S/1997/00284, the NATO Collaborative Linkage Grant PST.CLG 974737 and the University of Newcastle (Small Grants Panel). This work benefited from the use of NASA's Astrophysics Data System Abstract Service.